Cutting Edge Fuel Injection Systems & Components For Over 65 Years

Electronic Fuel Injection Technical Instructions and FAQs

What difference does ram tube length make?

IR manifolds are unique in that changes to intake runner length are easily achieved. Changing this length can have a profound effect on where the engine makes idea power for your particular combination. In general, shorter ram tubes will provide more top end power at the expense of low speed torque, while a taller ram tube will increase low speed torque while losing some top end power.

Racing Applications:

Racing applications involving light-weight cars with high revving engines, such as FED or Jr. dragster, will typically employ a shorter ram tube to move the available power band to the higher RPM. Heavier applications such as door cars, or other racing venues that involve getting on and off the throttle will typically use a longer ram tube to achieve a balance of power throughout the operating range.

Street Applications:

With street applications, many use the ram tube as a focal point. Whether you want them sticking out of your hood, or under the confines of a stock hood, length plays a critical function aesthetically. This typically means that a compromise of sorts has to be made with respect to the ideal ram tube height for that particular engine combination. But what we have found is that the torque generation of an IR manifold is so much better than that of a common plenum manifold, that for most street applications ram tube height has little bearing on performance. So put the ram tube height where it looks the part and enjoy the additional torque regardless of that height.

Ideal Ram Tube Height:

What is the ideal ram tube height for my combination is a common question we get but is very hard to identify. Sure, there are numerous computer models available along with dyno testing which are all popular tools to help identify ram tube height. But even with these tools at our disposal, they have the inability to factor in all of the dynamic conditions of a race car. It is only through track testing that the ideal ram tube height can be determined.

The Ram Tube Bell:

At the top of a ram tube is the bell, and this is where all the magic begins. It has been identified on Indy car engines that the entrance speed at the bell of the ram tube exceeds the speed of sound which would not be possible without a properly designed bell. The bell provides the proper transition of the air into the ram tube which is an important process for the high air speeds generated by an IR manifold. We would like to suggest that the bell not be removed from the ram tube if at all possible.

Manifold Installation

The information below is provided to help in the installation of your injector. If you have any questions or problems during your installation, please call the EFI tech line at 215-643-4607.

Important! Throttle Linkage/Return Spring Placement
It is important to design the throttle linkage and the return spring assemblies so they both pull from the same point on the throttle shaft. This point shuld also be right next to one of the throttle stops. Failure to do so WILL result in poor idle and throttle tip in performance.

Braided lines
All braided lines need to be thoroughly flushed after assembly. The use of a hacksaw, cutoff wheel, etc. creates a tremendous amount of debris, therefore the use of a braided steel hose cutter is recommended.

Injector installation (3-piece models)
Trial fitting your injector is recommended before final installation. Use silicone for the end rail gaskets

3-piece injector installation:

  1. Trial fit injector and plate assembly
  2. Glue intake gaskets to head and trim gasket if required
  3. Install one half of the injector and torque to spec
  4. Lay a large bead of silicone on all 4 edges of the lifter valley plate (gluing the plate will ensure that the bead of silicone stays below the surface of the plate while providing a good seal).
  5. With a helper, hold the lifter valley plate in place without touching the previously installed injector half and the engine. Slightly tilt up the plate to allow room for the other injector half to slide down onto the head
  6. Together, slide the injector half and the plate down to their final resting place
  7. Torque injector to spec and tighten lifter valley plate screws

BBC additional instructions
All oval ports will require the use of a rectangular port intake gasket such as FelPro #1275. BBC applications require the lifter rails to be drilled and tapped to accept (2) 1/4-20 screws to mount center plate.

351W additional instructions
Engines with the three piece 351 injector require the lifter rails to be drilled and tapped to accept (4) 10-24X3/4 screws to mount center plate

348/409 additional instructions
348/409 Distributor Installed Height: Oil pump tang engagement and distributor gear engagement must be verified using the supplied distributor adapter. If required, this adapter can be machined to set distributor height and is o-ringed for a leak free seal.

348/409 Valley Plate Installation: Due to the unique design of the lifter valley rail, additional sealant will be required on the left rear corner of the valley plate to ensure a leak free installation. 348/409 applications will also require the lifter rails to be drilled and tapped to accept (2) 1/4-20 screws to mount center plate.

Throttle Return Springs
Due to the possibility of twisting the butterfly shafts and/or causing drivability concerns, it is recommended that throttle return springs be attached to the throttle arm or a secondary throttle arm that is mounted next to the throttle arm. A throttle stop should be positioned next to the throttle arm to eliminate shaft twist from the return springs. Return springs are not supplied by Hilborn.

Idle Speed
Idle speed is adjusted using the idle stop screws on either side of the injector. Which screw to use to set the adjustment depends on where the throttle linkage/return spring is located. It is recommended that the idle screw positioned next to the throttle linkage/return spring be used as the main idle adjustment. The secondary stop is correctly adjusted when light return pressure is applied by hand to the throttle stop and the throttle blades only slightly move.

Throttle arms are provided as a means to attach your vehicles linkage or cable to the injector. Brackets may need to be constructed. When constructing your linkage please remember:

  • Cable linkage, such as those available from Lokar Products, provides the greatest flexibility.
  • There should not be any bind in your linkage system
  • A mechanical throttle stop should be used. Do not use the injector as the stop or damage to the shafts and couplers could result
  • It is recommended that the throttle return spring be attached to the same point on the injector as the throttle linkage.
  • Avoid using the hex link as it is needed for adjustment
  • Throttle shafts and couplers can be easily bent and care should be taken not to introduce twist into these assemblies. Design your linkage placement and return spring arrangements accordingly

Vacuum Kit
On naturally aspirated applications the vacuum kit is comprised of the rubber lines attached to a block mounted in the center of the fuel injector assembly/manifold. A -6 AN fitting is provided to supply vacuum to those accessories that require it. A PCV valve is not recommended. For blown applications, a single -6 fitting is provided.

! Warning:  Do not attach a fuel line to the vacuum nipple.

Hilborn Injectors are supplied with (2) -8 nipples for coolant flow out of the engine. These nipples must be positioned at the front of the engine or engine damage may result. If additional coolant flow is required, larger nipples may be substituted. Additional nipples may be added to the rear holes of the injector to help cool the rear of the engine. When using a remote thermostat housing, a 1/8 hole should be put in the flange of the thermostat to allow air pockets to exit.

Magnetos are not compatible with EFI due to electromagnetic interference (EMI). If a magneto is desired, consult your magneto manufacturer for electronic conversions.

In order to keep the battery in a fully charged state it is recommended that the alternator output wire be attached directly to the battery. This will keep voltage drops to a minimum allowing the full potential of the alternator to be used. A minimum of an 8 gauge wire is recommended.
! NOTE: One wire alternators produce a large amount of ripple, therefore it is highly recommended that the output be attached directly to the battery to provide adequate buffering of the ripple.

View our Manifold Adjustments Video.

Manifold Adjustments

For correct operation of your Hilborn injector the manifold will need to be tuned. This includes linkage and butterfly adjustments. If the injector is removed from the engine after being tuned, the process will need to be repeated. Although injector linkage is preset at the factory, adjustments will need to be performed after installation. Three piece injectors, by nature of their design, will need to be adjusted before initial startup and when warm.

