Backfiring while decelerating on the track
Thread Starter
8th Gear
Joined: Aug 2016
Posts: 8
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Backfiring while decelerating on the track
I have a 2016 PGTS. Recently on the track when I braked for a curve, I heard 3 or 4 loud backfires. This happened many times after it first started. Is this normal? The dealer couldn't find any thing wrong.
Any suggestions or advice would be gratefully appreciated.
Any suggestions or advice would be gratefully appreciated.
Intermediate
Joined: Nov 2015
Posts: 43
Likes: 0
From: NJ
Turn off the sport exhaust and he car will become quiet and then you'll miss it and turn it back on! :-)
Instructor
Joined: Aug 2013
Posts: 211
Likes: 63
From: SF, CA
You're doing it right. lol. That's Porsche Vario-Cam doing it's job! I'm actually surprised a stock GTS with stock cats will pop and crackle.
I heard and felt it on a stock 991.1 TT which likely have very aggressive cam tuning. It feels amazing when you lift off and it cracks along with hearing the blow-off valve divert the air, plus a unsettling fluttering from the excess spooled air in the intake system (but that's for another thread!).
Backfire you're hearing is the excess combustion process in your exhaust system due to running rich and/or aggressively tuned overlapping cams (for performance reasons). Adjustments made by the car's engine management unit to use more fuel during high throttle applications and adjust cam timing causes incomplete combustion. Combustion isn’t complete before the exhaust part of the engine cycle. When the exhaust valve opens on that cylinder, the extra air allows the unburned fuel to explosively burn and the popping noise of a backfire is heard through the entire exhaust system. If you run catless, you'll at this point see flames also shoot out.
It's amazing, have you attended a Weathertech/Tudor IMSA or a PWC race event before? The backfire and crackle on those racecars = orgasm.
I heard and felt it on a stock 991.1 TT which likely have very aggressive cam tuning. It feels amazing when you lift off and it cracks along with hearing the blow-off valve divert the air, plus a unsettling fluttering from the excess spooled air in the intake system (but that's for another thread!).
Backfire you're hearing is the excess combustion process in your exhaust system due to running rich and/or aggressively tuned overlapping cams (for performance reasons). Adjustments made by the car's engine management unit to use more fuel during high throttle applications and adjust cam timing causes incomplete combustion. Combustion isn’t complete before the exhaust part of the engine cycle. When the exhaust valve opens on that cylinder, the extra air allows the unburned fuel to explosively burn and the popping noise of a backfire is heard through the entire exhaust system. If you run catless, you'll at this point see flames also shoot out.
It's amazing, have you attended a Weathertech/Tudor IMSA or a PWC race event before? The backfire and crackle on those racecars = orgasm.
Last edited by mdosu; Aug 23, 2016 at 08:37 PM.
Instructor
Joined: Aug 2013
Posts: 211
Likes: 63
From: SF, CA
Again, Sport Button probably adjusts throttle sensitivity, which might adjust cam timing, or at least create situations where overlapping exhaust cams open earlier.
It's not a exhaust piping thing, which adjust valves in the muffler area of the exhaust, that's what gets louder exhaust noises.
It's not a exhaust piping thing, which adjust valves in the muffler area of the exhaust, that's what gets louder exhaust noises.
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Instructor
Joined: Aug 2013
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From: SF, CA
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Instructor
Joined: Aug 2013
Posts: 211
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From: SF, CA
Don't take this as an insult, but here's a really good simple visual and explanation how cams work and also how overlapping cams work.
It's the overlap (earlier, quicker opening) of exhaust cams that's creating the backfire you're hearing. But again this what Vario-Cam is designed to do. Because Racecar.
http://auto.howstuffworks.com/camshaft.htm
Now you can tell your wife and friends all about it as you will find it all very fascinating.
It's the overlap (earlier, quicker opening) of exhaust cams that's creating the backfire you're hearing. But again this what Vario-Cam is designed to do. Because Racecar.
http://auto.howstuffworks.com/camshaft.htm
Now you can tell your wife and friends all about it as you will find it all very fascinating.
Three Wheelin'
Joined: Jul 2009
Posts: 1,469
Likes: 37
Yes, the "sport-plus" button of our cars is a kind of "manual" version of the VW software : when you are in Normal, ..you pass the emission control, but when you are on Sport-Plus, the emissions are "different" and ..the MPG too !
ROTFL, big time !
ROTFL, big time !
Pro
Joined: Mar 2002
Posts: 586
Likes: 2
From: Boston
Good explanations here. I love the exhaust overrun on downshifts, whether in Sport Plus, Sport or Normal. When I had my 991.1 TTS on the track the exhaust overrun was noticeable (Sport Plus), but does not compare to the PGTS or the 991 GT3. I'll stick to NA Porsche engines as long as possible. FWIW, it is sounding from press reports of exchanges with Porsche engine people that the next PGTS will unfortunately be turbo charged. No doubt the sound engineers tune the exhaust note as much as possible and still remain compliant, but NA sound and torque curve are what I prefer. Not as fast as turbo models, but on roads quite fast enough.
Banned
Joined: Jan 2013
Posts: 26,140
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From: Chicagoland Area
Lots of this with the 991:
https://rennlist.com/forums/991/9500...d-farting.html
Instructor
Joined: Feb 2007
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From: Bay Area, CA
Intermediate
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