Track Backfire

[wilke3169]

While at the track this weekend my car backfired. This was no little backfire. A mechanic at the track said he thought someone shot a gun.
This happened once in the fall. Now, it happened twice this weekend. Odd part is it has been in exactly the same place on the track.
At Carolina Motorsports Park you come out of turn 9 and just as I shift to 4th the car backfires. First time this happened in the fall I had no idea. The first time this weekend I actually had the clutch in and thought I may have "coasted" while making a deliberate shift. Second time, I shifted and when I got back on the throttle there was nothing there. Knew what was about to happen this time and warned my instructor.
It acts as if the rev limiter is actuated.
All igniton and fuel systems are stock on this car.
This has never happened anywhere else except this one spot on the track.

Any thoughts or suggestions?

[KTL]

Backfires like that on full throttle lift to change gears are typically related to exhaust leaks

[wilke3169]

I don't think it's an exhaust leak because the car loses power. Actually, watched rpm's drop from 5500 to 4800. Then a big bang and we were off and running again.

[KTL]

Oh. I missed the rev limiter comment in your first message.

A friend had the same problem on his 3.2. Not quite as pronounced as your issue, having a miss at ~5000 rpm. Turned out to be a bad ignition coil. His was consistent and not at just one location on the track.

[SCrescue]

I agree with Kevin that it is likely ignition related. One of my other cars (not Porsche) was firing off rifle shots (mostly on hot restarts, sounded like a 30-06) and eventually traced it back to bad spark plugs. Sounds like your ignition is breaking down under load, loading up the exhaust with raw fuel, then when the ignition kicks back in it fires off with a very loud report...

[Dixie]

At first, I thought Kevin shot me, and was going to toss me out. Cars go faster if they weigh less...