Adjustments should be made in the following order:

  1. Centering Butterflies
  2. Hex Link Adjustments
  3. Butterfly Arc and Adjustments
  4. Advanced Tuning

The first three adjustments will be performed with the ram tubes, throttle linkage and return springs removed. After you are satisfied with these adjustments, reinstall the throttle linkage, return springs and set idle speed. Recalibration of the TPS may be required. The fourth and final adjustment will then be performed. All adjustments should be performed with the engine at operating temperature.

Centering Butterflies (Stack Injectors) :
This operation verifies the butterflies are centered in the bore which removes the risk of sticking along with ensuring that the butterflies are in a fully closed position for the arc adjustments and for adequate idle speed control.

  1. Loosen lock nuts and back out idle stop screws on both ends of the injector.
  2. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
  3. Loosen the allen head pinch clamp screw on only one bronze throttle arm.
  4. Loosen all the butterfly screws on one side of the engine.
  5. Lightly tap the butterflies closed with your finger while lightly sliding the throttle shaft back and forth against its’ stops.
  6. Center throttle shaft side to side. Tighten butterfly screws using caution not to over tighten.
  7. Perform same adjustments on other side of engine.
  8. Tighten the pinch clamp screw on bronze throttle arm.

Centering Butterflies (Blower Hat or Scoop Injectors) :
This operation verifies the butterflies are centered in the bores, which removes the risk of sticking along with ensuring that the butterflies are in a fully closed position for the arc adjustments and for adequate idle speed control. No other adjustments are required for hat or scoop injectors.

  1. Loosen lock nuts and back out idle stop screws on both ends of the injector.
  2. If your injector is electronic, do not loosen the pinch clamps that control the TPS linkage or improper function of the TPS may result. If you have upset this factory adjustment please call the EFI tech line.
  3. Loosen all the butterfly screws.
  4. Lightly tap the butterflies closed with your finger while lightly sliding the throttle shaft back and forth against its’ stops.
  5. Center throttle shaft side to side. Tighten butterfly screws using caution not to over tighten.

Hex Link Adjustment:
This adjustment verifies the hex link, along with the bronze arms, is at the optimal starting point for the butterfly arc adjustments.

  1. Loosen lock nuts and back out idle stop screws on both ends of the injector.
  2. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
  3. Verify starting hex link length. The ideal starting length of the hex link is to make the centers of the heim joints the same center dimension as the butterfly shafts. An eyeball adjustment is all that is required at this time. NOTE: One side of the hex link will have a groove around the outside of the hex link; this to identify a left hand thread.
  4. Verify correct bronze arm angles by opening the throttle from idle to wide open, making sure the tip of the bronze arms are split on either side of 12 o’clock with relation to the butterfly shafts. In other words, idle will be at 10 o’clock, half throttle at 12 o’clock, and wide open will be at 2 o’clock. If not, loosen the two pinch clamp screws on the bronze throttle arms and adjust as necessary.
  5. Verify that both sets of butterflies are in the fully closed position by loosening one bronze throttle arm, allowing the butterflies to fully close. Tighten the pinch clamp screw and begin butterfly arc adjustments.

Butterfly Arc and Adjustments:
Insuring the butterflies on each bank are opening at the same rate is critical for correct throttle tip-in and part throttle drivability. The use of feeler gauges for adjusting the hex link for correct engine “tone” side to side will not correctly set the butterfly arc, and will result in a rich/lean condition on each side of the engine and poor drivability.

  1. Two pieces of round stock are required for gauging. Verify the gauging that you will be using to be within .001 of an inch from each other. Two philips head screw drivers from the same manufacture or socket extensions are ideal.
  2. Insert gauge #1 between the center of the throttle blade and wall of injector making sure that the gauging material stays in contact with the machined portion of the throat. While supplying light pressure with your finger to hold the butterfly against the gauge, insert gauge #2 between an open butterfly on the other side of the engine. If gauge #2 has a light drag without opening the butterfly against gauge #1, the arc is correct, remove gauging and proceed to “Final Tuning”. If gauge #2 is either loose or tight, see “Butterfly Arc Adjustments” for further adjustments.

Butterfly Arc Adjustments:

  1. Remove gauging from the injector.
  2. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
  3. Loosen lock nuts on both sides of the hex.
  4. Either lengthen or shorten hex link 1/4 to 1/2 of a turn.
  5. Lightly tighten the lock nuts on the hex link.
  6. Note: The following MUST be completed or the adjustment will not be correct. Loosen the allen head pinch clamp screw on only one bronze throttle arm, lightly tap the butterflies closed on either side of the injector and tighten the pinch clamp screw.
  7. Insert gauge #1 between butterfly and wall of injector. While supplying light pressure with your finger to hold the butterfly against the gauge, insert gauge #2 between an open butterfly on the other side of the engine. If gauge #2 has a light drag without opening the butterfly against gauge #1, the arc is correct, proceed to line 9.
  8. If gauge #2 is closer to the ideal drag, repeat steps 4 thru 8 until correct spacing is accomplished. If gauge #2 identifies you are moving away from the ideal drag, reverse the direction you are turning the hex link and repeat steps 4 thru 8.
  9. Without introducing bind in the heim joints, fully tighten hex link lock nuts.

Final Tuning:
Final tuning, or balancing of the butterflies insures that each cylinder is performing the same work at idle and at part throttle. When accomplished, your engine will start and idle extremely well. For EFI applications, this step will ensure there is a good exhaust note and provide the best in drivability and engine acceleration. For Mechanical/Racing applications this step will ensure that all cylinders are at the correct temperature for the quickest front half times or optimal acceleration when exiting a turn. We have found that the use of a Synchrometer, allows us to maximize adjustments with the least amount of time. Before getting started, attach return springs and set idle speed. The engine and injector should be at normal operating temperatures.

  1. Injectors with pinch clamps for ram tube retention will need to have the horizontal and vertical cuts in the casting taped from the inside to insure correct reading of the Synchrometer.
  2. Place the Synchrometer on one of the throats and adjust the air flow restrictor so the needle is centered in the gauge. There is no correct starting place for the needle since we are looking for equal airflow not an airflow number.
  3. Verify averages are consistent bank to bank. If they are not, remove throttle linkage and return springs and back off idle stops. Loosen a gold arm for the hex link and, while supplying slight closing pressure to the bank with the higher air flow values, tighten the gold arm for the hex link. This will allow both banks to open the same amount and even the airflow from bank to bank. It may take a couple of attempts before you are satisfied.
  4. Identify butterflies that need to be adjusted to provide equal air flow. Open butterflies and identify which end of the blade points up. Mark the up end with a marker. Butterflies are beveled and need to go back in with the correct orientation (see Butterfly Installation). Also, adjustments to the butterfly are typically done on the end that points up.
  5. Remove the two screws and lock washers that hold the throttle blade, taking care not to drop them into the engine.
  6. Open throttle shaft to wide open throttle while grasping the end of the throttle blade to remove it. Protect the throttle blade from vise damage by using a piece of aluminum on either side. A rod vise works very well also.
  7. Using the throttle shaft witness marks as a guide, insert the butterfly between the aluminum, lining it up with the witness mark. The end of the butterfly to be adjusted should be sticking out of the vise and both throttle shaft witness marks should be hidden.
  8. Lightly tap the blade in the direction required. If the cylinder requires more airr, adjust butterfly as to open it in the throttle bore, and conversely, if the cylinder requires less airr, adjust butterfly to close it in the bore. Remember that .010 of an inch is a lot.
  9. Reinsert butterfly into the throttle shaft. Install and tighten the butterfly screws.
  10. Repeat as necessary. The closer the values, the better your engine will perform.

