toddk911

I have been wondering about this and have read in here in the past that you want to run the lowest possible fuel pressure and tune and add fuel, vs. raising the fuel pressure up super high; i.e. more fuel volume is better than more fuel pressure.

But assuming it is not an issue for injector sizing but for performance, would the higher fuel pressure be better for atomization and to offer sort of a "direct injection" by running much higher fuel pressure? Like maybe 55-65psi?

Has anyone done testing to see, much like 12.5:1 being the best a/f for power, is there an ideal fuel pressure for best performance/power?

m73m95

I have seen this thread somewhere before?

The matrix has changed something.....

Rogue_Ant

You don't wan't to run FP as low as possible... Fuel injectors need a decent amount of pressure to properly spray. Most injectors have a certain range that is acceptable, and we want to stay in that range.


-Rogue

951_RS

I responded in the bizarro thread =[
Anyways, so I guess that means that it all depends on the injectors you're using I take it.

Rogue_Ant

More pressure works the injector harder. But newer injectors are pretty tolerant of a 40-80psi range, IMO (remember that boost adds pressure @ 1:1 rate to the injector).

V2Rocket

injectors want to be at a certain pressure so that fuel can disperse into the air in a sort of mist, if it wasnt high enough it would be more of a stream. same reason why meth injection needs to be high pressure

theedge

Theres multiple reasons not to run super high pressure in most situations...


-Injectors cant always handle it (most will do 60-80PSI, after that it depends). Remember that for your peak fuel pressure, you take your base pressure and add your max boost pressure. So at 45PSI (approx 3 bar) base pressure + 20PSI boost, youre at 65PSI.

-Higher pressure means more fuel for a given pulse width on the injectors, ie if the injector is open for 2mS at 45PSI pressure, itll inject X amount of fuel. Double the pressure, and at 2mS youll have 2X roughly. This is an issue because injectors have a minimum pulsewidth they can handle accurately. Too small of a pulse width to account for too much flow, means you get crap idle. This is the same issue that plagues large injectors without sequential injection control. This may not happen on 951s with smaller sized injectors, its hard to say.

-Fuel pump flow drops off with increased pressure. Fuel pump may/will (depends on which pump you have and how old it is) not flow enough to keep up with the injectors at peak pressure which is (you guessed it) high boost. Kaboom.

-Fuel pump current draw goes up with higher load from higher pressure, causing more voltage drop in the crap stock wiring, and more pump strain and then even MORE struggle to keep up with flow at higher pressures.

-Run too low of a pressure, and the injectors wont atomize the fuel into a nice mist. This is usually around 25PSI and below that it gets iffy.


In short, unless you have a setup that needs and can cope with high pressure, stick to 2 or 3 bar as a nice safe easy number.

TurboTommy

I think the fuel flow only goes up by the square root of the pressure increase.
So, in this case; 2 times the pressure, fuel amount will go up by the square root of 2.

Yeah, and I've definitely experienced the "high fuel pressure, not enough pump flow" syndrome:
Many moons ago, I once tuned for significantly higher fuel pressure to prevent running out of fuel due to maxed out duty cycle, only to find out that I was now running out of fuel pump ( I seen a gragh somewhere, where it illustrates the expodential decrease of flow as pressure increases)

Chris White

Keep in mind (for those of you running 20+year old computers and wiring) that the higher the fuel pressure the harder it can be for the driver to open the injector. Normally not an issue but with aging equipment it can be a problem.

Also – for those of you running high boost engines you have to be careful about running out of injector dynamic range. From idle to 20 lbs of boost can use up the full range from minimum injector on time to full saturation (you don’t want to run an injector at 100% duty cycle…).

In a properly tuned system the fuel pressure will be dictated by injector sizing.