MECHANICAL FUEL INJECTION 101
One of the main goals of this website is to put valuable information for set-up and tuning of Mechanical Fuel Injection (MFI) at the fingertips of the user and also help a potential user feel more comfortable and informed before they make a decision.
Please take the time to read over the below information and if at any time you have questions please call or email. We have made every attempt possible to outline everything you need to know about these systems in simple terms on this one page.
There are FIVE BASIC COMPONENTS to an MFI system.
1. FUEL PUMP
The first and most important component is the mechanical fuel pump. These pumps are capable of very high pressures and are typically driven off of the crankshaft (belt drive) at half of the engine speed. There is also a cam driven option available. The fuel pump is sized to output over twice the amount of fuel the engine requires. Due to this the system needs to be thought of in terms of BYPASSING FUEL.
2. FUEL CELL
The fuel cell is more important in an MFI system than any other system. As mentioned above, this is due to the fact that over half of the fuel delivered by the pump is bypassed from the engine back to the tank. It is imperative that the location, size, orientation of the fuel cell and the sizing of the hoses are correct. Please keep the below items in mind in regards to the fuel cell.
Fuel Cell Location - Place in front of the engine if at all possibe. If you cannot mount the fuel cell in front of the engine in a door car application due to space limitations we have a surge tank option you can read about on this page. Distance away from the engine makes no difference so it is OK for the fuel cell to be in the nose of a dragster.
Fuel Cell Capacity - KillerRONS.COM recommends that the fuel cell capacity be twice that of the usage per pass. The typical injected alcohol engine uses 1.5 to 2.0 gallons per pass from "trailer to trailer" so a 3 gallon minimum fuel cell is required.
Fuel Cell Orientation - The pick-up of the fuel cell needs to face towards the rear of the vehicle so that upon acceleration it is covered by fuel. It is not acceptable for the pick-up to be towards the front or side of the vehicle as this could lead to cavitation of the fuel pump and a "lean" condition at launch.
Fuel Cell Vent - Due to the large amount of fuel bypassed back to the tank it is important that fuel cell be allowed to "breathe" with an appropriate vent. KillerRONS.COM recommends an OPEN -6 (3/8" ID) hose as the minimum. If you feel that you must use a roll-over valve or filter it must be oversized and working properly. Let it be known that this will be the first thing we ask you to remove if you have an ill-running car after after the installation of an injection system.
Return Hose - All return hoses that carry bypassed fuel should dump onto the top of the fuel cell as far away from the pick-up as possible. No anti-aeration tubes submerged into the fuel should be used.
3. THROTTLE BODY
The third component is the throttle body that sits atop the intake manifold. The throttle body is typically made up of one (Flying Toilet) or multiple (Terminator) throttle blades. The function of the throttle body and throttle blades is nothing more than to control air flow to the engine from idle all of the way through wide open throttle.
4. BARREL VALVE
The fourth component of the system is the barrel valve. In simple terms the barrel valve's primary responsibility is to set the idle mixture by bypassing un-needed fuel back to the fuel cell. Thus, producing an idle mixture that is just rich enough to prevent a "lean" stumble or hesitation when the large throttle body is opened quickly to partial and wide open throttle.
An incorrect idle mixture (excessively rich) along with improper use of the fuel shutoff cable can result in excessive fuel consumption and diluted oil which is the single-most form of user error in these systems. You can read more about how to PROPERLY use the shutoff cable in the second bulletpoint on this page.
CHECKING THE IDLE MIXTURE WITH A LEAK DOWN TESTER
New systems have the idle mixture set from the factory using this method. You can also measure and set the exact percentage of fuel bypassed at idle with a LEAK DOWN TESTER. If you do not own a fuel injection leak down tester you can see purchase information here. If you do not have access to a leak down tester please scroll down to "The Snap Test Section" of this page for information on how to do this manually.
Below you will find step-by-step instructions for using the leak down tester. Note that this procedure can be done on the engine with the hoses hooked-up or removed from the injector bodies. Please call or email KillerRONS.COM for a baseline setting before getting started.
Cap the return side of the barrel valve so no air can escape.
Install leakdown tester to the barrel valve inlet.
Set blade gap to recommended setting with a "feeler's gauge" on the barrel valve side the throttle body only. Please back the throttle cable idle adjuster off and be sure to tighten the jam nut. Later when your idle (blade gap) has been finalized at the track you can bring this idle screw back up.