You have now completed the injector manifold adjustments. Install your ram tubes and enjoy your Hilborn Fuel Injector.

Can I run a HILBORN EFI injector on a street car? What about a mechanical injector on the street?

Because of EFI (electronic fuel injection), the Hilborn Injector has become the favorite choice for many types of motorsport enthusiasts that enjoy driving their cars on the street. This ranges from show cars that drive 500 miles a year to street rodders driving 35,000+ miles a year and everyone in-between. The joining of EFI and a Hilborn injector has produced what we feel is the ultimate induction system for any show car or street rod. We took the neck snapping throttle response, the increased torque and power, and the awesome engine acceleration common to a Hilborn Injector, controlled the fuel with EFI, and found we had an unbeatable combination of calm drivability and unmatched performance. But the best was yet to come. We found that our manifold design, which removes the distribution concerns associated with a common plenum, allowed even the most aggressive engines to have extremely good part throttle manners, while still retaining their aggressive exhaust note. Yes, that’s right, our race inspired manifolds actually promote part throttle drivability. This would be our most important benefit since you spend most of your time at part throttle in a street car. So not only is the answer, Yes, you can!, but for the perfect induction system, it has to be a Hilborn.

NOTE: Our products are not legal for sale or use on emission controlled vehicles.

What about a mechanical injector on the street?

A Hilborn mechanical injector is classified as a constant flow system and was designed to operate at WOT under load. As a constant flow system, pressure and volume are controlled by the main jet, or pill, along with pump speed (engine rpm) and nozzle size. The barrel valve, which controls idle fuel and transitional fuel from idle to WOT, can be compared to a ball valve much like the one that turns off the water in your house. The basic design and lack of fuel control of a barrel valve does not allow us to control the fuel at part throttle especially no load part throttle. If you consider the fact that an engine’s fuel requirements are based directly on load, and that we can have many different loads at different rpms all at the same throttle angle, the lack of fuel control for street applications becomes apparent. A mechanical system does not employ enough fuel control in the operating range where you drive your street car and, therefore, is not recommended for street use.

Of course we have all heard the stories of mechanical system working on the street but few if any actually worked correctly. The use of a dial-a-jet, additional bypass valves, and home center ball valves have all been used to provide fuel control for adequate street use, but fall far short of the fuel control required as part throttle load is constantly changing. The constant manual adjustments needed, as one guesses the current fuel requirements of the engine, leaves very little time to actually drive the car and, at best, is incredibly inaccurate. Blown applications appear not to be as affected by the lack of fuel control of mechanical injection, possibly due to the load placed on the engine to drive the blower, but is still not recommended for those looking for the best all around drivability.

The use of alcohol helps because of it’s large tune-up window, but fails to provide drivability due to loading up, mileage (in gallons to the mile) and severe oil dilution. Claims from those that run injected engines on stands or dyno’s stating they can make mechanical injection streetable, are unable to simulate a fraction of the different part throttle load scenarios your engine will see, nor provide the required fuel control. Interestingly enough, engineers have devised a way to electrically control these valves and bypasses…it’s called electronic fuel injection.

Common Plenum vs. Individual Runner

An intake manifold comparison by Andrew Starr


For maximum power to be realized, all cylinders of the engine must do the same work, or produce the same power. For many years, research has developed new designs of cylinder heads, intake manifolds and other engine internals to provide increases in horsepower. In fact, gains in engine efficiency are some of the factors in these large-scale power increases. Some of these efficiencies can be defined as improved distribution. Distribution, in relation to the internal combustion engine, can have several definitions. For the purposes of this report, we will concentrate on distribution with regard to equal distribution of air and fuel at a given air/fuel ratio, along with distribution in regard to equal amounts of the air-fuel supplied to each cylinder. This helps us in our goal to make peak power in each cylinder and therefore maximum power from the engine.

In the extremely competitive world of prostock racing, engine development advances have contributed to 500 cubic inch engines making in excess of 1300 horsepower. Many of these advances have also redefined the distribution in the engine and therefore its efficiency. For example, siamesed intake ports gave way to symmetrical intake ports, factory valve angles led to reduced valve angles for increased line of sight to the intake valve, and then from rectangular ports to oval ports. Fabricated intake manifolds with huge plenums, along with short straight runners, replaced cast intakes. These developments were years ahead of their stock counterparts and contributed to the massive power levels seen today. So how does one increase engine output without a prostock racer’s budget? The easy answer starts with fuel mixing and intake manifold selection. The eight-stack injector disposes of the concerns of the carburetor and the v-style common plenum intake manifold. And because of this, efficiency and distribution are improved along with power output and throttle response. The eight-stack fuel injector originally pioneered by Stuart Hilborn in 1948 was simple in design, yet it produced fantastic results in the racing world against carburetors, making the carburetor extinct in some racing venues even today. Today, advancements in carburetor and intake manifold technology still waver in comparison to the original design of the fuel injector from many years ago.

What makes the Hilborn injector superior to the carburetor and superior to other EFI systems available today?  I will explain.

The Carburetor

The carburetor can best be described as an air/fuel mixer that uses a differential in pressure to provide fuel at an established metered amount. Although easy to define, the actually workings of a carburetor are complicated, enough so that very few people are able to maximize its potential.

With air movement toward the intake valve in the intake manifold, a result of piston movement and valve timing, the main venturie of the carburetor has air flowing past the booster creating a pressure drop. This pressure drop causes fuel to be pushed into the booster supplied by the fuel bowl via the main well of the carburetor. The shearing of the fuel as it enters the air stream out of the booster causes the fuel to separate into smaller particles, where it is picked up by the air and carried into the intake manifold.

The legendary Smokey Yunick states, “The carburetor is a big restriction in the intake system”(1). This is because in a round column of airflow, flow speed is fastest towards the center of the column (2).The design of the carburetor places the booster towards the center of that column in order to receive the strongest signal, or pressure drop at the booster, for maximum booster performance. Since airflow is impeded, the direction of part of that flow is diverted into eddies of spinning air that will disrupt the rest of the airflow around it.

Float bowl volume is essential for correct air/fuel ratio. With a needle and seat size a nominal .110 of an inch for gas, coupled with fuel pressure of six to eight pounds per square inch at the needle and seat, it is difficult to keep the bowl filled. Carburetors use atmospheric pressure to provide lift of the fuel, therefore as the bowl empties, an increase in pressure drop is required to lift the fuel up the main well into the booster, compromising consistent air/fuel ratio. Fuel bowl problems also manifest themselves in applications that incur aggressive changes in direction, as in road-race and autocross applications.

