Performance EFI

How to properly modify your EFI fuel system for performance applications
By: Jim Roal

Since the mid-1980's, carburetors have basically disappeared from production cars and trucks.  The main factor driving this was emissions compliance and fuel economy standards but everything has improved because of it.  Today you can build a 10 second daily driver that can get good fuel economy and your grandmother could drive it.  You no longer have to battle with constant carburetor problems, points that wear out, poor fuel economy, and all kinds of driveability problems.  Today you can drive to the drags (and get well over 20mpg getting there), run consistent 12 second quarter mile passes all night (or even quicker), and drive home.  You probably don't even need to bring tools.  Most of this has come about because of electronics.  Modifying a late model EFI system for performance is quite different from the old carburetors of yesteryear however.  In this article I will attempt to explain how a typical EFI system (Bosch type, for Otto cycle engines) works and how you can modify it to support more power.


The main components of most EFI systems includes fuel injectors, an engine control module (ECM), an electric fuel pump, a fuel pressure regulator, and several sensors.  The typical EFI fuel system uses 40psi fuel pressure across the injectors.  The reason I say across the injectors is because it uses a pressure regulator to maintain 40psi pressure difference between the fuel pressure in the fuel rail and the air pressure at the injector tip.  This is done by connecting a vacuum line between the air side of the regulator diaphragm and the intake manifold.  When the manifold is in a vacuum, the fuel pressure in the rail (what you would measure on a gauge) will be 40psi plus manifold gauge pressure.  For instance, if you have 20 inches of Mercury vacuum in the manifold (approximately negative 10psig) you will have 40psig -10psig = 30psig fuel pressure in the fuel rail.  If the intake manifold has boost, say 6psig, you will have 40psig + 6psig = 46psig fuel rail pressure.  The reason for this is to have a constant pressure across the injector so the flow rates will be stable for a given injector pulse width.  This simplifies programming for the engine control and eliminates the need for a fuel pressure sensor.  Many new EFI systems however, have eliminated the fuel pressure regulator and now have a fuel pressure sensor.  These systems control fuel pressure by modifying the power to the fuel pump.

The ECM reads engine data from several sensors and uses this information to calculate and deliver what is called a pulse width to the injectors.  The injectors have battery power to one of the 2 electrical pins whenever the key is ON.  The other electrical pin is connected to the ECM.  The ECM grounds this pin for a certain period of time which will deliver a specific amount of fuel through the injector.  The injector is pulsed (or fired) relative to engine speed.  Older systems fired injectors in banks (4 at a time for a V8) but most newer systems fire each injector independently.  On a bank-fired system, each injector will will fire every other crank revolution.  Fuel flow is directly related to pulse width.  A longer pulse width will deliver more fuel.  A pulse width is the time in milliseconds the injectors is ON (fired).  The duty cycle is the percent of ON time relative to the maximum amount of available ON time.  As the engine speeds up, there is less time between injector firing events because they are happening quicker.  100% duty cycle means the injector is ON the maximum possible time.  Injectors are rated by fuel flow at 100% duty cycle, generally pounds mass of fuel per hour.

The ECM needs to know how much air, by mass, is entering the engine so it can calculate the proper amount of fuel to achieve the proper air fuel ratio.  There are several ways to do this.  You can measure air flow and density (temperature and pressure)(this is a vane air flow, or VAF, system), mass air flow (mass air flow or MAF system), calculate air flow based on engine characteristics, speed, and air density (often called speed/density), or other less common methods.  Most early port fuel injection systems used the VAF approach.  It had a simple air flap door that was pulled open by incoming air.  There was a temperature sensor in the system to measure inlet air temperature, and most systems used an atmospheric pressure sensor as well.  Air density was calculated using the air temperature and pressure.  Flow was then measured using the VAF meter and the mass of air entering the engine was then calculated.  The speed density system required data from a dynamometer to determine how much air entered the engine based off of air density, engine speed, and intake manifold pressure.  These systems did not tolerate internal engine modifications because the air flow would no longer be accurate based off of the manifold pressure.  MAF systems actually measure mass air flow directly, eliminating the need for more complicated calculations.  They are the most tolerant of internal engine modifications.

That is the very basics of how the EFI systems works to deliver proper fuel flow to maintain the proper air fuel ratio.  There is quite a bit more to the ECM than this but this will get you up to speed enough to understand basic fuel control.


