Injector sizing.....Food for thought

[pickens]

I believe I read that the stock dsm turbo injectors are rated at 450 cc/min at a standard fuel pressure of 43.5psi. Now if it is true that the stock injectors flow that much fuel at that fuel pressure, then they are really flowing more fuel in reality. Most dsms run a base fuel pressure of 41-43psi (vacumm hose disconnected). As the boost rises, the fuel pressure also rises linearly. Therefore at say 15psi, the fuel pressure is around 56-58psi. At that fuel pressure, the stock injectors should be able to flow around a calculated 525cc/min. If you bump up the base few pressure a few psi w/ an adjustable fuel regulator, then you get a flow rate of 550cc/min which is good for around 325HP at the crank. So, why is everyone so quick to say they need larger injectors to run relatively small in size 16g turbos. Please discuss.

 

[kpt4321]

The reason the fuel presure rises, is to offset the pressure in the intake manifold. When boost goes up 1 psi, the fuel pressure also goes up one psi, and that way the pressure differential across the pintle is always the same.

Also, keep in mind that injectors are rated at 43 psi, but 1g base fuel pressure is really only 37 psi. That means in reality you only have about 420 cc/min of fuel flow.

 

[Groomz]

You have to remember too that the smaller injector is going to have to work against that high fuel pressure to stay closed as well. A larger injector will not need as much fuel pressure and pulse width to deliver the same fuel, which means less heat and better fuel control. The tuning margin on a high pressure smaller/stock injector would be hell. Go big with injectors and a pump, and make sure there's an AFPR somewhere in the equation.

Remember, too much fuel is a whole lot less dangerous than not enough.

 

[pickens]

You both are correct, but. Although the base fuel pressure is 37psi, at WOT under stock boost (12psi) conditions the fuel injector should be able to spit out 475cc/min max. I think that is what is missing in alot of injector sizing situations, the rising rate regualtor. Under WOT, the base fuel pressure is much less than the actual operating fuel pressure. I bring this up because at 17-18psi on my setup I have logged many times and am still running rich which would make sense because at 18psi my fuel pressure should be around 58psi,which would equate to my injectors spitting out a max of 525cc/min. Thats more than enough uel for the power I'm making. Obivously, the car is not asking for more injector yet based upon my logs. Make no mistake though, I am NOT saying that a set of bigger would not help. They would definately help by tricking the ecu into giving more timing advance and reducing fuel cut, but they are not needed to avoid running lean as many people suggest time after time.

 

[kpt4321]

No. You're wrong.

The fuel pressure regulator is NOT rising rate. It is a 1:1 ratio, so the PRESSURE DIFFERENTIAL across the injector is ALWAYS the same.

The base pressure is called such because it is measured with zero psi of boost in the manifold, or atmospheric pressure. Thus, the pressure in the fuel rail, and behind the injectors, is the differential pressure, since the pressure in the manifold is zero.

As manifold pressure rises, the pressure AGAINST the fuel coming out of the injector goes up. If you didn't do anything about this, then the injectors would flow less fuel the more boost you had, because they would have to push against all this pressure.

Instead, the fuel pressure goes up in a 1:1 ratio with boost, so that the DIFFERENTIAL pressure across the fuel injector is ALSO the same thing, which is equivalent to the base fuel pressure. The flow rate of the injector is constant, no matter what the manifold pressure is. That's the point.

The fuel pressure changes, but that doesn't do shit. What matters is the diferential pressure across the pintle, which is ALWAYS CONSTANT.

Originally posted by pickens
You both are correct, but. Although the base fuel pressure is 37psi, at WOT under stock boost (12psi) conditions the fuel injector should be able to spit out 475cc/min max. I think that is what is missing in alot of injector sizing situations, the rising rate regualtor. Under WOT, the base fuel pressure is much less than the actual operating fuel pressure. I bring this up because at 17-18psi on my setup I have logged many times and am still running rich which would make sense because at 18psi my fuel pressure should be around 58psi,which would equate to my injectors spitting out a max of 525cc/min. Thats more than enough uel for the power I'm making. Obivously, the car is not asking for more injector yet based upon my logs. Make no mistake though, I am NOT saying that a set of bigger would not help. They would definately help by tricking the ecu into giving more timing advance and reducing fuel cut, but they are not needed to avoid running lean as many people suggest time after time.

 

[pickens]

I stand corrected.