CFM ratings for carburetors were introduced as a way to identify correct carburetor sizing for specific applications. In actuality, only the throttle blade size is needed to identify size, since a venturi will only flow so much air at a certain depression. Any published CFM ratings, regardless of product, should also include their depression in inches of water. Unlike the cylinder head aftermarket where the standard for CFM ratings is 28 inches of mercury for pressure drop, there is nothing published for the carburetor aftermarket. As an example, all of Holley’s 750CFM carburetors use a 1.375 primary and secondary throttle blade, yet there are many companies who will claim up to 950CFM with the same throttle blade size. These types of exaggerated specifications only lead to confusion of the end user on what size carburetor will fit his needs. Qualified carburetor shops will sell a carburetor by throttle blade size and not by exaggerated CFM ratings.

At idle when air speed is low, the fuel droplet pulled from the booster is much larger than the one pulled when air speed is greatest or wide-open throttle. This has a profound effect on mixture distribution

If a carburetor’s venturi is sized for low to midrange torque and strong acceleration, it will be too small to produce top end power; conversely, if the venturi is sized for top end power, low to midrange torque and throttle response will suffer (3). The correctly sized carburetor for some applications will be a compromise of low to midrange torque and top end horsepower.

The V-Style, Shared Plenum Intake Manifold

As stated earlier, for maximum power to be realized, all cylinders of the engine must do the same work, or produce the same power. Although many factors control this quotient, none are more important than the intake manifold, specifically the shared plenum style used today.

First it is important to keep in mind that the air and fuel have not mixed into a homogenous mixture in the venturi of the carburetor or in the runner of the intake manifold. A homogenous mixture is defined as a mixture whose physical properties are uniform throughout. Fuel in the manifold is not uniformly mixed as this only happens under the extreme heat and pressure contained in the combustion chamber.  In reality, fuel in the intake tract uses air as a carrier; therefore, it is relatively easy for fuel to fall out of suspension, causing mixture distribution concerns.

Accelerating air has passed through the carburetor venturi, picked up its metered amount of fuel, and then is deposited into the plentinum. As the air/fuel enters the plenum, a reduction in air speed is created due to the large increase in plenum area, which causes the outer boundary layer of air to slow dramatically. This reduction in air speed creates eddies in the outer layer causing fuel to fall out of suspension and onto the port walls and floor. Meanwhile, the faster moving center column of air also slows, but is still on a collision course with the manifold floor. The first concern happens when the column of fuel and air collide with the floor. The law of physics states that an object in motion tends to stay in motion comes into play, since air and fuel have weight. Since fuel is much heavier than air, fuel falls off the carrier and puddles onto the floor, although the air carrier has corrected and is entering the intake runner. Correspondingly, this same air will also change direction and creates additional eddies into the plenum. In some applications turbulent air will now disrupt the incoming air/fuel affecting carburetor metering (4).There is also the air/fuel that has made the transition and is proceeding into the intake runner. Part of the distribution concern is evident by the raw fuel on the floor and plenum walls. This fluid will be picked up by passing air, put into suspension, and carried into the cylinder. It is important to note that these fuel particles will be of different size than the ones still in suspension and will contribute to distribution concerns especially on the four inner cylinders of a single plane intake where the runners are shorter and are provided a stronger signal, or faster airflow, than the four outer runners.

The advent of carb spacers and shear plates have been used for years to combat some of the concerns associated with air/fuel transitioning from plenum to intake runner and reversion concerns of this type of intake.

But a lesser-known approach of solving the problem of plenum floor transition was the introduction of the “Turtle,” a concept pioneered by manifold designer Jean Dittmer. As you can see at left, the Turtle, with multiple channels and a raised center, is used to help reduce the sharp transition of the plenum floor along with helping direct the air/fuel in a more efficient means into the intake runners (5). The Turtle is an excellent example of the methods required to fix the poor distribution concerns associated with the plenum floor. Ultimately, due to its specific intake applications and concerns with installation, the Turtle never made an impact in mainstream racing.

Although the effects of reversion have been mentioned, it is important to understand the impact it has on this type of intake. Reversion is the reverse pumping of the air/fuel in the intake, due to the intake valve opening while the piston is approaching TDC. This timing event happens two times in a four-cycle engine. The first reversion pulse is during the exhaust stroke as the piston is about to reach TDC. The intake valve opens as the exhaust valve is closing, commonly know as valve overlap. Air, rushing out of the cylinder, due to a low pressure at the exhaust valve, is enhanced by the movement of the piston towards TDC. Theory states that when the intake starts to open, the air/fuel will be drawn into the chamber exiting with the burnt gases to flush the cylinder of the remaining spent air/fuel and provide the momentum for the air/fuel in the intake manifold to start filling the cylinder. It is at this time that some reversion makes its way into the intake runner, diluting the fresh intake charge with the spent air/fuel mixture.

The second reversion pulse is much more significant and happens on the intake cycle. As the piston travels past BDC, heading towards TDC, the cylinder continues to fill. The momentum of the air/fuel continues to pack the cylinder even though the piston is moving up the bore. When the intake valve closes, the piston has already moved past BDC by up to 20 degrees or more. This reversion pulse is very obvious in the lower rpm range, and typically the larger the camshaft, the more reversion in the lower rpm range.

Reverse pumping in the intake can be identified as the bounce commonly seen in the vacuum gauge, or engines that do not “clean out” until higher rpm’s.

Reversion, fuel falling out of suspension, eddy currents, long runners versus short runners, high-speed right angle turns. All of these conditions in the intake manifold do little to promote good distribution. It then becomes impossible for all the cylinders to do the same amount of work and therefore the engine to make maximum power.

With regard to Multiport Fuel Injection that uses a convention shared plenum intake, much of the same negatives apply. Regardless of how and where the fuel is introduced into the intake tract, concerns with air direction changes, sharp runner turns and the negative pulses from all cylinders will also negatively affect any mulitiport EFI system.

The Eight-Stack Manifold

In a test I conducted with a common plenum intake against the same engine with a Hilborn eight-stack manifold, with both adjusted to provide the same air/fuel ratio, the eight-stack manifold made more power (6). There are numerous reasons why.

Without a carburetor’s booster in the way, the eight-stack manifold will flow more air than a comparably sized carburetor venturi. Without the need for a pressure drop to supply fuel for the engine, the bore size of the eight-stack can be increased to supply the required air for top end horsepower, yet have increased throttle response along with increased low to midrange power. The mild turning radius of the eight-stack intake helps promote line of sight for the air/fuel into the cylinder head. Since there is no common plentinum, the cylinders no longer need to digest an air/fuel mixture that is contaminated with pulses from companion cylinders. Since each cylinder is separate, there is no dilution to companion cylinders from reversion, eddy currents, fuel puddles and the tight bends for the air/fuel to follow causing mixture distribution concerns.

The eight-stack manifold employs a converging tract design, meaning the larger ram tube top is reduced in size as it enters the lower portion of the manifold before entering the opening of the cylinder head runner. With this design, air/fuel speed is increased when going from the larger opening to the smaller opening, ensuring that the air/fuel stays in suspension, unlike the tract of the common plenum manifold, which allows the air to slow when going from the small venturi of the carburetor to the larger opening of the plenum. Around the point that the intake tract is constricted, fuel is injected under pressure, allowing it to take maximum advantage of the air speed. In the converging tract manifold, most of the fuel that falls out of suspension as the wall is constricted is picked back up and put into suspension instead of being pushed along the walls.


