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Join Date: Dec 2011
Location: Atlanta, GA
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Another overboost situation

Gang,

Got the car back from shop after top end rebuild. Here's what I have done so far:

3.3 Stock P/C and stock 7/1 compression
Heads port and polished and increased head size to 38mm
GT35R turbo
B&B headers/ Rarlyl8 street muffler
Full bay Garretson intercooler
Rarlyl8 adjustable WUR/ Fuel head modified for more fuel
New fuel injectors
964 cams
964 BOV
swapped to 1 bar spring to stock wastegate
New clutch/pedal assembly/clutch cable
AFR gauge installed


Started breaking in the engine and I THINK I'm hitting the overboost switch. Before the rebuild I had .8 bar spring which netted me .9 on the gauge due to better flow from my headers. Now after switching to 1 bar spring and other things (see above), I get 1.1/1.2 bar and I feel I'm tripping the overboost sensor. (fuel seems to stop flowing and I loose all power, face to the windshield experience, with a large backfire(almost gave my wife a heartattack)). Curious to know what my options are and recommended path to take.

Also, still working to solve my rich AFR mixture. Not sure if this affects overboost.

Thanks for help,
Sean
Old 06-16-2014, 08:43 AM
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Join Date: Oct 2002
Location: Baltimore, MD, USA
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I sent you a PM.

I would be able to offer a nice solution.

Thanks,
Fred
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 06-16-2014, 10:48 AM
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skimp when i went with short tube headers i use a .8 bar spring because the routing for the wastegate on short tube headers sends a weakened pulse to the wastegate as opposed Brian's equal length headers or stock manifold, jmho.
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'87, Modified WUR, Billet valve, 965 downpipe, Kokeln intercooler, GHL headers, Garrett T04B K27 hybrid, Tial w/.8 bar, WEGO IV, RarlyL8 dual out, LSD
Old 06-16-2014, 05:59 PM
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stock over boost sensor? if so, it is doing its job. you can remove the wire and ground it to bypass it.

check the rubber boot from the AFM to the turbo. (vacuum side, no boost pressure).
if it is soft, it is collapsing and stopping air flow.
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
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Old 06-17-2014, 05:20 AM
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Overboost switch is there for a reason. 1.2 bar on stock pistons/cylinders/compression and CIS is rather high. Consider installing stock spring for a while, and adjusting the boost upwards with bleed, instead of harder wastegate springs.
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Thank you for your time,
Old 06-17-2014, 07:43 AM
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Thanks for the suggestion everyone. I got a few PMs and someone suggested a Ruf 0.9 bar spring for my stock wastegate. I wasn't even aware there was a 0.9 bar spring for a stock wastegate. That might fit my needs the best. Keep the overboost switch intact and achieve my 1 bar spring goal. Anyone installed one on their cars?
Old 06-17-2014, 11:47 AM
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