I've read conflicting entries on what the fuel pressure should be and how to check it... can anyone enlighten me on this.

'87 TPI 5.7, 24# injectors, TPIS intake, Top Proline Heads

I've read that fuel pressure should be anywhere from 38 - 46 lbs. I also keep reading about 43.5 being stock (something about "3 bars"). Should I be higher or lower pressure from stock with the bigger injectors ?

Also do you check the pressure with the car running with it shut off (key on), with or without the vaccum line connected to the regulator ?

I'm confused ...

 

The GM shop manual says the fuel pressure should be check with the key on engine off and should be at anywhere between 41 - 47 psi. Since most injectors are rated at 43.5. If you have a AFR, 43.5 psi should be your starting point. I run 45 - 47 psi on mine and have good performance. Be aware that when you change fuel pressure you also change the flow rate of the injector. There is a special formula to calculate your new flow rate. I cant find it right now but its something like New flow rate = Square root of (new pressure/old pressure) x old flow rate. This is important when doing your own prom tuning.

 

Thanks for the info Derek !

Anyone else running the factory specs ... and is this how everyone else is checking there fuel pressure ?

That is interesting about the formula for flow rate ... it you find it, would you mind posting it ...

 

I'm assuming since no one else has posted that everyone agrees with Derek's process for checking the pressure ? No offense to Derek, I just like to get multiple opinions.

I just want to know for sure ... cuz' my dad has been checking it with the motor running. I don't want to have to re-tune this car after we find out we set the pressure wrong.

 

If the motor is running, manifold vacuum is altering the pressure (at idle, for instance, the pressure typically sits at 37-39 PSI, depending on your settings). Rev the engine and you'll see what I mean.

Derek is correct. Key on, engine off. Make sure the ECM has primed the pump. It might be worth keying off for 20 seconds and then on a second time to re-prime.

With that said, this is the DIY-PROM forum. Your question might see more responses in the General Engine forum.

 

I will try to post in the proper place next time. Now that I know the proper way to check this, it's off to the garage ... thanks!