Fuel supplied into the nozzle of an EFI Hilborn injector can be as high as 60 pounds per square inch, allowing the injector to spray the fuel in a fan pattern. It also allows a consistently smaller sized droplet, which is much more consistent than that of a booster. Therefore, the airspeed will not dictate the size of the droplet, which in turn allows the fuel to mix in better proportion with the air, providing improved engine efficiency and therefore making more power.

Since a Hilborn injector does not require a bowl for fuel storage, it is an ideal system for any application that will create excessive fuel slosh as in road race and auto cross applications.

Separation caused by the fuel crashing into the plenum floor is reduced due to the individual throat design of the Hilborn injector. Also important is the reversion from one cylinder has no effect on the other cylinders. This allows each cylinder to act on its own, greatly improving the engine’s ability to produce equal power from the fresh intake charge each cylinder receives on the intake stroke. Since each cylinder has its own throat, the line of sight to the intake valve is improved, resulting in a straighter shot for the air/fuel mixture. The fewer turns the mixture has to make, the more the fuel stays in suspension, providing enhanced distribution into each cylinder.

The Hilborn injector has numerous advantages over carbureted systems. A carburetor requires a correctly sized venturi for its application in order to pull fuel out of the booster. However, the Hilborn, with its injected fuel, can maximize airflow potential by removing the restriction associated with the booster while enlarging the throat diameter allowing maximum breathing for the engine. This increase in throat size does not impede low speed engine performance but enhances throttle response, and engine acceleration. This allows for maximum engine rpm output without sacrificing low speed performance.

For maximum power along with razor sharp throttle response, it is quite apparent that the Hilborn Fuel Injector is the right choice.

1 Smokey Yunick, Smokey Yunick’s Chevy Engine Guide, Hot Rod High Performance Series Vol. 4 Number 3 (1987): 59.
2 Heinz Heisler, Advanced EngineTechnology, (Warrendale: SAE International, 1995) 233
3 Yunick 71.
4 Yunick 68.
5 Brodix Catalog, May 1998, 44. Used without permission.
6 Andrew Starr, “Field Evaluation Report,” December 2002.

Can I run an IAC Valve?

On 8 stack applications using a MAP sensor, a second vacuum kit is recommended for correct engine operation using an IAC. Installation of the second vacuum kit is not available from Hilborn Fuel Injection. An IAC valve is NOT required for proper idle operation of your EFI system but, on the other hand, it is highly recommended for blown systems.

Holley TPS

Screen Shot 2016-02-09 at 11.19.59 AM



Screen Shot 2016-02-09 at 11.40.38 AM

Fuel System #1 Schematic


  • Street rodding/cruising with stock to mildly modified engines (low to moderate horsepower).

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Fuel System #2 Schematic


  • Street/strip competition with modified to high performance engines (moderate to high horsepower)
  • Street rodding/cruising with stock to mildly modified engines (low to moderate horsepower) !NOTE: This fuel system requires additional fuel fittings, not supplied in efi kit, that must be purchased separately.Screen Shot 2016-02-09 at 11.51.24 AM
Fuel System #3 Schematic


  • All racing applications with high performance engines (very high horsepower)
  • Street/strip competition with modified to high performance engines (moderate to high horsepower)
  • Street rodding/cruising with stock to mildly modified engines (low to moderate horsepower) !NOTE: This fuel system requires additional fuel fittings, not supplied in efi kit, that must be purchased separatelyScreen Shot 2016-02-09 at 11.55.37 AM
Manifold Adjustment Video

Synchrometer Tuning Video

Butterfly Installation


Is your injection streetable?

Very! Combined with EFI technology, our fuel injectors offer superior drivability under all types of conditions associated with aftermarket street performance. Although they perform very well on the street, they are not “street legal” or suitable for emission controlled vehicles, so we recommend that you check your local laws first.

Which electronics systems do you offer with your kits?

We currently offer three ECUs with our kits, the FAST XFI and the Holley HP or Dominator. An ECU is only as good as the chip set and all three use a very high quality and fast chip set. They all feature robust wiring and wide band O2 sensor control along with Windows based mapping. As part of the kit we also supply base programming of the ECU.

Can I convert my old mechanical unit?

Although it is possible to convert your old Hilborn Injector to EFI, we have found that after spending the time and money required to accomplish this, it makes better financial sense to buy a new injector already set up for EFI. We’ve redesigned our original castings at the foundry level to accept all the necessary EFI components, which in the long run, can save you money by eliminating costly re-machining for conversion. Because of this, we do not offer machining conversion services at Hilborn Fuel Injection. However, most blown mechanical fuel injectors, whether scoop or hat style, can be easily converted to EFI.

Isn’t the Hilborn manifold technology “old school”?

In actuality nothing could be further from the truth. When one examines the dynamic inside a common plenum manifold, it is easy to understand how the disruptive reversion pulses from adjacent cylinders slam into the columns of laminar flow, causing fuel to fall out of suspension and separate with the air. This affects low speed torque, top end power and acceleration rate since the working fluid is supplied to the combustion chamber in a less burnable state and with reduced volume.

On the other hand, the isolated runners of a HILBORN individual runner (IR) manifold completely eliminate these reversion pulses, allowing the working fluid to remain in laminar flow, generating tremendous air speed which fills the cylinder for maximum power potential. In racing venues where IR manifolds are legal they are the dominate induction system. Super car manufacturers, including Ferrari and Maserati, use IR technology to build maximum potential for their street cars, proving that there is no better induction system for your application.

I don’t race, I’m more concerned with drivability. How does your system perform at low engine speeds?

Although we have many performance advantages, it is our low speed drivability that really shines. The same attributes that provide our mid and top end power advantages are also why we have excellent idle, throttle response and part throttle drivability. Since we no longer have to worry about those disruptive reversion pulses, our manifold is the perfect choice for smooth part throttle cruising on the open road. We have found this to be true whether your engine is stock all the way to heavily modified engines with large camshafts.

Will your manifold remove my camshaft thump at idle?

When the butterflies are correctly adjusted, the engine may not be as rough at idle as with a carburetor, but the camshaft thump will be very pronounced. The trade off is a positive though, as there is a dramatic difference in low speed throttle response, torque and acceleration rate easily making up for the slight difference in sound.

Why is your IR manifold better than others?

HILBORN manifolds are bred from our racing heritage and are designed to make maximum power. Our manifolds are a true individual runner (IR) manifold and are not conversions, instead, they are engineered at the foundry level to accept the EFI components. We have also found that one size doesn’t fit all, so our manifolds are flexible enough to fit engines with unique heads and various deck heights. Also, we offer a choice of butterfly sizes that allows us to tailor the manifold to meet, or exceed, the power expectations of your engine without compromise to drivability. No one gives you more choices than HILBORN.

What is a true individual runner manifold?

