My project is coming along nicely and I am getting close to fighering this puppy up... One of the few things remaining is to set my adjustable fuel pressure regulator...

The Fiero is an 88gt with a stroked 3.1, 272 cam, ported/polished & gasket matched intake, ported exhaust, stock injectors & pump. Basically anything you could do to the 2.8 I did...

I need a good starting PSI and the best way to set it. I can get a pressure gauge to attach to the valve, but do not want to turn the engine over or start it at this time...

BTW, the Fiero has been sitting for 15 years. The tank was emptied ever since...

Any pointers would be appreciated

Thanks,
AL

For a Fiero V6, adjust the fuel pressure to 43.5 psi +/- 3 psi at the Schrader valve on the fuel rail with the engine not running. You can get the fuel pump to run continuously by applying +12 volts at the fuel pump pin of the ALDL connector.

The fuel injectors are a constant-flow-rate design, so they are relatively insensitive to small fuel pressure variations. You can't compensate for the increased engine displacement by simply increasing fuel pressure.

Since you have a 3.1, I might go with a higher setting.
I have a 3.4L (crate engine) and set the fuel pressure
to 50psi (with engine off) and it runs at approx 45psi when
running and warmed up.

[This message has been edited by Larry Nakamura (edited 09-27-2009).]

quote Originally posted by Marvin McInnis: You can't compensate for the increased engine displacement by simply increasing fuel pressure.

Why not? Wouldn't a higher pressure force more fuel through the injector?

quote Originally posted by ALJR: Why not? Wouldn't a higher pressure force more fuel through the injector?

As I stated above, "the fuel injectors are a constant-flow-rate design, so they are relatively insensitive to small fuel pressure variations." I've actually done the experiment, and Fiero fuel injector flow is relatively constant over a pressure range of +/- 20% from nominal.

Your assumption would be correct if the fuel injectors were simple pulsed orifices. If the injectors were simple orifices, you would expect the flow to vary directly with roughly the square root of the pressure differential across them.

quote Originally posted by Larry Nakamura: I ... set the fuel pressure to 50psi (with engine off) and it runs at approx 45psi when running and warmed up.

That's because the fuel pressure your gauge measures varies with manifold vacuum. Actually, the fuel pressure regulator continuously varies the absolute pressure in order to maintain a constant pressure differential across the injectors ... i.e. (absolute fuel pressure - absolute manifold pressure = constant). According to GM, the design point for the Fiero V6 injectors is a 43.5 psi (3.0 bar) pressure differential.

[This message has been edited by Marvin McInnis (edited 09-27-2009).]

With all thet being said, will my stock injectors work with my 3.1L build? Will the ECM be able to compensate, ie: not run lean?

I have the newer 3.1l but with the stock injectors it seems to be running a bit lean so im thinking about bumping up to either tpi or mustang injectors which are 17 and 19lb injectors.

quote Originally posted by mtownfiero: I have the newer 3.1l but with the stock injectors it seems to be running a bit lean so im thinking about bumping up to either tpi or mustang injectors which are 17 and 19lb injectors.

Newer 3.1?? With the stock Fiero intake or the 3.1 intake? Are the stock fiero injectors 14.7lbs, I forget its been so long...

Either way, I am thinking I should hold-off on installing my upper plenum untill I figure this out (don't want to waste the gaskets)...

Thanks for the help guys! This has been an ongoing project for the past 15 years; I don't want to screw things up when I am in the home stretch

quote Originally posted by ALJR: With all thet being said, will my stock injectors work with my 3.1L build? Will the ECM be able to compensate?

I have a 3.1 that is otherwise completely stock, and it runs fine with the standard 2.8 (~14 lb/hr) injectors. The stock ECM does raise the BLMs up above 128 (into the 140s) to compensate for the additional displacement, but that doesn't help full throttle operation (which is always open loop). I have changed the fueling tables slightly and burned my own EPROMs to get the BLMs back down around 128 where they belong, but I'm still not 100% satisfied.

IMHO, 17 lb/hr injectors are about right for a 3.4 but would flow too much fuel for a 3.1, especially at idle and low throttle openings. The stock ECM has a lot more range available to richen the mixture than to lean it down, i.e. the allowable BLM range runs from 120 to 150, with 128 being "nominal."

quote Originally posted by Marvin McInnis: I have a 3.1 that is otherwise completely stock, and it runs fine with the standard 2.8 (~14 lb/hr) injectors. The stock ECM does raise the BLMs up above 128 (into the 140s) to compensate for the additional displacement, but that doesn't help full throttle operation (which is always open loop). I have changed the fueling tables slightly and burned my own EPROMs to get the BLMs back down around 128 where they belong, but I'm still not 100% satisfied. IMHO, 17 lb/hr injectors are about right for a 3.4 but would flow too much fuel for a 3.1, especially at idle and low throttle openings. The stock ECM has a lot more range available to richen the mixture than to lean it down, i.e. the allowable BLM range runs from 120 to 150, with 128 being "nominal."

Do you think I would have any trouble running the stock injectors with my 3.1 that has been built pretty aggresivly while running the stock ECM?

quote Originally posted by ALJR: Do you think I would have any trouble running the stock injectors with my 3.1 that has been built pretty aggresivly while running the stock ECM?

No way to tell in advance. By modifying your engine from stock you have become your own test pilot, and there's no substitute for your own data from your own engine. Use WinALDL and look especially at the BLM table for the major part of your answer. Somebody else here on PFF who has built a similar engine to similar specs may be able to provide guidance.

[This message has been edited by Marvin McInnis (edited 09-28-2009).]

quote Originally posted by Marvin McInnis: No way to tell in advance. By modifying your engine from stock you have become your own test pilot, and there's no substitute for your own data from your own engine. Use WinALDL and look especially at the BLM table for the major part of your answer. Somebody else here on PFF who has built a similar engine to similar specs may be able to provide guidance.

this is the probably the best way to go.
but, overall - the 3.1 if not a huge leap from the 2.8, and, as long as you are using the stock intake system - you wont ever need to worry about overrunning the stock 15's