EFI for the Masses

Is electronic fuel injection for everyone?

By Mike Petralia

If you paid attention to nothing more than what type of vehicles the OEMs are building today, then you'd think there wasn't any other way to fuel your car besides electronic fuel injection (EFI). The fact that a carbureted car has not left Detroit since 1985 would lend one to think that carburetors are surely a thing of the past. But are they? And why has EFI become the staple of auto manufacturers worldwide? Is it that much better than a carburetor? It surely can't cost less, because you can tell just by looking at the stuff that EFI is expensive. And aren't the OEMs always looking for the cheapest way to build the best cars they can?

In reality, the automakers have one major factor to thank for EFI: smog. That's because without the strict emissions rules placed on the manufacturers back in the '70s and '80s there would never have been a reason to further develop EFI. We use the term "further develop" here because EFI was around automobiles long before 1985. Several companies tried to mass-produce EFI systems for the mainstream many years ago, but at the time they were too expensive and impractical to last. Then, Big Brother stepped in and told the OEMs they had to clean up their act. Since most vehicle emissions are produced when the car IS NOT at wide open throttle--the big pollution comes from cars idling and cruising around town--the OEMs had to figure out a way to clean up emissions in those highly critical areas. EFI was the answer.

WHAT IS EFI?
It's important not to confuse today's electronic fuel injection with the mechanical fuel injection systems used in the past and still in use on some of the highest-output race engines in the world. The fuel injection that powers our cars today is called EFI because the injector pulses are controlled electronically, not mechanically. That means there's a small, yet very powerful, computer in your car that controls how much fuel the engine gets. That same computer also controls your ignition advance so there are no more little springs and weights in the distributor (that is, if your car even has a distributor). Ironically, the computer still needs input from both you and the engine to correctly meter the fuel and set the advance curve.

Things like throttle position, temperature of the air inside and outside the engine, manifold pressure or vacuum, oxygen content in the exhaust, coolant temp, and even the amount of air flowing into the engine all play a critical role in how the ECU meters fuel. One thing that the computer does not control, except in a few limited OEM applications that have just come out, is fuel pressure. That's still controlled by a good old-fashioned regulator, which may be pressure-referenced off of the intake manifold. Typically, a pressure-referenced regulator will increase fuel pressure at zero intake manifold vacuum or, in the case of a supercharged engine, when there's boost in the manifold. The natural effect of more fuel pressure in an EFI system is more fuel sprayed for the same injector pulse duration (we'll explain more about that in a minute) and a richer mixture. It's kind of like the power valve enrichment circuit in a Holley carburetor. At WOT the engine needs much more fuel than it does at cruise, so the power valve, or in EFI's case the extra fuel pressure, adds that much-needed extra fuel. Ironically, it's one part of an EFI system that is still mechanically controlled.

CONTROLLING EFI
The computer used to control EFI is called the ECU, or Electronic Control Unit. It's a self-sufficient device that, once properly programmed, can tune the motor on its own and will keep it running the most efficiently under all conditions. The ECU uses a pre-programmed fuel map (see Fig. 1), which the user/tuner inputs into the ECU to decide how long to hold the injectors open (known as injector pulse width). This directly controls how much fuel gets into the engine and is displayed in milliseconds in the boxes on the fuel map. By increasing or decreasing pulse width on any part of the map, the engine will run either richer or leaner.

The ECU is constantly altering the injector pulse width in its search for the perfect air/fuel ratio, which it measures in the exhaust using an O2 sensor.An easier way to understand what the ECU does would be to think of it as your own personal little carb tuner hidden under your hood and the fuel map as his playing field. While you're driving around town, your personal tuner runs up and down his playing field looking for how much fuel to give your engine, based on the demands you've placed upon it. All around his playing field are buttons he can hold open for any length of time to inject fuel into the engine. Let's say you're just cruising around. He goes to the "cruise" area of you fuel map (which, in reality, will be somewhere in the a high-vacuum, low-rpm, and low-throttle position vicinity on the map, middle left in this case) and sees that it tells him to inject fuel for around 30 milliseconds at each injector. He holds the button down for 30 milliseconds (the little guy works fast), but then you want to pass someone. You mash the throttle about 3/4 of the way to the floor. Inside your engine, the vacuum drops, the throttle opens farther, and the engine's speed starts to climb.