A true IR manifold is one where the individual throats are completely segregated from the adjacent throats, allowing the highest possible air speed, for not only maximum efficiency, but also low speed drivability. It is fashionable today for other IR manifold manufacturers to incorporate a common plenum, making them a hybrid design, muting the advantages of an IR manifold (maximum power and torque, engine acceleration rate, and especially low speed drivability associated with our tremendous cylinder fill) while at the same time, introducing the disadvantages of a common plenum manifold. The primary reason for this hybrid design is for the use of an IAC valve. Since it is not feasible to build the plenum large enough to not be affected by the operation of the IAC, a reduction in resolution for the MAP sensor, along with noticable decrease in low speed drivability, is experienced. A second reason for the hybrid style intake is, it is thought to eliminate the need to correctly synchronize the butterflies. Unfortunately, this is not the case as butterfly synchronization is also required for the hybrid designs.

What is required to synchronize the butterflies?

We understand how critical butterfly adjustments are in order to realize the low speed benefits you will receive with an IR manifold. We have developed techniques for butterfly adjustments and open source this information on our web site, see Synchrometer Tuning Video. We also supply a synchrometer with every manifold as it is required tooling for correct butterfly synchronization.

Why are your fuel rails located outside on some applications?

Most of the manifolds in the Hilborn line are a curved throat design. This design, based off of racing experience, allows the air to transition smoothly into the intake runner to help generate maximum power, but affords very little room on the inside for fuel rail installation. Outside placement of the fuel rails also allows our injector nozzle bosses to be raised in the throat and situated to spray the fuel into the wall. This process conditions the fuel by smashing it into a smaller droplet allowing it to enter the air stream in a more burnable state. This also helps the fuel to pick up heat off the wall to start the expansion process, providing extremely quick response in the combustion chamber for a tremendous increase in acceleration rate. We have found this process to be far superior than spraying the fuel directly onto the intake valve which promotes a larger fuel droplet and a reduction in combustion chamber response.

Why do your manifolds produce more power than other IR EFI manifolds?

With our true individual runner curved throat design, multiple bore sizes and optional porting, no other induction system can make as much power as a Hilborn IR manifold. In most applications it is the intake manifold that is the primary restriction of air flow but our IR manifolds remove a majority of this restriction allowing the engine to breathe and to make maximum power throughout the operating range. The most important facet of our IR manifold is a dynamic that is never discussed and that is engine acceleration rate. We are able to condition the air/fuel charge and optimize cylinder fill which makes the combustion process extremely efficient resulting in a dramatic increase in how fast the engine responds and accelerates.

How many CFM does your IR manifold flow?

A very common question! CFM ratings have been all the rage for quite some time, but it is only one small tool used in engine development. Since an engine is a dynamic device, using just one facet, such as CFM, without considering air speed, can lead to an incorrect conclusion about a product. In most cases, intake manifolds typically don’t have a CFM rating as they are an extension of the cylinder head. Since a Hilborn injector functions as an intake, we are unable to provide any type of usable flow numbers. Instead, we base maximum performance on the size of the butterfly.

Why can’t I get rubber hoses or individual metal lines as fuel lines for your EFI manifolds?

Although, at one time, we supplied fuel hoses instead of fuel rail, we have discontinued doing so for several reasons. Primarily, it is a safety issue. With a hose type fuel supply, a cap is typically attached to the nozzle (fuel injector) with a simple spring clip that is locked into a plastic groove. This is far from being as secure as a fuel rail, and if for some reason the clip, cap, or groove fails, high pressure fuel is sprayed into the engine compartment.

The second reason is more mundane but quite important to the drivability of your EFI system. The fuel supply in your EFI system is a dynamic device in that when the nozzle opens to spray fuel we have initiated the movement of this fuel. When the nozzle closes, the fuel in the line continues to flow (an object in motion tends to stay in motion) crashing into the nozzle. Its’ resulting force pumps a pulse back up the line upsetting the rest of the fuel supply. Due to the smaller inner diameter of the hose, this pulse is magnified as it makes its way back and forth in the injector line, mixing in with the fuel supply, as fuel tries to get to the nozzle. This pulsing creates a condition when, at various times, the nozzle opens and there is lack of fuel pressure, and other times excessive fuel pressure, resulting in poor fuel control. This can negatively affect part throttle drivability along with other isues. EFI is all about fuel control, and we have better success with the built in reservoir of a fuel rail that acts to dampen those reversion pulses. In this case, function over form.

Which electronics systems do you offer with your kits?

Please see our ECU FAQ’s.

What cam profile works best with your IR manifolds?

When tuning in speed density, a camshaft that promotes the highest engine vacuum is typically employed with the use of a common plenum manifold. But this can be a compromise for some performance engine builds. With the isolation of the runners in our IR manifold, we can be more aggressive with camshaft selection, without reducing or eliminating resolution. We have numerous aggressive combinations running in speed density with perfect street manners.

What is an IAC valve and can I run one with a Hilborn IR manifold?

An idle air control valve or IAC is easily described as an electrically controlled vacuum leak. It is not a sensor, instead it’s defined as an actuator since the ECU controls it. This variable vacuum leak allows the ECU to help maintain the programmed idle speed.

With Hilborn IR manifolds running in Speed Density (MAP sensor), a second vacuum kit is recommended for correct engine operation using an IAC. Installation of the second vacuum kit is not available from Hilborn Fuel Injection. An IAC valve is NOT required for proper operation of your EFI system but, on the other hand, it is highly recommended for blown systems, and is easily attached with a -8 hose.

Other manifolds have a built in plenum for the IAC, why doesn’t Hilborn?

Our manifolds do not employ a built in plenum which is popular today. We have found that this hybrid design reduces the primary attribute of increased cylinder fill of an IR manifold by exposing the laminar airflow to opposing pulses from other cylinders, negatively affecting power and low speed drivability. Also see “What is a true individual runner manifold?”.

Is installation of your IR manifold difficult?

Hilborn manifolds can come as either a single piece or three piece design depending on the application. Single piece manifolds install the same as any other intake, but the three piece manifolds will require drilling and tapping of the lifter valley rail (or China rail) in order to bolt down the center valley plate. We can offer technique help for those who have an assembled engine for a clean and trouble free installation. Regardless of manifold type, butterfly adjustments are required before initial start and after warm up for proper operation. Please see our Manifold Adjustment Video.

Do I need a return line?

Yes, our injection systems require a return line. Please see fuel schematics for recommended placement for the return line.

Can I get my injector polished or plated?

Our standard finish is a clear anodize which produces the timeless nostalgia grey finish. If polishing your manifold is preferred, we can have the process done or we can send the manifold to you. If we polish your manifold, we also offer a ceramic clear coat which eliminates touch ups as the polished surface starts to oxidize. Our ceramic finish is not only heat resistant but also scratch and impact resistant, keeping your manifold looking new for years.

Due to the machining tolerances, the casting cannot be chrome plated.

Do I need to run a thermostat?

We do recommend the use of a thermostat with all EFI applications. Due to some of our manifolds being multi-pieced, we include a remote thermostat housing in our kits to aid in installation of a thermostat.

How much more power will I make?