Engine controls are designed to meet the needs of the engine they are installed on.  Fuel injectors and flow meters (MAF or VAF) are chosen based on the engines power output.  A 600HP engine will need twice the air flow and fuel flow of a 300HP engine.  The best way to achieve this is on a VAF or MAF system is to replace the injectors and flow sensor (VAF or MAF) with a set that can support the 600HP.  For most systems, you can increase the capability at least double it's current by changing only the injectors and flow sensor.  Many flow sensors are available that are designed to be installed in conjunction with a specific injector size.  For a given engine air/fuel requirement, the ECM will read a lower airflow than actual and deliver a shorter pulse width.  The shorter pulse width will cause the injector to deliver less fuel.  If the air flow meter is properly sized, the reduction in pulse width will cause the new, larger injectors to deliver the same amount of fuel as the stock injectors did at the original pulse width.  Lets say the air flow meter and injectors are designed to support double stock engine power.  The pulse width at idle will be about half of what it was before.  The new injectors at 50% duty cycle will now flow the same fuel as the original injectors at 100% duty cycle.  The new injectors at 100% duty cycle will flow twice the fuel.  The chart below shows this relative comparison.

Many people think this will result in an overrich condition.  Here is why it won't.  Lets say the engine is put on a dyno and the throttle set to deliver 60HP (part throttle) on a 225HP engine.  Lets say the stock fuel system had a 30% duty cycle at this setting.  The new system will have a 15% duty cycle and flow the same amount of fuel.  What we have gained though is the fuel control system has now been extended to support double the power.  The air fuel ratio will still be controlled as before so you will not be too lean or too rich.  This can all be done without changing software (or a chip).

Fuel injectors should generally be operated in a 10% to 90% duty cycle range.  Below 10% and above 90% the flow may not be linear.  In other words, there could be a too much or too little flow outside this working range.  This can be a problem at idle where, if you increase the injector size a large amount, the duty cycle may drop below 10%.  Another side effect is the changes will not work completely for transitional modes.  For instance, when you apply the throttle quickly, the ECM richens the fuel mixture a bit to prevent a lean spot.  The new system will richen up even more.  This is generally not a problem though.  I always like to install an adjustable fuel pressure regulator along with the MAF and injectors for fine tuning.  By adding fuel pressure, you can richen the system up when it is in open loop (cold start, WOT, etc).  This helps compensate for the high vacuum appearance the ECM will get from the larger MAF.

On OBDII equipped engines (all 1996 and newer), it is quite likely that these modification will result in the "check engine" light coming on and staying on.  The reason is OBDII uses model based diagnostics.  It looks at all sensors and compares actual reading with expected readings generated from a mathematical model of the engine.  When the injector pulse width is below expected, and the mAF reading are lower than expected, fault codes will be generated and the "check engine" light will illuminate.  This will not cause a problem and the system is still not polluting but OBDII will set the faults anyway.  The best way around this is with a software reflash (see below).  Since it does not really cause a problem, many people just remove the "check engine" bulb.

One other side effect is timing control.  The ECM uses the mass air flow rate, along with other parameters, to calculate engine load.  This engine load calculation is used to determine enrichment during heavy loads, and to calculate the ignition timing.  The timing affect is similar to the vacuum advance of yesteryear.  When you have high manifold vacuum, more timing advance can be dialed in for better fuel economy.  The modified EFI system will appear to have lighter engine loads and more timing will be added, especially at lighter loads.  If this system is going to be used with forced induction, this could result in detonation at lower engine speeds.  The best way to fix that is install an ignition boost retard system.  Crane and MSD both sell boost retards for under $200 that will work with most applications.  Since forced induction engines need premium fuel anyway, the result is actually good.  More timing is added at light loads (not under boost) and the premium fuel compensates nicely.  If this is a naturally aspirated engine, you generally would not modify the EFI unless you have installed higher compression, cams, heads, etc.  For an engine with a larger cam , this system will work quite well too.  Generally, the added overlap from a larger cam will allow more timing at low engine speeds anyway and the modified EFI system is happy to accommodate.

The ECM has power transistors (also called injector drivers) in it to control the switching of the injectors ON and OFF.  These injector drivers can only handle a certain amount of power or they will fail.  This power is related to the injector impedance.  A high impedance injector will result in lower power passing through the injector drivers.  As injectors get larger, their impedance drops and the power increases.  Make sure the ECM can handle the impedance of the injectors you install or you could fail the ECM.  You can buy high impedance injectors over 36lb/h now, I have seen as high as 50lb/h in high impedance injectors.  There are places that will change the injector drivers in your stock ECM so you can run low impedance injectors.  The table below shows what injector size is needed for each cylinder for a given power output on various multicylinder engines.

