Fire67

iTrader: (3)

I was getting ready to purchase my new 75# injectors from racetronix today... Then I realized that 75's and 85's were the same price.

My wife pointed out that this is the 3rd time Ive bought injectors for this car (over a period of 5yrs and 3 setups). And suggested that I buy big enough to grow with if the need arises.

According to the math, with 75lb/hr injectors @ 80% duty cycle and 43.5psi base FP, My pumps will flow about 25gallons per hour more than neccessary to feed the injectors. With the way the motor combo is now, the 75# injectors will be able to supply plenty of fuel for my power expectations.

Should I get the 85's and dial base FP down to make them flow 75ish lb/hr for this setup? My pumps will flow more at the lower pressure, so thats not a problem. And when boost comes up, FP will be 15psi higher anyway... Mainly I will have the ability to grow with power levels by simply swapping pumps, instead of pump and injectors next time.

Can anyone think of any possible problems that may arise out of running the larger set to leave room for future expansion?

Old Geezer

iTrader: (3)

"According to the math, with 75lb/hr injectors @ 80% duty cycle and 43.5psi base FP, My pumps will flow about 25 gallons per hour more than neccessary to feed the injectors. With the way the motor combo is now, the 75# injectors will be able to supply plenty of fuel for my power expectations.

** At 3 bars, the additional fuel you mention, [25 GPH], will only support an additional 96FWHP @ .6 BSFC. Your next "step up", may exceed this??

Should I get the 85's and dial base FP down to make them flow 75ish lb/hr for this setup? My pumps will flow more at the lower pressure, so thats not a problem. And when boost comes up, FP will be 15psi higher anyway... Mainly I will have the ability to grow with power levels by simply swapping pumps, instead of pump and injectors next time.

** Dialing the FP down is not the way to "correct" the flow. The fuel requirement should be controlled with the ECM tune. With the correct inj constant, the ECM will make the changes, using the pulse width.

**The FP rise, due to boost reference on the reg, will not increase the flow. It only serves to compensate for the boost seen at the inj nozzle.

Can anyone think of any possible problems that may arise out of running the larger set to leave room for future expansion?

** Get the bigger injs and let the tune operate them as it should....

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Fire67

iTrader: (3)

Old Geezer-

So your saying that I dont need to lower fuel pressure, just compensate with the tune.
Effectively lowering the duty cycle?

I misquoted the excess flow capacity I have with the pumps by a bit. I will actually end up being only 25lb-hr above whats neccessary to feed the 75's at 80% duty cycle.

According to the results of a calculator I found, lowering the duty cycle of the injector effectively lowers the flow. Keeping me safe on the 'limited' pump supply.

If I need to go higher in duty cycle I will have to upgrade pumps, but that wont be for a while hopefully.

Thanks, just the info I needed to get past this mental I had goin on!

Old Geezer

iTrader: (3)

"So your saying that I dont need to lower fuel pressure, just compensate with the tune.
Effectively lowering the duty cycle?"

That's the way it works.
Whatever Fp you are going to use as a baseline, calculate the inj flow rate at that Fp, and use that as the constant.

New flow rate = old flow x sq rt of new psi/ old psi



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