A common question but one with a varying answer. Depending on the induction system we replace, we have seen upwards of 100+ ft/lbs and almost 100 horsepower with these torque increases much earlier in the curve.  Although those numbers are not typical, we do generally see at a minimum, an increase of 30-40 hp and 45-55 ft/lbs of torque. All applications will respond with a tremendous increase in throttle response and, more importantly, a substantial increase in engine acceleration rate.



Can I get my ram tubes polished or plated?

For most applications our standard ram tube finish is cad-plated steel, which produces a dull silver finish. We can also have the ram tubes powder coated or chromed while our aluminum ram tubes can be polished.

Do you have air filters for my application?

We offer many filter applications especially for the more popular injector manifolds. These include our billet aluminum ram tube air filter combinations, bootie and tube top filters along with bug domes. We also supply box style sprint car filters from K&N or R2C. Another option is ram tube seals; an economical way of attaching a filter base to the ram tubes, thus, allowing a custom filter box to be designed. View our catalog for more information on air filtration products.

Should I dyno test my system?

We advise dyno testing to get the most out of your EFI unit. We prefer a chassis dyno, instead of an engine dyno, since all the subsystems (fuel system, ignition system and injector) are installed on the car. A chassis dyno also allows you to work out part throttle tuning, saving the additional expense of two dyno sessions.

I would really like an IR manifold, but it’s out of my price range. Do you have any other options?

We sure do! As EFI experts, we also supply common plenum style throttle bodies and intakes and can help you choose the right one for your combination or budget. Hilborn Fuel Injection is your one stop shop for all of your EFI needs.

Click to see our line-up of throttle body systems.

Which one is the BEST ECU?

Every EFI system is unique whether for the application, type of engine or builder preference. We offer a choice to make sure you get the best ECU for your application. There are many ECUs on the market today and we have teamed up with what we feel are the best ECUs with regards to performance, ease of use and “bang for your buck”. Since we also provide the technical support, we have limited the choices so we can tech them efficiently. It is best to contact our EFI Tech line to discus your needs so we can pick the electronics package that is the best fit for you.

Click to contact our EFI Tech line.

How do these ECUs work?

Simply put they use inputs from the O2, air, coolant and MAP sensors and then determine the correct injector duty cycle in conjunction with inputs from the fuel table. The fuel table is user adjustable and is adjusted to provide a smooth transition from cell to cell as you accelerate.

Can I get a brief overview of these ECUs?

A brief overview of each ECU is available here: FAST XFI, HOLLEY HP

What if I’m not very computer savvy?

The windows based software of both systems is much easier to learn and use compared to systems of yesterday. To get you started, we supply a custom base start-up calibration and go over the details. Our tech support is just one phone call away and we can walk you through any start-up issues you might have. Unlike our competitors, we do not charge for this service. Our technicians are unable to remotely tune your system for you, instead, we can supply tips on tuning technique so you can have the confidence to make the changes you want. However, if you are interested in a top notch tuning service, ask us for details about Hilborn Fuel Injection’s on site tuning program.

Please click to contact our EFI Tech Line.

Do your systems self learn?

Yes, mostly. We have found the Holley’s self learn to be very robust and active but it has certain requirements for it to be active. Learning will not be active under throttle movement or fast MAP sensor movement. An IR manifold has very fast MAP movement even during light throttle tip-in which relegates the learning to being most helpful at cruise and at wide open throttle. Regardless of how aggressive the learn is, most applications will need some touch-up tuning with a lap top.

What does the self learning actually do?

The job of a tuner is to not only find the ideal air fuel ratio and timing curve but to also tune the fuel table or map. The fuel table has numerous cells that call out the fuel supply to the injectors and it is the tuner’s job to not only eliminate as much of the O2 correction as possible but to ensure that there is a smooth transition from cell to cell. The Holley self learn builds a background fuel table with corrections values for the main table in order to achieve the air fuel ratio inputted and to keep the cell transition smooth. It is a tremendous technology but still imperfect especially when used with an IR system.

What is the difference between a wide band and narrow band O2 sensor?

A narrow band O2 sensor can only correctly measure the stoichometric air fuel ratio. For gas this ratio is 14.7 to 1, but this air fuel ratio is not ideal for all applications. Since the ability to measure a specific air fuel ratio other than 14.7 to 1 accurately with a narrow band O2 sensor is not possible, a wide band O2 sensor is employed. This type of sensor can accurately measure air fuel ratios from 8 to 1 up to 16 to 1, and is preferred over the narrow band. Both ECUs offered use a wide band O2 sensor.

What cam profile works best with these ECUs?

Since Speed Density uses engine vacuum to identify load AND the greater the vacuum signal between part throttle and wide open the better the resolution is for tuning, a camshaft that promotes the highest engine vacuum is typically employed. But can be a compromise for some performance engine builds. As cams become more aggressive we loose engine vacuum at idle and the allowable resolution this signal offers. This is all true for a common plenum MPI injection system.

But with a HILBORN IR manifold, we eliminate the reversion pulses that negatively affect the low speed and idle vacuum allowing the engine build to dictate the cam specifications, not the EFI system. So pick the camshaft that works best for your application and we’ll do the rest.

What is the difference between Speed Density and Alpha N?

The Alpha N tuning strategy uses input from TPS and RPM to set the fuel curve, while Speed Density uses a MAP sensor and RPM. A MAP sensor in essences is an electrically controlled device that reads engine vacuum to identify engine “load”. An engine’s fuel requirements are directly related to load and corresponding RPM more so than the throttle angle input of Alpha N. Therefore, the ability to tune to the constant change in engine load via Speed Density is desirable compared to Alpha N.

What is the difference between open and closed loop control?

In Open loop, the ECU uses information from user defined inputs, such as the fuel table, to set injector duty cycle, and will not automatically correct to match your target air fuel ratio.

In Closed loop, the ECU will monitor the A/F ratio and automatically adjust the injector duty cycle in order to meet the target A/F ratio. However, one must still tune the fuel table to keep the correction percentage as low as possible for smooth transition from one cell to the next.

Do I need two O2 sensors?

Both systems have the ability to accommodate dual O2 sensors but come standard with one. One is typically all that is required for correct operation but certain high horsepower or racing applications may benefit from two.

The FAST XFI will allow the second O2 to be data logged, but the ECU will only correct off of one when run in closed loop.

To run two O2 sensors with a Holley the Dominator ECU is required. It will also datalog both O2’s and can be programmed to correct off of either or an average of both.

Can I run an O2 sensor with zoomies?

Yes it can, but it may require open loop operation of the ECU. At idle, cam overlap sucks fresh air into the exhaust and due to the short length of the pipe this will resulting in a false reading at the O2 sensor. Some tuning by “ear” will be required at idle, but typically as engine RPM increases the ECU closed loop function can be turned back on.

What distributor do I need?

We feel the best input signal for any EFI system is a crank trigger but understand that not all applications can accommodate a crank trigger. A discrete hall effect type distributor would be the next best option and plug and play styles from Holley and FAST are available. Due to the potential for noise and operating concerns we attempt to shy away if all possible from the MSD magnetic pick-up type distributors.

