North American Turbocoupe Organization

Full Version: Boost falls off at high rpms
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I noticed something strange the other day. If I floor it in 5th gear going 75 or so, I get about 17 pounds of boost. That's with a Gillis valve and K&N cone, BTW.

When I downshift to 4th, I get about 15. If I'm accelerating from a stop and floor it in first gear, the boost kind of peaks at 3000 and gradually falls off around 4000 or so. At 5000-ish, it actually falls below 15 psi.

Is that normal? Is that an anomaly of the Gillis valve, or perhaps something like an exhaust restriction? It isn't really a problem, but I'm just curious. I noticed the passenger-side muffler is hanging really low so I'm guessing the exhaust was replaced at some point, possibly with a restrictive el-cheapo system. That might explain it.
I know that my overboost buzzer on the daily driver never goes off in first, seldom in second, and quite regularly in 3,4&5. (Boost gauge broke) I think it's a matter of "load".
The more load you put on a turbo the higher (within reason) the boost goes. In first gear there just isn't enough load on the engine (before you have to shift it) to make maximum boost. I will check my boost gauge in the project car and see if I get the effect you are having. I'll try to post this in a day or two. I don't think the effect you are seeing is due to backpressure.

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88 TC X 2, 86 SVO, Main TC with K&N, 3"DP to 2.5"duals through Dynamax, Ric valve at 17+ and disconnected KS.
Elite Bodega 16" chrome wheels. Autometer pod w/ A/F and Vac/boost gauges.140 mph Motorsports Speedo New engine: Total Seal rings and TRW pistons, ported and polished head w/ cc'd chambers,1.59"exhausts, SS valves, gutted upper, knife edged lower, A-230 cam, Race Engineering Adj Cam Sprocket, Crowlers, ARP head studs, and rod bolts. Walbro 255 HP pump and Kirban adj FPR, T-3. Centerforce II, KB subs and jack rails.
I put a bleeder valve on and am able to run up to 17.5 psi in every gear.

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1988 TC 5 spd K & N, Bleeder, half of the sensors are new but I'm still gettin' a Code 42!
Thanks Pete.. The engine load is probably it, first gear is pretty short and doesn't allow for much spoolup time at WOT anyway.

Turboy, I had a Code 42 too. It was the fuel pressure regulator. Take the vacuum hose off of your regulator. If it smells like gas, it's definitely bad and explains your rich condition (gas being sucked into the intake). It can still be bad without the gas smell, too.
I just replaced the FPR and now I am getting it at idle too! I think that I am gonna have to go back and verify the VAM setting at idle. Thanks for the input though. Also, now after I replaced the FPR the car is running better than ever - no stumbling through first and full boost always at WOT! WOOHOO! I can't wait to pull up next to my first vic...I mean Mustang GT!

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1988 TC 5 spd K & N, Bleeder, half of the sensors are new but I'm still gettin' a Code 42!