Choosing Fuel Injectors

How much fuel do you need?

First of all, you need to know about how much power you will be making. Come up with a realisic horsepower number. After that, you should decide on a BSFC or Brake Specific Fuel Consumption for your engine. Approximate BSFC's are 0.45 to 0.50 for naturally aspirated engines and 0.60 to 0.65 for turbocharged engines. I was told that I could assume a BSFC of 0.525 for our stock engine based on the fact that it is a recent DOHC I-4 with reasonably high compression (9.6:1). This means that the engine will use 0.525 lbs. of fuel per hour for each horsepower it produces. Use the equation below to figure out what approximate fuel flow rate you need per injector in cc/min [cubic centimeters per minute]. This isn't what size injector you should buy, just how much fuel you need.

A word on duty cycle and fuel pressure

I used the acronym DC in the picture above. I use DC to say "duty cycle", when I'm in a hurry. How long each injector remains open per unit time is called "duty cycle". For example, if an injector is open for 0.85 seconds over a 1.0 second period, the injector was open 85% of the time that it could have been open. Thats an 85% duty cycle. Injectors have mechanical parts that can't open and close but so fast. As you ask the injectors to open for longer and longer periods [higher duty cycles], there can be some error in how long they actually stay open. For example, if you ask for a 95% duty cycle, it may stick open [100% duty cycle]. So, if you want accurate control of your fuel, don't ask your injectors to do more than about an 85% duty cycle. Using bigger fuel injectors allows you to spray the same amount of fuel as a smaller injector, but at a lower duty cycle. The actual fuel flow out of the injector is dependent on the fuel pressure too. Higher fuel pressures will cause more fuel to be squirted out of the injector each time it opens. Most turbo kits bump up fuel pressure as fuel demands increase. This is fine, except when you reach high pressures. Anything over 100 psi fuel pressure can give questionable reliability. The injector may jam shut, starving the engine of fuel. Not to mention that regular fuel hose isn't designed for those pressures. So using bigger injectors can also let you lower your fuel pressures down to more reasonable levels.

Now pick an injector size

You already know how much fuel you need. The next thing you need is to decide on your maximum duty cycle explained above. Anything from 80-85% is acceptable. Just divide your fuel flow rate by the maximum duty cycle to get an approximate injector size. If you choose a duty cycle that is less than about 100%, then you will need a way to fool the ECU into running that lower duty cycle. No matter what DC you plug into that formula, the ECU will still try to run the injector near 100% DC under maximum load. You'll need a S-AFC or similar to trick the ECU into running the lower duty cycle.

Fuel injectors are generally rated at a differential pressure of 43.5 psi. The differential pressure is the fuel pressure minus the pressure in the intake manifold. Turbocharged cars will have intake manifold pressure greater than 0 [of course]. So if you have a turbocharged car and choose a fuel injector that is the same size as the approximate injector size that you calculated, you will need to run a fuel pressure that is a little higher than 43.5 psi [to keep the differential pressure at 43.5 psi]. The equation below shows how fuel flow rate changes with fuel pressure [and intake manifold pressure].

So you don't have to buy an injector that is the same size as the approximate injector size that you calculated. You can also use a slightly smaller injector at a slightly higher fuel pressure and get the same fuel flow rate. For example, you can prove that a 370 cc/min injector running at 61.5 psi differential pressure will flow just as much fuel as a 440 cc/min injector running at 43.5 psi differential pressure. But why use the smaller injector? Smaller injectors may offer more precise fuel metering when only small quantities of fuel are being injected [read "idle"]. So using too big of an injector might make the engine run rough while idling. Also keep in mind that there is somewhat of a lower limit on fuel pressures. It is a good idea to try to keep your fuel pressure above 30 psi (even at idle). The fuel injector may not spray properly at lower pressures. Some injectors may operate fine at less than 30 psi and others may not. Its just safer to stay out of that zone. Like I said, it depends on the injector. Only experimentation will tell you. So its decision time. Consider your engines future fuel needs, the maximum fuel pressure that you feel safe with, and the approximate injector size that you calculated. Then decide on what size injector and fuel pressure will serve your purposes. Once you have decided on the injector size and fuel pressure will give you enough fuel at maximum load [full boost], you need to work on providing the proper amount of fuel at lower loads. Check out the Choosing a Fuel Pump and Regulator page for help with that.

Contributed by Corbin
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Cars Modifications Power Turbo Fuel_System How to Choose Fuel Injectors

Document statistics: Last modified on 2009-05-19 21:27:24 by Corbin