Your tuner runs to the spot on the fuel map that closely approximates the area you are in, and it tells him to now inject fuel for 60 milliseconds. This cycle constantly repeats itself the whole time you're driving (the little guy never gets tired). The ECU is always recalibrating the air/fuel curve to make the engine run its best. But how does it know when the engine is running its best? That's controlled by the parameters pre-programmed into the ECU, and in fact, most ECUs have the ability to go beyond those parameters, because they know that no computer program can make the engine run perfectly all the time. The ECU can "self-adjust" to better match conditions. What that means is that the ECU may only be sending 85 percent of the fuel map's recommended injector pulse width, because the conditions that day are not meeting the specified parameters of the ECU's map. The engine could be hot, or the air going into it could be cold, or any numbers of factors could be calculated, and injector pulse is constantly adjusted based on what the ECU determines to be the correct setting. As you can tell, programming the ECU properly is pretty critical to performance. And it's not something that can be done with just a few clicks on the keyboard. Tuning the ECU to its maximum potential is done best on the road and at the track. With a laptop plugged in and a computer-knowledgeable passenger punching the keys, drive all over the place keeping throttle positions constant while tuning takes place.

PROPER INJECTOR SIZING
Fuel injectors are rated by how much fuel they can flow wide-open in 1 hour. However, that fuel is not measured in gallons or liters; it's measured in pounds-per-hour. For best operation it's critical to use the properly sized fuel injectors. Running too small of an injector will limit horsepower potential and could damage the engine by causing it to run too lean, while running too big of an injector could give you idling problems and make the engine less responsive. Horsepower output; the engine's Brake Specific Fuel Consumption (BSFC), which is the pounds of fuel consumed per horsepower per hour by your engine; and the injector's duty cycle, which we'll explain in second, all determine proper injector size. Since most of this is way too confusing for the average guy without a dyno in his back pocket to calculate, we like to assume a BSFC figure of around 0.500 and a minimum injector duty cycle of 0.85 for most street engines.

That means that at max power the injector will be open 85 percent of the time in order to provide enough fuel to make the horsepower needed. This is also affected by fuel pressure, because as pressure goes up, volume goes up, and as pressure goes down, volume goes down. Most injectors are rated to operate around 43 psi, but you can certainly go higher or lower than that. But it's a relatively safe pressure for a mild-to-healthy street engine. Higher compression race engines will usually run a BSFC in the 0.400s at around 45 psi because they're more efficient and will make the same horsepower as the typical street engine would using less fuel. Use the following formula to estimate injector size for your engine.

Injector Size = (HP x BSFC) / (Number of Injectors x Duty Cycle) Ex: 400hp engine w/ 8 injectors
BSFC = .500
400 x .500 = 200
8 x 0.85 = 6.4
200 / 6.4 = 31.25

Always round injector size up to next largest available injector. A typical 400hp street engine with 8 injectors and .500 BSFC operating at around 43 psi would need a minimum 30 lb/hr injector, but 32 lb/hr injectors would be better in case you may add a bigger cam or something later to make just a bit more power. The cool thing about changing injectors is that it's easy and only takes slightly longer than changing jets in a carburetor.

Keep in mind that horsepower estimates in all these calculations are always at the flywheel, not the rear wheels. The less efficient an engine is (i.e. higher BSFC figure), the bigger the injector it will need.

FUEL SYSTEM
It is critical that the fuel pump, lines, and filters flow enough fuel. You want a pump that is rated at least 25 percent more than your peak fuel needs (measured in pounds of fuel flowed per hour). The filters must also flow enough. It's best to run a filter (40-100 micron) before and a high-pressure filter (10 micron) after the pump to protect the fuel injectors from clogging. Remember, fuel pump flow DECREASES with increased pressure. You need to know the flow at the pressure you run. Voltage greatly affects pump flow. A pump will flow a lot less at 12 volts than at 14 volts. Always use the correct gauge wire and a separate relay for the pump.

We have literally just tapped the surface of EFI here but will continue to bring you more in-depth EFI tuning stories in the coming months. While it is still rather expensive and may seem a bit daunting at first glance, EFI really is a cool way to cruise in today's world. If you had it on your hot rod or daily driver, you'd realize how much more fun it is to be able to just hop in, start the engine, and drive anywhere, in any weather. Perhaps the biggest irony of any EFI system is that once tuned correctly, it won't require you to lift your hood even once, but you'll always have it open just to show your friends that you're cruising in style.

EFI Acronyms:
Here's some basic EFI
terminology defined.
ECU- Electronic Control Unit
EFI- Electronic Fuel Injection
IAC- Idle Air Controller
MAP- Manifold Air Pressure
MAT- Manifold Air Temperature
MPFI- Multi-Port Fuel Injection
TBI- Throttle body Injection
TPI- Tuned Port Injection
(GM trade name)
TPS- Throttle Position Sensor