fuel pressure question?

[nstycpe93]

i have some more questions on fmu and fuel pressure.

i took the fmu out today and want to know what does everyone have their fuel pressure set up with vacuum off.

i have edelbrock rpm heads and rpm intake, long tubes, e cam, novi 1k 8 lbs, 30lbs injectors, 255 pump, with the fmu i had the pressure up to 42lbs vacuum on and 48 with it off.

any one have any idea what would be a good pressure to try.

thanks.

[468LC]

Ken have you driven the car with the FMU out of it yet?
You will be cutting it close with the 30s but it should drive great.
KEEP YOUR EARS OPEN TO PINGING. I can't really recommend a fuel pressure for ya though. Your car has enough parts on it to warrent a good dyno tune with a wideband just to make sure your not going to hurt anything. Be careful.

[nstycpe93]

steve, i drove the car for the first time today with the fmu off.
the idle was better, seems the gas needle didnt take a dive after a few miles. i got on it pretty hard but not for a long time, still got some snow on ground here and streets are wet.

but i do plan to dyno it jan 18, so should find out soon what the a/f is. i also bought a air/fuel gauge,
do u happen to know what wire it is hooked up to on the sensor on the pipe. thanks

[fanglemeister]

Not to sound too obvious but the pressure setting depends on how much horsepower you are making. If you have a 1/4 mile speed and race weight we can fairly accurately estimate your horsepower.

Also much depends which variety of MAF meter you are running, some are calibrated to run leaner than others (cough C&L cough), which makes more power for advertising purposes but doesn't help to accurately fuel boosted apps very much. Other types of meters may be factory calibrated for boosted apps and run way too rich overall. YMMV.

I run 30#s, an eec-tuner and Pro-M 30# Bullet meter, and still find it beneficial to fine tune my fuel pressure. Time on the dyno, &/or with a wideband O2 setup will tell. You want to set it up with a conservative tune, as once you get on the street and get some heat into the motor, believe me you will need the extra fuel cushion. Regarding the A/F gauge, I hate to say it but they are a waste of money, tuning wise. They will not give you any indication other than if you are above or below stoich which is 14.67:1 A/F ratio.

One other comment regarding MAF meters and the eec calibration: If you are not running an eec-tuner or similar, the stock eec's calibration won't indicate flow over 4.78 volts from the MAF meter, which roughly translated for a 30# calibrated MAF meter is about 410-415 horsepower at 11.78:1 a/f. If you run an eec-tuner or similar, you can increase the headroom of the MAF meter signal up to 4.99 volts, which will then allow the MAF to accurately meter air up to about 460-470 HP. The stock eec calibration commands 11.78:1 a/f under WOT which is fairly safe at boost levels under 9 psi.

Another item you have to watch is the rpm level of your power peak. This concerns the duty cycle of the injectors. At 5000 rpm you have @ 20ms to inject the required fuel, and as rpms increase, this window of time decreases. What this means to us is that 30s might be fine for making 400hp at 5000 rpm, but are way over the edge trying to make the same 400hp at 6200 rpm.

Back to the topic at hand: Generally speaking (not taking into account your MAF meter type and rpm level), 30#s at stock pressure are fine for 375 to 400 horsepower. If you are making 400-425 HP from 8psi boost you should start out running 44-48 psi base pressure. If making 425-450 HP you should be running closer to 48-50 psi base. At this point it might be a better plan to install a 6:1 FMU and drop the pressure back to stock. Reason being: when you jack up your fuel pressure more than 10% or so, over the long term, the eec will start dropping injector pulsewidth during all modes of operation including, eventually, WOT. For this reason it would behoove you to reset your eec memory at least once a week or so to keep the fuel correction only in short term memory, so that the fuel correction only effects closed loop and light load open loop operation.

HTH