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Electronic Fuel Injection - EFI For The Masses

Is Electronic Fuel Injection For Everyone?

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.

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