geeteego

iTrader: (1)

To start:

Car is a 2004 GTO, 364 LQ9 with a Vic Jr/90mm ported TB, ported 243's, Patriot Golds, 7.400 pushrods, stock rockers, mild reverse-split cam, and a rear-mount T76 intercooled turbo running at WG pressure (0.8bar). Tuned with HPT (SD, 2-bar MAP).

We are having what is turning out to be a consistent stumble at around 6k, almost sounds like a jake brake, on WOT tuning pulls. We changed the plugs, gapped them to .032, and it went away for 1 run. The next run, it came back, after the car warmed up...it almost seems like it's dropping a cylinder, with a correlating stumble on the graph.

Fuel pressure is there, AFR is conservative between 11-11.5:1. The graph for the most part is unchanged from a previous tune in March of this year, just now has this odd "stumble". Last compression test showed all cylinders to be 165-175, with one at 135. Plugs that were pulled looked a little crusty, but that is probably from when I lifted the heads losing the WG reference line a month or so ago. All 8 were uniformly so.

Any ideas? Valvespring, etc?

SMOKINV8

iTrader: (6)

Any changes made around the time the stumble started?

The compression test sounds a little out of range for that one cylinder...

geeteego

iTrader: (1)

No, what we found was that the previous tune, while conservative, in March was VERY rich today. Took a little bit of fuel out, and same thing, just a little more power.

I agree on the cylinder, but the car's run fine to this point...

edcmat-l1

iTrader: (10)

Might wanna try tightening up the gap a bit more, and increasing the coil dwell.

Schwanke Engines

iTrader: (1)

Could be valvetrain issues. If you start going into valve float, you will loose all power, especially being boosted.

geeteego

iTrader: (1)

Thanks guys for the input. It is baffling, to say the least.

Schwanke Engines

iTrader: (1)

Do you have a dyno graph? Maybe some logs?

geeteego

iTrader: (1)

I'll see what I can do about getting them up here. Thanks!

geeteego

iTrader: (1)

To add:

Last compression test (I found the actual #'s.)

1. 155
2. 135
3. 155
4. 145
5. 170
6. 140
7. 140
8. 130

geeteego

iTrader: (1)

Well, despite the issue, I took it to the track today.

98mph in the 1/8, and 129mph in the 1/4 shifting at 6200 on the street (12psi) tune. Felt really good up top, despite still fighting lag in the first 330. If the motor's hurt, it sure didn't show today.

I'm buying HPT Pro tomorrow so I can start logging on the street.