For those that want the look of a magneto for their EFI system,  HILBORN Fuel Injection is proud to offer a Joe Hunt Conversion EFI distributor. More than just a conversion, these hand built distributors start out as brand new bodies which are then treated to a proprietary hall effect pick-up and rotor. This distributor was designed specifically for EFI applications eliminating signal issues that plagued converted magnetos. Please click here to learn more about our Joe Hunt magneto look alike distributor

HILBORN Fuel Injection can supply all of your distributor and ignition needs. Please feel free to call our EFI Tech Line to discuss distributor options for your application.

Which is the correct MAP sensor for my application?

All naturally aspirated applications need only a 1 bar sensor. Boosted applications will need a 2 bar or higher, depending on the boost pressure. Since 1 bar is equal to 14.7psi, then a 2 bar will handle boost up to 14.7psi, a 3 bar 29.4psi and so on.

What is an IAC valve?

An idle air control valve or IAC is easily described as an electrically controlled vacuum leak. It is not a sensor, instead it’s defined as an actuator since the ECU controls it. This variable vacuum leak allows the ECU to help maintain the programmed idle speed.

Can I run an IAC valve?

When running a HILBORN IR manifold in Speed Density (MAP sensor), a second vacuum kit is required for correct engine operation if you intend on using an IAC valve. Installation of the second vacuum kit is not available from HILBORN Fuel Injection but we can supply the necessary parts required. An IAC valve is NOT required for proper operation of your IR EFI system but, on the other hand, it is highly recommended for blown systems, and is easily attached with a -8 hose.

So how does an engine with one of your IR manifolds idle without an IAC valve?

When used with a 4 barrel type common plenum manifold, an IAC valve is an integral part of controlling warm idle and decel rpm. This is due to the turbulence created in a common plenum manifold. A HILBORN IR manifold on the other hand, does not have this disruptive turbulence providing very stable idle and deceleration characteristics even with large camshafts. Our manifolds reduce the roughness at idle dramatically, allowing idle speeds down in the 650-700 rpm range without adversely affecting the exhaust “hit” we all want.

Can I run my vacuum accessories?

Yes, we provide a vacuum junction block for all of your vacuum accessories but they must be of the closed variety. Closed vacuum accessories include MAP sensor, brake booster and transmission modulators. Open vacuum accessories including PCV valves and IAC valve are not recommended as they will reduce or eliminate resolution for the MAP sensor.

If you would like to run a PCV or IAC valve, a second vacuum kit can be installed on the manifold to accommodate those accessories. Please contact our EFI Tech Line for more information.

Will I see a mileage increase?

Yes! EFI allows finite fuel and spark tuning at any rpm and load. Used in conjunction, one is able to maximize fuel economy without sacrificing performance. In some cases, customers have reported doubling their mileage over their previous carbureted application.

Can I run alcohol or E85 with electronic fuel injection?

Yes, both types of fuel can be run with an EFI system. As a rule, alcohol is very corrosive, and unless you are prepared to constantly maintain your fuel system, we do not recommend it. At double the volume over it’s gas counterpart, larger or even multiple injector sets  and possibly a mechanical fuel pump may be required.

E85, on the other hand, has proven itself as a versatile fuel for performance street cars, whether naturally aspirated or blown. But much like alcohol maintenance is required especially if there are periods of inactivity with your vehicle. On average, E85 applications need a 30% increase in fuel volume requiring larger injectors and possibly fuel pump compared to gas applications

Should I dyno test my system?

We advise dyno testing to get the most out of your EFI unit and prefer a chassis dyno instead of an engine dyno, since all the subsystems (fuel system, ignition system and injector) are installed on the car. A chassis dyno also allows you to work out part throttle tuning, saving the additional expense of two dyno sessions.

For most applications, the self tuning algorithms available today reduce the need to have the vehicle dyno tested. All that is needed is a little more seat time and some manual fine tuning to achieve the desired results. Ultimately the decision to dyno test is up yo you.

What is noise, electrical interference or EMI?

EMI is short for Electromagnetic Interference and is also known as noise or electrical interference. All wiring exerts an electrical field that is best described as an invisible slinky around the wire that grows as current is increased. EMI is when higher voltage wiring emits a larger slinky and it jumps into an adjacent wire, creating an unwanted signal. This creates a problem with all types of electronic equipment such as ECUs. The remedy is to keep high power wires separated from signal wires, and to attach the power wires of the ECU directly to the battery. The battery offers a natural filter to absorb those unwanted EMI pulses.

Do I really need to run the power wires all of the way to the battery? I’ve run a large cable up front and it should be just as good right?

The battery is a natural capacitor making it capable of filtering unwanted signals from power lines. It is these unwanted signals that can interrupt proper operation of the ECU causing drivability issues. But in order to receive the benefits of this filtering, the power wires from the ECU MUST go directly to the battery terminals and not to a master disconnect, fuse block or junction block. Wiring that is not attached directly to the battery terminals are no longer filtered by the battery. It is highly recommended that the ECU be powered directly to the battery.

NHRA/IHRA Safety Rules:

A common misconception is that wiring the ECU directly to the battery will not allow the engine to be shut down with a master disconnect. This is not the case since it is the the switched 12v line that turns the ECU off and on. The switched 12v line is typically installed after the master disconnect keeping the car within the NHRA/IHRA  Safety rules.

Are your systems plug and play?

Our EFI systems come with the correct connectors, pre-terminated for all the sensors except the TPS and IAC (if your system uses one). Detail schematics are provided to attach these two components or see the links below. We understand that most people do not want to fill the engine compartment with extra wiring, so we are able to offer tips and tricks that allow you to extend only the wiring needed into the engine compartment for that clean look. In order to achieve this, we will need to lengthen or shorten wires which will require basic soldering iron skills. Since every application is considered custom, it is impossible to provide a one size fits all wiring solution. For those with advanced wiring skills unterminated harnesses are also available.

Holley TPS Schematic

FAST TPS Schematic


What if there isn’t a dyno shop in my area?

In most areas we are able to recommend dyno shops that can expertly tune your EFI system. But if you want to tune your EFI system yourself, you will be able to use the data logging capabilities that both systems have to pin point where to fine tune. The Holley system has a very robust self tuning algorithm that is exceptional at part throttle and even wide open throttle, but it will not tune every facet of an IR or blown system and will require final tuning with a lap top.

I read that IR manifolds always pop out of the ram tubes and that it can’t be fixed. Is this the case?

Popping out of the ram tubes typically indicates a lean misfire and is an issue that is easily remedied.

In most cases it is merely due to improper synchronization of the butterflies. The process of synchronization includes initial manifold adjustments and then fine tuning with the use of a synchrometer. See our videos on both subjects below. These procedures are normally performed with the engine and manifold at operation temperature, so it is not uncommon to have some slight popping out of the ram tubes when the engine is cold.

Another issue is improper design of the throttle linkage where the throttle arm, throttle stop and return spring are at different locations of the manifold, which results in twisting of the throttle shaft and popping. Insure that the throttle arm, throttle stop and return spring are all located at the same location of the manifold.

Finally, adjustments to the Fuel Table or AE Fueling in the ECU may be required to eliminate popping.

When an IR manifold is installed and tuned as recommended, popping out of the ram tubes is a non-issue.

Manifold Adjustment Video

Synchrometer Tuning Video

UPDATED!!! Manifold Adjustment Video


Experience IR…

Individual Runner, Incredible Ride!