Fuel pressure question.....

dnsyang · 11-03-2002, 07:04 PM

My fuel pressure reading says 50psi when it idles...and when full boost WOT(20psi) kicks in, my fuel pressure goes all the way up to 70psi...Is that normal?

jw · 11-03-2002, 07:18 PM

Yes, what you are seeing is perfectly normal for a 1:1 FPR.

larryd · 11-03-2002, 11:17 PM

where do you have the gauge that you can read it when your at 20lbs of boost out of curiosity?

jw · 11-04-2002, 08:50 AM

Larry, you can get an electric fuel pressure gauge with a sender unit. You dont want mechanical gauge to go into your cockpit

candela · 11-04-2002, 10:26 AM

One time my friens connection on his mecheanical oil pressure gauge came loose from the back of the gauge from the damn AZ heat. It was not really squirting, but it was coming out oil all over his dash and carpet...it was hilarious!!

jw · 11-04-2002, 10:33 AM

He should be lucky the thing didnt catch fire. At that pressure, the fuel catch fire easily. I had that happened to me once when I put on the gauge without tightening it properly. It caught fire immediately after the car started. That's why I carry a fire extinguisher around nowadays

larryd · 11-04-2002, 11:39 PM

damn that would suck.. where do you tap for the electrical fuel gauge out of curiosity?

GSX90 · 11-04-2002, 11:55 PM

Quote:

Originally posted by larryd
damn that would suck.. where do you tap for the electrical fuel gauge out of curiosity?

I tapped my electric sending unit right on top of the fuel filter banjo bolt

EvuLFleA · 11-12-2002, 04:29 PM

Quote:

Originally posted by larryd
damn that would suck.. where do you tap for the electrical fuel gauge out of curiosity?

I tapped mine on my test port on my fuel rail. If you want a CHEAP electric fuel pressure gauge watch ebay for a autometer recalled gauge. I got mine for $60, sent it to autometer and they returned me a $180 full sweep gauge.

SPEEDY · 11-12-2002, 07:23 PM

I notice in his mod list that he is running a walbro 255lph fp with no adjustable fpr, with a FP Green. If 50 psi of fuel pressure is normal for him at idle, would that then mean his fpr is not getting over-run by the big walbro 255 fp like alot of people claim?

pneumo · 11-12-2002, 09:56 PM

Ha! Nice catch, Speedy. With a stock fpr normal fuel pressure at idle should be about 34 psi. Also the fuel pressure should increase 20 psi going from zero boost to 20 psi boost. That's 1:1. Under engine vacuum the fuel pressure should drop another 9 to 10 psi, less if you have cams.

SPEEDY · 11-13-2002, 04:50 PM

Quote:

Originally posted by pneumo
Ha! Nice catch, Speedy. With a stock fpr normal fuel pressure at idle should be about 34 psi. Also the fuel pressure should increase 20 psi going from zero boost to 20 psi boost. That's 1:1. Under engine vacuum the fuel pressure should drop another 9 to 10 psi, less if you have cams.

I agree! I think I understand what you are saying, Thanks! Well... I have a 2g and if my base fuel pressure is suppose to be 38 psi for a 2g, correct me if I'm wrong here, and he's running 50 psi at base idle with a walbro 255lph fuel pump, would that, or would that not be fpr over-run caused by the big fuel pump? As far as I can tell from his mods, he doesn't have an adjustable fpr from what I can see. Thanks for the info!!

talonted_one · 11-13-2002, 05:41 PM

actually base fuel pressure on a 2g is 43 psi. he is getting 50psi at idle so I would have to say yes his FPR is being over run.

SPEEDY · 11-13-2002, 06:05 PM

Quote:

Originally posted by talonted_one
actually base fuel pressure on a 2g is 43 psi. he is getting 50psi at idle so I would have to say yes his FPR is being over run.

Well hell...that's strange, wonder why my cheesy chiltons manual says 38 psi for? Of course I could be reading it incorrectly though. Thanks

jw · 11-13-2002, 06:23 PM

42-43psi for 2G is correct. 36-37psi for 1G. Non-turbo is 47psi or something.

EvuLFleA · 11-13-2002, 10:06 PM

Yes we have to run about 47-50 for 2gnt. Our fuel systems are so much fun to work with.

BlueShark · 11-14-2002, 08:40 AM

Correct me if I'm wrong, please.
Even if his pressure at idle is higher, he shouldn't have a problem if he is maintaining 1:1 ratio?

I'm wondering ### I do not have a FPR yet and I'm running 255 as well. Tunning is little tricky but I haven't experienced any "major" trouble so far.

pneumo · 11-15-2002, 12:31 AM

Yeah, that's the thing, if you have a fuel pump so big that the fuel return line can't flow enough back to the tank at idle, then it's not 1:1 anymore. As soon as you open the throttle a little, some of the extra gas volume gets bled out through the injectors, and the fuel pressure actually goes down even tho pressure in the manifold goes up! ( that's where the fpr gets it's pressure signal from) Somewhere between 5% and 20% throttle opening the fuel pressure will return to 1:1 ratio. Try to tune with that happening! And what if you have 5% throttle at 3000 rpm vs. 10% throttle at 1000rpm- what's the fuel pressure gonna be then? What correction do you use on the AFC at 1000 rpm? 3000 rpm?? Jeez that sounds like the "word problem" from hell. BTW this matters because extra fuel pressure makes the car run richer.

UncleBens · 06-08-2003, 12:28 AM

Old thread but for the archives, I would say that in conclusion, dnsyang does have an aftermarket FPR. Otherwise his pressure would start at a 50 bfp (overrun) and end up at 63psi (43psi + 20psi) after the stock fpr catches up.