Flow Peak Power Potential (Horsepower) at 100% Duty Cycle
lb/h cc/min 4 cylinder 6 cylinder 8 cylinder 10 cylinder 12 cylinder
10 105 73 109 146 182 219
14 147 102 153 204 255 306
19 199 138 207 277 346 415
24 252 175 262 349 437 524
30 315 218 327 437 546 655
36 377 262 393 524 655 786
40 419 291 436 582 727 873
42 440 306 458 611 764 917
48 503 349 524 698 873 1048
50 524 364 546 727 909 1091
54 566 393 589 786 982 1178
60 629 436 655 873 1091 1309
70 734 509 764 1018 1273 1527
80 839 582 873 1164 1455 1746
90 943 655 982 1309 1637 1964
100 1048 727 1091 1455 1818 2182
120 1258 873 1309 1746 2182 2618
130 1363 945 1418 1891 2364 2836
140 1467 1018 1527 2036 2546 3055
150 1572 1091 1636 2182 2727 3273
170 1782 1236 1855 2473 3091 3709
200 2096 1455 2182 2909 3636 4364

Note: see my Free Software page for a spreadsheet like this that will calculate injector, fuel pump, and FMU requirements.

Fuel Management Units (FMU)

Most aftermarket supercharger kits have a different solution to adding the required fuel to support large power gains.  The use a device called a Fuel Management Unit, or rising rate fuel pressure regulator.  The way this device works is it increases fuel pressure at a higher rate than the 1:1 you get from a regular fuel pressure regulator.  The FMU has no affect under vacuum so the stock fuel pressure regulator does it's normal job.  Once you start getting boost though, the FMU will increase fuel pressure much higher.  A 10:1 FMU will give you 10psi fuel pressure gain for every psi boost pressure.  At 6psi boost pressure, your fuel rail pressure will rise to 60psi with a 10:1 FMU.  Now you will have over 40psi pressure difference across the injector and the flow will exceed the injector rating.  The difference in flow can be calculated using the following equation: flow increase = sqrt((new injector pressure - 40psi)/40psi).  Notice that doubling the fuel pressure will only increase flow by about 33%.  Fuel pumps have a flow rating measured at 40psi.  When the fuel pressure exceeds 40psi, the flow drops below the rated flow.  There are a few problems with the FMU method.  The fuel pump will need to be much larger to handle not only the additional flow, but the additional pressure as well.  If you use an FMU it is best to add an additional in-line fuel pump in the system to support these high pressures.  The other problem is that it adds fuel based on manifold pressure only.  At low engine speeds, when you are still well within the factory fuel systems flow capabilities, the FMU will cause an overrich condition on MAF and VAF EFI systems.  The airflow meter measures the additional airflow and the injector pulse width is increased to support it.  Now the FMU jacks the fuel pressure up too and you have too much fuel.  The FMU system works OK on speed density systems because they can't measure the increased airflow anyway so the FMU will add the required additional fuel flow.

Chips, Software, Aftermarket ECM's

Another method sometimes used is to replace the injectors with larger ones and install a chip or software to compensate.  The problem with this approach is the air flow will exceed the stock air flow meter capabilities.  At that point managing the pulse width of the injector to achieve proper air fuel ratio is a guessing game.  It can be done if you have a dyno and a lambda meter but you are back to the concepts used by speed density systems, mapping injector pulse width based mostly off throttle position and rpm.  The proper approach would be to install a larger MAF that can properly read the additional airflow to calculate the proper fuel flow as described above.

Another approach, the best one really, is to install a larger MAF and injectors, and reflash the ECM software with special software tuned for your particular application.  Saleen now offers this sort of service for Ford EEC-V ECM's.  If it is done properly, you can optimize the timing and air/fuel ratio across the entire operating range.  If you want to go even one step further, Ford SVO and LaRocca's sell a piggy-back ECM called the Extreme Performance Engine Computer (EPEC).  This will let you add a boost pressure sensor and dial in your own MAF, ECT, ACT, TPS, and many other transfer function and tune the system for you application.  By having the boost pressure sensor, you can optimize timing and air fuel ratio for all situations without any guessing in the software.  You can measure boost directly instead.

For further info, try these sites:   Do It Yourself EFI website   Smok'em up has some calculators and info   Bowling & Grippo website.  Great resource of info, calculators, and programs.  Also, a complete do it yourself EFI system. The Ford EEC Website  Superstang article about tuning an MAF  TwEECer tunable chip for Fords

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