antman89iroc

I noticed that my fuel pressure drops to about 40 PSI at WOT past 4500 RPM.

I have a stock fuel system from the in-tank pump (probably the original) to the injectors with the exception of the adjustable regulator and fuel rails that came with my HSR. The pressure is set to 50 PSI with vacuum line off and idles about 45 PSI connected. It drops to 40 PSI regardless of where I set the adjustable regulator.

Is it normal for the fuel pressure to drop that much? Do I possibly have a fuel delivery problem?

Orr89RocZ

iTrader: (20)

i've never watched my fuel pressure on my car, but i dont think it should drop that much. you may have bad holley regulator.

I know my car is about 46psi vacuum off, and it idles at like 40-41psi. Doesnt drop/jump around either

antman89iroc

Update. Changed the filter and it seems a bit better. Held 50 PSI to 4500, 45 PSI to 5000 and 42 PSI at 5500+. This is with an in-car gauge going through the gears. Mostly in 2nd and 3rd. Seems to be running great. I'm concerned since the pressure drops when the HSR should be coming on. And the idle pressures seems a little high at 48 PSI.

antman89iroc

Update 2 I wanted to see if the regulator may be pulling out fuel pressure due to manifold vacuum increasing at high RPM. Set pressure to 48 PSI with vacuum line disconnected and plugged. Ran car with vacuum line plugged so manifold vacuum would have no affect. It still dropped to about 42 PSI above 5000 PRM or so. I figured that the pressure should remain the same all the way to the max RPM. Which in this case is about 5500 or so. (shift at about 5900)

Do you think I may have a pump problem or am I worrying about something for nothing?

Orr89RocZ

iTrader: (20)

how old is the fuel pump? are you getting full voltage back there?

Also, when you shut the car down, does fuel pressure bleed off quickly or does it hold?

antman89iroc

Pump is probably the original. Pressure holds on shut down 40 PSI all night. Voltage? That's a good question. I noticed today that the pressure holds at 46-48 at WOT thru 5500. The only difference from last night is the lights and fans were running. Thought I might run a hot wire to the pump and retest.

If the stock pump is new/in good condition- shouldn't it hold up to 300+ RWHP or would I need a high output unit?

RednGold86Z

iTrader: (1)

It's DEFINITELY your pump that's flowing too little. A little better voltage will not be enough to cover that big of a problem. Pumps should always be able to flow more than enough, by a safe margin, even at lower voltages. If it can't, get a better one.

p.s. make sure your fuel lines are all in good condition, i.e. not kinked, not leaking inside the tank.

Orr89RocZ

iTrader: (20)

make sure you have a new fuel filter too. that can get clogged and impede flow.

antman89iroc

Fuel filter is new, lines look good, no external leaks etc. Hooked up a "straight wire" directly to the pump and retested. Pressure is set to 48 PSI with vacuum line disconnected. Re-ran test and pressure held 48 PSI to about 5000 and dropped to 44 PSI at 5500 and 42ish at 6000.

Bear in mind these the RPM's quoted are with the stock tach which seemed to read about 500 RPM high when compaired the dyno's figures. So the pressure is starting to drop in the 4500-5000 range. (it's kinda hard to compare the fuel pressure gauge to the tach when you're banging through the gears!)

It just doesn't seem right and I haven't heard many people discussing dynamic fuel pressure. Isn't that when the fuel pressure actually matters?!? I think this fluctuation would play havoc when trying to "dyno-tune" the vehicle. I have low(er) pressure just when I need it most.

Besides, I'm looking into a wet nitrous kit and figure this needs to be worked out first.

Anyone recommend a good fuel pump. Preferrably an in-tank type.

Orr89RocZ

iTrader: (20)

Walbro GSS340M from Racetronix. i got that pump with their hotwire kit that runs off the alternator voltage. i'm getting good voltage back there.

if the fuel presssure drop off is consistant, then you can tune for the lack of fuel in the chip. but it shouldnt be changing pressure like that

antman89iroc

Thanks for all your input Orr, Red too.