Fuel pressure testing under load?
Thread Starter
Burning Brakes
Joined: May 2009
Posts: 1,014
Likes: 7
From: Ft Wayne IN
Fuel pressure testing under load?
Do you want to do the "load" or full throttle testing with the vacuum to fuel pressure hooked up or unhooked and plugged? What are the acceptable numbers (psi) swing? Thanks!
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
that's a natural self adjusting item. Its not a lot. As vac changes so does the fuel pressure regulator. If you have an adjustable the screw sets the MIN pressure so it never gets less than that. Vac pulls against the spring to add fuel based on what the intake vac is doing. Or, go back to the preset point when throttle is released.
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
If your testing pressure at idle then unplug the line the the regulator and test the pressure. If you want to see what your pressure is while driving, then hook up the gauge and tape it to the windshield so you can read it while your driving. In this case, leave the vacuum hose to the regulator plugged in.
I am guessing for your car, but idle pressure somewhere near 43 PSI.
I am guessing for your car, but idle pressure somewhere near 43 PSI.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
If you take the vacuum line off the fuel regulator, the pressure goes up 5 LBS.
the idea behind that is when you give it gas, the manifold vacuum goes away, and the regulator will then kick in better fuel atomization and possibly more fuel at 5 PSI higher. when you let off the gas, and the vacuum returns in the manifold, the fuel pressure goes back down to 42-45 PSI.
I usually leave the vacuum off my regulator. it starts better like that anyway.
the idea behind that is when you give it gas, the manifold vacuum goes away, and the regulator will then kick in better fuel atomization and possibly more fuel at 5 PSI higher. when you let off the gas, and the vacuum returns in the manifold, the fuel pressure goes back down to 42-45 PSI.
I usually leave the vacuum off my regulator. it starts better like that anyway.
Thread Starter
Burning Brakes
Joined: May 2009
Posts: 1,014
Likes: 7
From: Ft Wayne IN
INFO: 450 hp 383 c.i. Bosch-III inj. OEM fuel pump/pressure reg.
I had PCM do a retune after installing heated O2 sensors. Was running on the rich side at idle with that gassy smell you get without catalytic converters. After retune I don't have as bad a smell and she idles a little better. They must have pulled some fuel? It doesn't seem to run as good under full throttle though.
I checked the pressure AT idle after the engine was built but never under a load. I hear a lot of 383 guys talk about upgrading the pump to a 255 lph to help maintain GPM. It doesn't fall flat or anything at W.O.T. but am curious if there is a little more to be had.
I am getting 42-ish psi under full throttle and drops to 35-ish for a couple seconds when I let off the throttle. this is with the vac. line on.
I guess the only way to really know would be to buy a pump and adj. regulator and rent a DYNO $$$. At the end of the day I don't know if it is worth the cost. I think I read the stock pump is around 160-180 LPH. What crank HP will the stock pump support? I have never Dynoed my motor but the specs are in-line with other claimed 450-hp 383 c.i. motors.
I had PCM do a retune after installing heated O2 sensors. Was running on the rich side at idle with that gassy smell you get without catalytic converters. After retune I don't have as bad a smell and she idles a little better. They must have pulled some fuel? It doesn't seem to run as good under full throttle though.
I checked the pressure AT idle after the engine was built but never under a load. I hear a lot of 383 guys talk about upgrading the pump to a 255 lph to help maintain GPM. It doesn't fall flat or anything at W.O.T. but am curious if there is a little more to be had.
I am getting 42-ish psi under full throttle and drops to 35-ish for a couple seconds when I let off the throttle. this is with the vac. line on.
I guess the only way to really know would be to buy a pump and adj. regulator and rent a DYNO $$$. At the end of the day I don't know if it is worth the cost. I think I read the stock pump is around 160-180 LPH. What crank HP will the stock pump support? I have never Dynoed my motor but the specs are in-line with other claimed 450-hp 383 c.i. motors.
