is my fuel pressure too high under boost?

bullitt843 · 04-25-2002, 05:59 PM

All right heres the deal,I have an 88 LX hb stock from the intake down.I have a Trick Flow street heat intake,65mm TB,76mm C&L mass air meter,30# inj.,K&N conical filter, Vortech S-trim puttin' out 9lbs of boost,Vortech FMU,190lph intank pump,T-Rex inline pump,Holley adj. fuel press. reg. set at 38psi.My question is, how high should my fuel pressure be under full boost?It will hit almost 80psi.The fmu is calibrated for 12:1,Does this mean, for every pound of boost, my fuel pressure is raised 12 psi?

Unit 5302 · 04-25-2002, 10:25 PM

The injector safety limit is about 100psi, so you're safe by that measure. If it was 12psi of fuel pressure for every 1 psi of boost you'd be at 9 x 12 = 108 + 38psi = 156psi.

The best way to know if your fuel system is functioning adaquately or if it's overpowered is to get some fuel/air ratios off a dyno run.

The analyzer seems to think your combo can support up to about a maximum of 600hp pushing your injectors to the limit.

bullitt843 · 04-25-2002, 10:46 PM

Thanks for the help. I will be building a 331 soon and hoping every thing will be adequate.

LaxMan · 04-26-2002, 01:37 AM

I think your calculations might be a little off. (Then again so might mine)

It is true that the fuel pressure will be raise 12 pounds for every pound of boost. But that is not in addition to your 38psi.
The FMU is on the down streem side of the fuel system so the FMU will block the fuel going back to the take at specifiec pressure.

For instance. If you are building 3 PSI, the FMU will block off the fuel until it reaches 36 PSI and then start bleeding it toward the tank. The fuel regulator that is set at 38 is still making it hold 38 psi at the injector. Once you go to 4 psi the FMU blocks off the fuel until it reached 48 psi. Now the FMU is regulating the Fuel pressure and not you regulator. It basically is like the fuel regualtor is not even there at this point.

If you are going to run 9-10 PSI I would have to say switch to a 8:1 FMU calibator.

gtsr515 · 04-26-2002, 07:32 PM

Forget all that, An 88 with a 76MAF and a blower? HHHHHHMMMMMMMMMMMMM?

04-25-2002, 05:59 PM	#1
bullitt843 Registered Member Join Date: Apr 2002 Location: n.c. Posts: 5	is my fuel pressure too high under boost? All right heres the deal,I have an 88 LX hb stock from the intake down.I have a Trick Flow street heat intake,65mm TB,76mm C&L mass air meter,30# inj.,K&N conical filter, Vortech S-trim puttin' out 9lbs of boost,Vortech FMU,190lph intank pump,T-Rex inline pump,Holley adj. fuel press. reg. set at 38psi.My question is, how high should my fuel pressure be under full boost?It will hit almost 80psi.The fmu is calibrated for 12:1,Does this mean, for every pound of boost, my fuel pressure is raised 12 psi?

04-26-2002, 01:37 AM	#4
LaxMan Registered Member Join Date: Jan 1999 Location: Sugarland, TX Posts: 458	I think your calculations might be a little off. (Then again so might mine) It is true that the fuel pressure will be raise 12 pounds for every pound of boost. But that is not in addition to your 38psi. The FMU is on the down streem side of the fuel system so the FMU will block the fuel going back to the take at specifiec pressure. For instance. If you are building 3 PSI, the FMU will block off the fuel until it reaches 36 PSI and then start bleeding it toward the tank. The fuel regulator that is set at 38 is still making it hold 38 psi at the injector. Once you go to 4 psi the FMU blocks off the fuel until it reached 48 psi. Now the FMU is regulating the Fuel pressure and not you regulator. It basically is like the fuel regualtor is not even there at this point. If you are going to run 9-10 PSI I would have to say switch to a 8:1 FMU calibator. __________________ 306DSS, AOD, T-Trim Vortech #3325 w/ driver 6.9882@100.33 1/8th

04-26-2002, 07:32 PM	#5
gtsr515 Registered Member Join Date: Nov 2001 Location: Sunny, Hot, Sebring, Florida Posts: 725	Forget all that, An 88 with a 76MAF and a blower? HHHHHHMMMMMMMMMMMMM? __________________ Remember...2nd place is 1st place for losers!

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04-25-2002, 10:25 PM	#2
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	The injector safety limit is about 100psi, so you're safe by that measure. If it was 12psi of fuel pressure for every 1 psi of boost you'd be at 9 x 12 = 108 + 38psi = 156psi. The best way to know if your fuel system is functioning adaquately or if it's overpowered is to get some fuel/air ratios off a dyno run. The analyzer seems to think your combo can support up to about a maximum of 600hp pushing your injectors to the limit.

04-25-2002, 10:46 PM	#3
bullitt843 Registered Member Join Date: Apr 2002 Location: n.c. Posts: 5	Thanks for the help. I will be building a 331 soon and hoping every thing will be adequate.