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SPC hiccups at high boost (>22 psi)??
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Posted by Stealthbomber (more from Stealthbomber) on Sun, 16 Nov 2003 18:16:47 Share Post by Email
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Greetings, my friends.

I got a chance to romp the backroads with my "modestly modified" SPG this weekend. She seems to be running great, EXCEPT that at very high boost there is a sudden bucking or hesitation. Less than a heartbeat but it's most disconcerting. As long as I keep the boost from going past the end of the red zone, she romps, but if I floor it, the boost surges past the end of the red zone and there's a very brief but very unpleasant hiccup in the power.

It happens as or immediately after the stock boost gauge pegs. I've measured it with a handheld gauge and the peg is about 22 or 23 psi. I've pegged the gauge from time to time before and I've never noticed this before. I have no clue how high it would boost beyond the peg (an aftermarket gauge is in the plans but not in the car yet).

Mods relevant to the situation include: 87 SPG, 24 lb/hr injectors, 3.0 bar FPR, SPG9's AID, highly tweaked red APC (APC knock detection circuits still supposedly intact), Dawes Device (set to hide initial boost from APC, opens at about 8 psi boost), open K&N filter, upgraded sport exhaust. Motor and tranny rebuilt a couple of years ago; still the OEM Garrett T-3 turbo with about 250k on it.

She did this with a tank full of "93 octane" on the way up and with a half and half mix of 93 and 91 on the way back. Ambient temperatures were in the high 30s F.

I don't think it is simply knock detector kicking in. That seems to drop boost more smoothly. Mor do I think it's the overboost kicking in... I had that disconnected ages ago. At least, I paid the mechanic to disconnect it and I've never experienced the classic "dash in the face" before... and I don't think that's what is happening here. Let's assume that is NOT the cause and consider what else might be the culprit.

Last possible clue: I was under the hood recently and up under the dash replacing/fiddling with vacuum hoses. I think they're all intact and a glance under the hood didn't reveal

I'm hoping that it's just a cruddy tank of gas that can't handle 23+ psi, but please share your thoughts on other possible culprits.

As always, thanks very much for your thoughts and suggestions. Good roads, good health and good friends to all.

Cheers!

Jim Watts
The Stealthbomber
87 SPG, modestly modified.


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