Fuel Pressure Under Throttle Ever Higher Than Vacuum-Off Idle Setting?

[Swarzkopf]

My question is in the title. While driving, does fuel pressure ever rise above that which it is set at idle with the vacuum line disconnected? I don't believe it does, however, I'm not sure the vacuum level becomes negative and raises the pressure.

Thanks.

 

[Davin]

No, having the vaccumm tube off is the same as WOT (wide open throttle). It should never go higher than this in driving condiditons.

 

[Davin]

Here is a little extra informaiton on how the FRP works:

The fuel pressue in the fuel lines is created by the fuel pump, not the FPR. The FPR controls how much fuel is returned to the fuel tank. Fuel enters the FRP and pushes up on a diaphragm (see attached illustration). The diaphagm controls how much fuel may return to the tank. A spring pushes down on the diaphragm. Fuel pressure is a tension between the pressure of the spring pushing down on the diaphragm and the fuel pushing up. The higher it goes, the less it blocks fuel returning to the tank. In stock FRPs, this is fixed. In adjustable FRPs the control screw determines how low the diaphragm can lower and thus, the maximum fuel available. When manifold pressure rises (i.e. more throttle and air flow with RPMs) the pressure above the diaphragm increases and pushes down on the diaphragm and limits the fuel that may return to the tank, thus increasing the pressure in the rails.

Davin

 

[LoudToy958]

Great info - curious - I just installed a Fuel Press gauge with an Aeromotive FPR - I set the static at 38psi and when vacuum is applied its about 42psi. If I throttle up should this drop due to fuel and pressure being used?
Just a little confused. I get how the reg works, I just don't get how the gauge should read under throttle load.

Any info is appreciated.

 

[HISSIN50]

An easy way to think of how Fuel pressure should react is to know that pressure basically goes up inversely proportionate to manifold vacuum.

Shane, do you plug the vac line to the reg when setting pressure? It makes a marginal difference (I dont think it matters here though).

That is pretty weird that you have more pressure with vac on the reg than with no vac. Kinda' backwards. If you put a handheld vac pump on the FPR nipple, does it hold vac?

 

[LoudToy958]

An easy way to think of how Fuel pressure should react is to know that pressure basically goes up inversely proportionate to manifold vacuum.

Shane, do you plug the vac line to the reg when setting pressure? It makes a marginal difference (I dont think it matters here though).

That is pretty weird that you have more pressure with vac on the reg than with no vac. Kinda' backwards. If you put a handheld vac pump on the FPR nipple, does it hold vac?

I just noticed that the vac line that was on the car is a little too large for the FPR. That might account for the issues I was having. I may have had a small leak at the FPR nipple. I got some smaller line now, but the car is apart at the moment. I did however plug the vac line when setting pressure.

What should pressure be at vac on psi vac off psi? I have it set at the normal 42psi vac on, but unsure what the static setting should be.

Didn't mean to jack your thread up top, but looking for some similar info myself. Gotta feeling things aren't kosher with the fuel system......

 

[HISSIN50]

What should pressure be at vac on psi vac off psi? I have it set at the normal 42psi vac on, but unsure what the static setting should be.

Not sure on a Cobra, but on a GT, the stock pressure with the vac line off and plugged is ~39 PSI. Once the vac line is reattached, the FP is generally drops to ~32 PSI (varies if someone runs a cam with a diminished vacuum signal, etc). IIRC, anything at or over 30 PSI, with the vac line attached, is within spec.

Good call with the vac line sizing. My line was totally vulcanized (it actually had to be cut off and broke into sharp chunks. ). I had some small line on-hand and tossed it on - not sure what size it was though. Like you, I like to run vac lines on the tight side.

Good luck with it Shane.

 

[LoudToy958]

Not sure on a Cobra, but on a GT, the stock pressure with the vac line off and plugged is ~39 PSI. Once the vac line is reattached, the FP is generally drops to ~32 PSI (varies if someone runs a cam with a diminished vacuum signal, etc). IIRC, anything at or over 30 PSI, with the vac line attached, is within spec.

Good call with the vac line sizing. My line was totally vulcanized (it actually had to be cut off and broke into sharp chunks. ). I had some small line on-hand and tossed it on - not sure what size it was though. Like you, I like to run vac lines on the tight side.

Good luck with it Shane.

Well, after looking at the directions for the regulator, I got it. I never read those. I am trial and error guy. Guess that will change now. Here are my findings so everyone can see. Pretty easy. Might take a minute to load though.

Swarzkopf - really sorry to get into your thread. Didn't mean to jack it. I hate when people do that. Looks like you ???s are answered though. If not read the directions in this link. Actually can go from 37-39 at idle to like 45 or so at WOT.

Aeromotice FPR instructions

 

[Swarzkopf]

No, having the vaccumm tube off is the same as WOT (wide open throttle). It should never go higher than this in driving condiditons.

That's what I thought. I just wanted to verify this.

Thanks.

 

[Swarzkopf]

Swarzkopf - really sorry to get into your thread. Didn't mean to jack it. I hate when people do that.

You didn't jack it. There's lots of good information here.