Poping sound
06-30-2003, 03:08 PM
#16
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If you have a "blow off valve" installed between the intercooler and the throttle body, it would make the sound the NZ guy mentioned as it blows off under decel in order to spool faster when you get back on it...
Gordon
Gordon
12-13-2006, 08:17 PM
#20
The popping, as stated is caused two things, first and formost overlap from the camshafts. This causes both the intake and exhaust valves to be open for the slightest second. In turn a bit of air is pushed through to the exhaust. This occurs most when your off the throttle because there is not burning taking place in the combustion chamber. The flames exiting the exhaust light this mixture causing a tiny explosion into the exhaust.
Why you do and don't hear it. The stock exhaust has so many baffles (so restrictive) that all noise is suppression. Those with cat-back boxes will hear it, but thats all. Those without catalytic coverters will possibly see this if you don't have much of a muffler either. Your most noticable pops will be upper RPM's, especially if you get a good downshift high in the rpms in a hot engine.
Wes
Why you do and don't hear it. The stock exhaust has so many baffles (so restrictive) that all noise is suppression. Those with cat-back boxes will hear it, but thats all. Those without catalytic coverters will possibly see this if you don't have much of a muffler either. Your most noticable pops will be upper RPM's, especially if you get a good downshift high in the rpms in a hot engine.
Wes
12-13-2006, 08:57 PM
#21
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I am without cat and muffler and get popping all the time when lifting off. Sounds great!
If I am coasting down a hill, say at 3K revs without any throttle it will pop and bang all the way, but if I press the throttle with a feather-light touch it doesn't pop, which proves its the closing of the throttle on revs that does it.
(If i'm feeling self conscious- eg. if near a police car, then just depress the clutch and the noise stops)
btw, I have no flames, although I can't see behind me of course!
If I am coasting down a hill, say at 3K revs without any throttle it will pop and bang all the way, but if I press the throttle with a feather-light touch it doesn't pop, which proves its the closing of the throttle on revs that does it.
(If i'm feeling self conscious- eg. if near a police car, then just depress the clutch and the noise stops)
btw, I have no flames, although I can't see behind me of course!
12-14-2006, 08:32 PM
#25
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Flames, oh.
But I would only want them for special days, not all the time. ;-)
PS: I installed the headers and yes flames when car running hot (on track) and even on road sometimes. Oh, such fun!
But I would only want them for special days, not all the time. ;-)
PS: I installed the headers and yes flames when car running hot (on track) and even on road sometimes. Oh, such fun!
Last edited by 911addict; 09-23-2009 at 05:17 PM. Reason: updated
09-20-2009, 07:52 PM
#27
The owner of a car I'm planning to purchase linked me to this thread when I asked him about the popping I heard from the exhaust on deceleration.
Read through the suggestions and felt I'd clear up some misinformation that's been put forth as fact.
A loud pop from the exhaust after high rpm or high load/boost operation is perfectly normal. The systems that control fuel delivery are metering out large amounts of fuel when the air supply is rapidly shut off (ie. throttle plate closes). Unburnt fuel makes it into the hot exhaust system and is ignited. One or two pops result. Normal.
What's not normal - repeated pops coming from the exhaust system while letting the car decelerate in gear to a stop. Modern fuel delivery systems are designed to cut fuel upon deceleration (ie. TPS reads zero) to improve gas mileage. If you are hearing pops - unburnt fuel combusting in the exhaust system - something in the system is not functioning properly.
Regarding intake/exhaust overlap suggestion - if there isn't any fuel in the intake charge, it's impossible to have fuel igniting in the exhaust system. No fuel in, no fuel out.
Confirmed my suspicions with a respected Porsche tuner (tuner who built the car, actually). A properly sorted engine management system (regardless of whether it's CIS, EFI or the mechanical injection systems on the 70's Alfa Romeo's) does not allow the repeated pops on deceleration.
Similar problems with the Alfas I referenced above were caused by a bad fuse. Suspect there is some similar cause for the Porsche.
Best,
John
Read through the suggestions and felt I'd clear up some misinformation that's been put forth as fact.
A loud pop from the exhaust after high rpm or high load/boost operation is perfectly normal. The systems that control fuel delivery are metering out large amounts of fuel when the air supply is rapidly shut off (ie. throttle plate closes). Unburnt fuel makes it into the hot exhaust system and is ignited. One or two pops result. Normal.
What's not normal - repeated pops coming from the exhaust system while letting the car decelerate in gear to a stop. Modern fuel delivery systems are designed to cut fuel upon deceleration (ie. TPS reads zero) to improve gas mileage. If you are hearing pops - unburnt fuel combusting in the exhaust system - something in the system is not functioning properly.
Regarding intake/exhaust overlap suggestion - if there isn't any fuel in the intake charge, it's impossible to have fuel igniting in the exhaust system. No fuel in, no fuel out.
Confirmed my suspicions with a respected Porsche tuner (tuner who built the car, actually). A properly sorted engine management system (regardless of whether it's CIS, EFI or the mechanical injection systems on the 70's Alfa Romeo's) does not allow the repeated pops on deceleration.
Similar problems with the Alfas I referenced above were caused by a bad fuse. Suspect there is some similar cause for the Porsche.
Best,
John
The popping, as stated is caused two things, first and formost overlap from the camshafts. This causes both the intake and exhaust valves to be open for the slightest second. In turn a bit of air is pushed through to the exhaust. This occurs most when your off the throttle because there is not burning taking place in the combustion chamber. The flames exiting the exhaust light this mixture causing a tiny explosion into the exhaust.
Why you do and don't hear it. The stock exhaust has so many baffles (so restrictive) that all noise is suppression. Those with cat-back boxes will hear it, but thats all. Those without catalytic coverters will possibly see this if you don't have much of a muffler either. Your most noticable pops will be upper RPM's, especially if you get a good downshift high in the rpms in a hot engine.
Wes
Why you do and don't hear it. The stock exhaust has so many baffles (so restrictive) that all noise is suppression. Those with cat-back boxes will hear it, but thats all. Those without catalytic coverters will possibly see this if you don't have much of a muffler either. Your most noticable pops will be upper RPM's, especially if you get a good downshift high in the rpms in a hot engine.
Wes
09-21-2009, 05:04 AM
#28
Racer
CIS = Continuous Injection System.
Those pops are absolutely normal for our CIS engine. Other then EFI, with CIS the fuel delivery is never shut down completely and even off throttle a small amount of fuel is injected steadily and this unburnt fuel ignites in the hot exhaust which causes the popping sound.
09-23-2009, 05:16 PM
#29
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This would mean that all of our cars are not functioning properly!
CIS = Continuous Injection System.
Those pops are absolutely normal for our CIS engine. Other then EFI, with CIS the fuel delivery is never shut down completely and even off throttle a small amount of fuel is injected steadily and this unburnt fuel ignites in the hot exhaust which causes the popping sound.
CIS = Continuous Injection System.
Those pops are absolutely normal for our CIS engine. Other then EFI, with CIS the fuel delivery is never shut down completely and even off throttle a small amount of fuel is injected steadily and this unburnt fuel ignites in the hot exhaust which causes the popping sound.
09-23-2009, 05:51 PM
#30
Rennlist Member