This must be done before any leakage measurements are taken or adjustments of the barrel valve turnbuckle.
Adjust gauge(s) to 100 psi with the shutoff in the full "OFF" position.
Move the shutoff into the fully "ON" position and note the readings. The gauge below reads 30% leak down (100-70=30). This means that 30% of the fuel is sent to the engine at idle and 70% is bypassed back to the fuel cell.
Note that if you cannot achieve or hold 100 psi with your compressor you will need to "do the math". Please contact KillerRONS.COM for the formula.
Making adjustments to the idle mixture is done with the BARREL VALVE TURNBUCKLE. To adjust the idle mixture, the user should loosen the two jam nuts to allow the tie rod to be adjusted easily. To lean the mixture the turnbuckle would be lengthened which rotates the spool clockwise toward straight up vertical or 12:00. To richen the mixture, the turnbuckle needs to shortened which rotates the spool counterclockwise.
Note: If you look closely at the above photo you will notice the index mark on the SPOOL is oriented just left of "straight up vertical". Under no circumstances should this index mark ever be oriented to the right of "straight up vertical".
THE "SNAP TEST"
As said above; the LEAK DOWN TEST is the KillerRONS.COM recommended way to check or set the idle mixture but it may not always be feasible. That being the case the "Snap Test" is the next option.
To perform this test you will need to bring the engine to temperature using the shutoff cable with the car in Park and on jack stands. Once at temperature, move the shutoff cable inro the fully "ON" position and give the engine a quick "snap" of the throttle. If the engine response is "crisp" lean the idle mixture by lengthening the turnbuckle 1/2 turn. Repeat this process quickly until the engine stumbles or hesitates. At this point your idle mixture is bypassing too much fuel and is slightly lean. Now it is time to make your final adjustment by shortening the barrel valve turnbuckle one complete turn. You should give the engine one more quick "snap" of the throttle to make sure the hesitation or bog is gone and the engine is "crisp". This is the correct idle mixture as the engine is slightly rich.
As mentioned previously, if you look closely at the ABOVE photo you will notice the index mark on the SPOOL is oriented just left of "straight up vertical". Under NO CIRCUMSTANCES should this index mark be oriented right of "straight up vertical" as in the photo below. The below photo os INCORRECT!
SETTING THE IDLE SPEED
Once the idle mixture has been established it is time to set the idle speed (RPM) by adjusting the amount of air shown to the engine at idle. This is accomplished with the IDLE ADJUSTMENT SCREW. Newer systems have an Idle Adjustment Screw on each side of the throttle body. For initial adjustments, it is best to adjust on the barrel valve side only. KillerRONS.COM recommends setting the idle speed no lower than 1000 RPM with the car in gear.
5. BYPASS PILLS AND NOZZLE JETS
The Main Bypass Pill controls the amount of fuel bypassed at partial and wide open throttle. Larger orifice pills lean the mixture by allowing more fuel to be bypassed back to the fuel tank ultimately decreasing the system pressure. On the other hand, smaller orifice pills richen the mixture by not allowing as much fuel to be bypassed back to the fuel tank therefore increasing system pressure. These pills can be changed in a matter of seconds with no gaskets or draining of fuel required.
The final metering in the system takes place at the eight Nozzle Jets that are installed in the nozzle bodies in each individual runner. The nozzle jets are the only components in the fuel system that do not work in a bypass condition. Larger nozzle jets allow more fuel to pass through to the engine and lower the system pressure. Smaller nozzle jets allow less fuel to pass through to the engine and increase system pressure.
The relationship between the orifice sizing of the Main Bypass Pill along with the eight Nozzle Jets determines the amount of fuel that is delivered to the engine at a specific pressure. New systems are shipped with a baseline setting for the engine's horsepower rating along with analyzing specific information about the engine and vehicle. KillerRONS.COM can also offer free baseline Main Bypass Pill and Nozzle Jet tune-up recommendations by filling out the questionnaire here.
In a small number of engine or fuel system configurations an extra bit of bypassed fuel is required at various RPM's. In these instances a Mechanical or Electronic Hi-Speed Bypass is utilized with a pill. These pills work the same as above in that larger orifice pills lean the mixture and smaller orifice pills lean the mixture.
In summary, these systems are quite simple if you think of them in terms of bypassing the excess fuel back to the fuel cell with the barrel valve and various bypasses.
