Electronic Fuel Injection System - EFI Basics

This is a high-flow-rate fuel injector that is also expensive (try $95.95 each for an ACCEL 55 lb/hr injector, PN 74612 from Summit Racing). This size injector is based on the standard fuel pressure of 43.5 psi. Let's say that you already have a set of 40 lb/hr injectors and you don't want to pony up $775 for these larger injectors. Raising the fuel pressure will increase the flow rate of a smaller injector. To determine the flow rate of an injector with a higher fuel pressure, merely divide the new line pressure by the original line pressure, multiply that number times the injector flow rate, and then take the square root of that figure to determine the new injector flow rate. Using the above example, we're going to raise the line pressure from 43.5 to 65 psi. This becomes:

This is not quite enough to satisfy that 700hp package, especially considering that the BSFC for the nitrous horsepower will generally be much less efficient (around 0.60 to 0.65), which will require even more fuel. So you might have to reduce your nitrous appetite by roughly 50 hp (or use a wet system) to give the injectors a little bit of head room to accommodate the fuel required to make that kind of horsepower.

There are also two different types of electronic fuel injectors. The most common is referred to as a high-impedance (12 ohm) injector. The ohm rating is the amount of resistance measured in the injector circuit. These injectors require a small amount of electrical current to operate, are most often used in production-style engines and generally range in capacity from 15 to 44 lb/hr. Higher injector flow rates require more electrical current to operate, creating what are called low-impedance injectors rated at 2 ohms. This additional current requirement also places a greater demand on the electronic injector drivers that are part of the ECM. This means that an ECM designed for high-impedance injectors cannot be used to drive low-impedance injectors. Many higher-end aftermarket EFI ECMs are designed to accommodate low-impedance injectors, but not all ECMs are the same. It's a good idea to check into this if your engine combination will demand injector flow rates above 44 lb/hr.

By Jeff Smith

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