ELECTRONIC FUEL INJECTION (EFI)

For many of our readers, electronic fuel injection is the staple of fuel delivery systems; it can be found under the bonnet of practically all ‘modern’ vehicles from the ՚’80s onwards. However, when it comes to what actually goes on, it can be a bit of a minefield of physics, thermodynamics, and maths — not our strongest subjects at school, we’ll admit. There’s a hell of a lot of potential for power hidden in there, as an engine really is based on how much air and fuel you can cram into it, safely. We wanted to find out exactly how EFI functions and what you need to know when upgrading things. Naturally, we turned to the team at Link ECU to get the low-down.

WHAT COMPONENTS MAKE UP AN EFI SYSTEM, AND HOW DO THEY WORK?

“Electronic fuel injection traditionally consists of some sort of highpressure fuel supply, be it a mechanical or an electric pump, which is regulated to supply fuel to a rail that is fitted with the injectors. Sensor measurements — things like an RPM trigger, engine speed and throttleposition sensors, or manifold absolute pressure (MAP) sensor — allow the ECU to predict airflow into the engine; when it does so, it can inject the right amount of fuel. This gives the ECU a broad idea of the airflow going in. An O2 sensor installed in the exhaust reads exhaust gasses to also provide valuable parameters in order for the ECU to determine whether the car is running lean or rich.

“In older-style systems, it was essentially broken down into a table where, if X amount of air was flowing in, the ECU would look for the cell on the table that related to the amount of air and receive an answer of how much fuel was to be fed per millisecond. As ECUs have developed, it’s more of a live calculation based on a number of factors.”

WHEN CHASING POWER, HOW DO I KNOW MY INJECTORS ARE MAXED OUT?

“This is best checked through some form of engine-management diagnostics, be it an aftermarket ECU or a factory system that allows you to look at parameters such as the injectors’ duty cycle. This will give you a rough window of where your injectors can operate.

“The whole idea of fuel injection is to try to mix the right amount of fuel with the air that is flowing into the engine, so, if your injectors aren’t big enough to flow an adequate air–fuel ratio, that can cause issues. We recommend that you don’t exceed an 85-per-cent duty cycle, which basically means that the injector is open for 85 per cent of the time and is reaching the limit of what it can physically flow. The higher the duty cycle, the less predictable it is on how much fuel is added.

“For example, you could bump your duty cycle percentage to 90 per cent, think, Oh there is more here, I’ll go higher, and up it to 95 per cent. However, there may be no difference, because the injector is open for nearly the entire time, meaning that there isn’t any more flow to use. More power requires more fuel; if your injectors can’t supply the correct amount of fuel for your air–fuel ratio, then the engine will lean out. Running lean usually means more heat, and that can cause piston wear, seizing, etc.”

HOW DO YOU CALCULATE WHAT SIZE INJECTORS ARE REQUIRED?

“A few factors need to be considered: number of cylinders/injectors, approximate target power, aspiration type, and type of fuel. Most injector manufacturers have calculators to suggest the correct size injector for your application, but it’s really down to basic physics and thermodynamics. There is such a thing as going too big, too, when issues arise at idle because the engine needs very little fuel but the injector cannot open for a short enough time to deliver the small amount required, causing the engine to overfuel. Many high-powered cars run multiple stage injectors, having only one at idle feeding the engine and turning on more as RPM and power increase.”

WHAT KIND OF SUPPORTING UPGRADES ARE GENERALLY REQUIRED?

“The main one is having a fuel pump that can supply the correct amount of fuel. Otherwise, most of the basic components usually have a lot of headroom in them to allow for power increases.”

HOW IMPORTANT IS AN ECU IN THE SYSTEM?

“Vital! The better the ECU, and how it is set up, the better the system will be controlled and run. Not only is the quality of the actual ECU itself important, but so is the support system behind it and whether you’ve got a good, useful dealer to communicate with for technical support and proper documentation for setting it up.”

WHAT ARE THE BIGGEST MISTAKES PEOPLE COMMONLY MAKE?

“Amateur wiring — where someone doesn’t understand the importance of electrical fundamentals and tries to tune it themselves without having the knowledge or training to work through it. A lot of the time, the most important thing is who is going to tune it — when taking it to a tuner, it’s important to consult them regarding what components they recommend and what ECU they are comfortable with tuning. If you take a car to a tuner and he’s not familiar with, or doesn’t like, what you’ve used, the final product won’t be as good.”