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How to modify your EFI for performance

Article by: Jim Roal E-Mail


This article is written by Jim Roal and describes how to properly modify your EFI fuel system for performance applications.


Performance EFI

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.


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