izzyz28

So here's the scenario. Ever since I put my nitrous kit on(Zex dry kit, 125 HP jetting, 5 degrees retard, stock fuel pump) the car has run like a champ on the street. It rips through all the gears and pulls hard. However, at the track this past weekend I had a little scare. I made one pass on the bottle and it ran a ~12.90 @ 114 MPH. Normally aspirated it usually runs 13.2 @ 107 or so. So I only picked up 7 MPH or so from a 125 HP hit.

I wait a while and make a second pass.This time the car runs a 13.1 @ 109 MPH, but...........it ran an 8.4 @ 94 MPH in the 1/8 mile, which tells me that it pulled WORSE on the top end than it does N/A. It only picked up 15 MPH in the second 1/8, which is horrible. I should be running ~117ish on a 125 shot I would think.

It also seems(on the bottle) that the car is down on power above 5K and especially in fourth gear. It still runs smooth and everything, it just doesn't pull very hard. I can't hear any pinging or detonation, and there are no abnormal signs whatsoever.

I am heavily leaning towards the fuel pump being the culprit, and I will be replacing it with the Racetronix unit next week. Like I said, the car performs well in the first three gears, and then seems to lay down, especially at the track. I'm sure the fuel pump will fix my problem, but are there other things you guys would recommend checking? I would think my fuel injectors would be okay, and my fuel filter is maybe a year old.

Ole1830

Do you have access to a dyno?

Dynotune with wideband A/F should answer your question absolutely. Around here they cost ~$75. Definately worth the money.

Good luck,

Matt

sssalah

Isn't that roo much retad for 125 shot? it's not recommended to take timing for the first 100 hp and only 2 degrees per 50 hp after that. Some brave soles have run more than that with retarding.

izzyz28

I'm sure it is too much, but I wanted to be extremely safe with this setup, especially since I'm having trouble like I described earlier. If I still had the stock tune I'd probably only take out a couple of degrees, but with my tune, 93 unleaded, and many uncertainties, I'll stick with the rule of thumb 2 degrees for every 50 HP increase. This car is my DD and is driven over 20K per year, so I'd hate to have a meltdown.

HarrisSpeedWorks

Either way I think you're making a good move with the Racetronix kit. The dyno suggestion was a good one, if you can get on do it. Another idea might be to hook a fuel pressure gauge up. See if FP holds steady or starts to drop.

Matt

Cmr0z28

Get a fuel pressure gauge and "test" it before you go tearing into things

izzyz28

I'll be hooking up a gauge today at lunch and takign the car for a little spin, so we'll see what happens. I'm sure the pump is the culprit here. Even if it was a new factory pump, I think ~500 FWHP is probably too much for it to reliably support. I'll post the results this afternoon. Thanks for the suggestions guys!

jdizzell

So you have a dry kit w/ no fuel upgrades? I think that is your problem. Also did you change your plugs and re-gap them?

izzyz28

Yep, no fuel upgrades. I knew my fuel system would be very marginal at best when I installed the kit. It worked fine for a while, but I've always known it's on the edge. I did install TR6's gapped at .035", so I'm good to go there. Mallory 685 box with window switch , five degrees of timing pulled, ZEX kit has built-in WOT switch, etc.. I think once I get a good pump it'll be just fine. I'm gonna go outside and do some fuel-pressure testing on my "test track"(new road in front of work) here in about half an hour. The fuel pump should be installed sometime next week.

izzyz28

So I went ahead and taped a fuel pressure gauge to the windshield and went for a little drive. At idle the gauge was reading right around 36 PSI, and it would climb to about 42 PSI or so under light loads and RPM. However, above 3K and at WOT it would plummet to ~30 PSI and sit there. This is N/A by the way. I can only imagine how lean the car was running last week at the track. Fourth gear + 5500 RPM + 125 Shot + 30 PSI = I'll let you all know how it runs in a couple weeks.

Oh yeah, real quick question. As you can see in my sig. I trap ~107ish N/A. What should I be trapping on a 125 shot? I'm guessing around 117-118 MPH. What do you guys think? Is 10 MPH a reasonable gain?