drperformance

I have a 1995 383 13:1 compression LT1 with a direct port nitrous system in a 87 firebird and everything was tuned to run at 50 psi. I run a progressively controlled 200 shot in 2nd and 3rd gear.

Last month at the track my car was falling on its face at the top of first gear after a couple of passes. So I tried to adjust my aeromotive AFPR and the pressure was acting weird would not adjust up and was around 40 psi with the vacuum line off when it was set at 50 psi? So I decieded it was time to upgrade the stock 24 lb injectors for some 36 lb SVOs and I replaced the Aromotive AFPR (JUNK) with a vacuum regulated holley. And I put the car on a dyno and tuned everything in and it was running great.

Went to the track today and after a few runs I developed some breaking up in second gear? I know it is a fuel issue, so I pop the hood to add 2 lbs of fuel with the AFPR and to my suprise it did the same thing, I pulled the vacuum line and it was flucuating around 40 psi when it was set at 50 with the line off?? And when I went to adjust it it would not adjust up. So I let the car set and cool for 15 minutes or so and then checked the pressure and it was 5 psi higher, could it be a heat soak issue with the 255lph intake pump?

I then drove the car home which is a half hour drive at 70 mph, where it bucked and studered at first which I related it to the inconsistent fuel pressure. But the further I drove the smother it got and when I got home I poped the hood and checked the fuel pressure and it was right where it should be 50 psi whith the vacuum line off? Do I have a failing pump?

By the way the car put down 375 rwhp and 385 rwtq and I am running a -6 feed line and a -4 return.

Injuneer

Sounds like you may have a fuel vaporization problem and/or cavitation in the pump suction. Is it possible your fuel lines are picking up heat from the exhaust or the engine compartment? Is your fuel tank fully vented, or does it run the vent through the EVAP cannister? When you pop the fuel fill cap, is there any excessive pressure, or a lot of fuel vapor venting out?

The -6AN feed line is fine, but the -4AN return sounds a bit small. That would possibly cause high pressure problems at low fuel usage, when most of the fuel is trying to return to the tank via that line.

drperformance

The tank is the stock tank and it is vented but no evap any more. I have 3/8 feed and 5/16 hard lines ran to the firewall then the -6 and -4 braided lines from there to the fuel rail.

If I go to say -8 feed and -6 return, I kind of defeat it because the lines out of my third gen tank are 3/8 and 5/16 and the same coming out of the LT1 fuel rail. And I realy do not want to start a hugh fuel tank/lines/fuel rail/pump/regulator modification project, $$.

And as far as I know there is no abnormal pressure released when I remove the cap. Any sugestions?

drperformance

After I read your reply I went and took a second look at my fuel lines. It turns out that my braided lines are about 1/2" from the header and the Earles fuel filter I am using is only rated to 70psi, when I saw the Aeromotive ones are rated to 200psi.

So my plan is to change the fuel filter to an Aeromotive, move and heat wrap the fuel lines and change out the braided -4 return that runs to the firewall to a 5/16 like it should be.

What do you think?

Injuneer

Might help, if that's the problem. Those were just the ideas I came up with, based on your description. I thought you were running a -4AN line all the way from the tank. You don't need an -8AN for the feed. I feed 800HP through a -6AN line from the tank to the back of the fuel rails. But I have a full -6AN return line from the regulator all the way back to the tank, so that is can return most of the 410 LPH pump capacity when its idling.

Not sure why the pressure rating of the fuel filter would be of concern. Just means the case isn't going to split from high pressure, or the seals aren't going to allow the fuel to bypass the filter. But you aren't anywhere near 70psi.

mdacton

what would the fuel vaporization do?

how would it act driving?

drperformance

Well the car would break up do to going lean, and when I would pop my hood to see the fuel pressure it would be irratic bouncing around 37-40 psi. My gage is not even a liquid filled one, and it is set to idel at ~44 psi and 50 psi with the vacuum off.

One other thing I remember is that when I took the evap can and lines out I plugged the rubber line with a bolt comming out of the tank, but there still is an other line with a plastic breather.

And with the fuel filter rated at 70 psi, I thought maybe it is restrictive just enough to heat the pump up over time? The Aeromotive are rated at 200 psi and say you will only see a .3psi drop through the filter.

drperformance

I am still not sure if I should have plugged the old evap hose coming out of the tank, could that build pressure? But I spoke to Earles and the fuel filter that Summit recomended is rated to 70psi but is a carberated filter.

Injuneer

When I deleted my EVAP canister, I left all the hoses intact, and stretched a fine mesh cloth over the end of the main vent hose to keep crawly critters out.

drperformance

By plugging the hose comming out of the tank with a both I could be vapor locking my fuel system. Could that cause low pressure after a hard pass?

mdacton

I would think if you blocked it totally it couldn't vent and would be hard to pump the fuel out