Spray Patterns

A fuel injector is basically a nozzle with an electronic door inside which opens and closes to control when and how much fuel is sprayed. It can open slightly to let small amounts of fuel in and the door can be swung open as wide as it is capable of to let as much fuel pass through as its size will allow. While this is a very simple way to look at it, there is much more involved in how injectors work and how you size an injector correctly.

Let’s look at how you figure out what size injector is correct for the amount of power being made and how much the injector depends on how efficiently the motor uses fuel for the horsepower it is making. Remember a 1600cc engine and a 5700cc engine will use the same amount of air to make the same amount of horsepower, but they might not use identical amounts of fuel because of differences in port efficiency. I know it sounds complex, but I promise it really isn’t all that bad.

The higher the number on the BSFC chart, the less fuel makes it to the motor. How is this possible if the injector is firing fuel under pressure in a tight little cone in the direction it is aimed?

Well this goes back to some of the auto shop basics. Once the fuel leaves the injector it enters the air stream in a fine mist which should atomize the fuel and disperse it into the intake charge as it travels its last few inches. If everything went well here we would be ok. But we’re not. Some of the fuel doesn’t atomize well and sticks to port walls or in severe cases might even collect in small pockets burning off causing detonation in the combustion chamber if the quench (area where the piston and cylinder head scoop up and compress the mixture to ignite it) area is poorly designed allowing unburned gasses to escape. The more efficient the port and combustion chamber design in general, the lower the BSFC numbers will be on a particular motor because it is using more of what it has. With the exception of forced induction motors which run rich because of higher combustion temperatures this rule is pretty standard.

Know Your Duty
Next in line when it comes to figuring out what size injector is right comes with knowing the Duty Cycle you’ll be running. Like I mentioned before, a fuel injector is basically a nozzle with an electronic door inside which opens and closes. The duty cycle is how long that door stays open at one time. The longer the door is open, the more fuel flows out. The technical name for the door is the pintal. The way the pintal is opened determines which of the two types of injectors you have: Peak and hold or the saturated type.

The peak and hold style injectors “shock” the pintal open with a three or four amp signal to and then hold it with only two amps. Saturated style injectors are similar but just send one or two amps to open the pintal and hold it in place. “99 percent of cars made now are peak and hold style injectors,” says John Park of RC Injectors in Torrance, California, “peak and hold injectors are the best style to use with extreme amounts of fuel pressure because the injector drivers use so much more amperage to open the pintel it can over power the higher amounts of pressure holding it down.”

Now what does all this have to do with the duty cycle? Duty cycle is important because it tells you how hard the injector is working. It is rated in a percentage. A 50 percent duty cycle is 50 percent capacity. A 70 percent duty cycle means the injector is working at 70 percent of its capacity and so on. It is important you do not run injectors beyond 90 percent because they may fail. The pintal could hang wide open or closed among other things causing catastrophic engine failure. I like to size injectors so they can supply the engines maximum power between 80 percent and 85 percent; this way there is some kind of buffer there.

Go With the Flow
So now that we know about the BSFC, appropriate duty cycles and what types of injectors there are out there, how do you figure out what size injector you really need? It’s actually pretty simple. This is what I came up with for a 2.5 liter turbocharged four-cylinder Dodge Caravan I used to own. At 22 pounds of boost on a seriously ported head with the right turbo we estimated it would make about 425 horsepower at the flywheel. Here is the formula we used to calculate injector size:

So we rounded up to an 83 lb/hr set of injectors and we knew they would supply the fuel we needed with the big Walbro pump installed a few weeks earlier. Converting that to cc/min we would use the formula 79.69 x 10.5 and get 836.325 cc/min.

In closing we would like to remind you the injectors are not alone when it comes to delivering fuel. You can not make 425 horsepower without a fuel pump capable of at least that much volume like a Walbro 255LPH in tank pump. A set of 83 lb/hr injectors will do you no good with a fuel pump, rail and fuel lines which will only support 200 horsepower worth of fuel. You can put all the pressure you want through it and it won’t do the job. Remember the fuel system is your motors lifeline. Don’t skimp out and buy generic parts because you can not afford good name brand equipment. Be patient and save. If you had enough money laying around to make the type of power that would require an injector upgrade then spending a few extra bucks on a good set of injectors or a good fuel pump will save you green in the long run when it remains reliable and doesn’t cost you a motor.