Exhaust Pop
06-11-2010, 07:12 AM
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Exhaust Pop
I can't seem to find a straight answer on this, so thought I'd get HA's opinion.
What causes exhaust "pop'? I notice this especially when I leave my work parking lot, which has a strictly enforced 10MPH limit. I'll be cruising out in 1st gear, and there are three speed bumps. At each speed bump, if I let off the throttle rapidly, I get a single "pop" sound from the exhaust. The sound is NOT a bang, like a traditional backfire.
Some Googling produced a bunch of people who claimed that even though this sounds nothing like a backfire, this is caused by excess unburnt fuel in the exhaust. Have to call BS there.
I *THINK* what happens is something along the lines of the exhaust/intake/engine system being under positive pressure when the throttle is applied. When the throttle is quickly released, the system quickly transitions to a vacuum state. That part seems reasonable. But what exactly makes the pop sound? Is it air rushing back in through the tailpipe? Is it really on the intake side? dunno.gif
What causes exhaust "pop'? I notice this especially when I leave my work parking lot, which has a strictly enforced 10MPH limit. I'll be cruising out in 1st gear, and there are three speed bumps. At each speed bump, if I let off the throttle rapidly, I get a single "pop" sound from the exhaust. The sound is NOT a bang, like a traditional backfire.
Some Googling produced a bunch of people who claimed that even though this sounds nothing like a backfire, this is caused by excess unburnt fuel in the exhaust. Have to call BS there.
I *THINK* what happens is something along the lines of the exhaust/intake/engine system being under positive pressure when the throttle is applied. When the throttle is quickly released, the system quickly transitions to a vacuum state. That part seems reasonable. But what exactly makes the pop sound? Is it air rushing back in through the tailpipe? Is it really on the intake side? dunno.gif
06-11-2010, 08:49 PM
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nope its a backfire, not a traditional old rusty cadillac backfire but its a backfire, when turbo cars downshift they backfire all the time and its just popping that you hear. its normal for any car when your slowing down in gear and usually only happens in a certain rpm range. my car does it all the time.
06-12-2010, 01:51 PM
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What is happening, I think.... Exhaust gas moving down the pipe has pressure, that is over atmospheric. This would mean gas is moving out the pipe and not in. Now.. take and make your exhaust larger than it should be for idling or putzing around at 10mph. The pressure inside the bigger pipe drops to or below atmo and the outside air rushes in and screws with the exhaust gases trying to gtfo. The pop is a result of that conflict. I think.....