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Cause of "put put " sound during deceleration on race cars?

Somebody was driving a race car down the street today while I was taking a walk. I think it was a McLaren, but not sure. It had a lot of stuff tacked on on to it, horizontal rear fins and the like, and a lot of graphics, that’s for sure. I guess the owner wanted to be noticed. And heard, it was pretty loud on acceleration. On deceleration it was loud too, but in a much more annoying way. It made this very loud “put put” sound — sort of like firecrackers going off – whenever the driver slowed down, even just a little. I’ve heard this sound coming from race cars when the tv sports is showing an auto race too. But there’s no corresponding sound coming from the local econoboxes driving down the street. Anybody know the cause of that loud “put put” exhaust sound when a race car decelerates?

Excess fuel in the exhaust stream when the throttle is closed lights off as it exits the exhaust. Wild cam timing helps that as well. It is a small series of backfires. Older carbureted cars did that a bunch.

Normal street cars have better fuel and milder cam contol and restrictive exhaust so they don’t do that.


Do you mean the % of the cycle the exhaust and intake valve are open at the same time is more in race cars than econoboxes?

There is definitely more “overlap” with a hotter cam.

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Just to add, also in interest of fuel efficiency, engine braking is minimized and cars tend to/are designed to coast more now when you let off the gas.

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Did you see license plates? Maybe there weren’t any. It might have been a real race car that the owner/crew chief took out for a quick spin to check that it was running properly. The shop might be close by.

My stock ‘83 GTI would do some of that on deceleration, not very loud, I liked it.

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Definitely Yes! The cam opens the valves early and closes them later that theory would suggest until you add resonances into the equation! Intake and exhaust are open at the same time for just a shot few degrees of rotation.

So the engine is a bit lazy at low speeds until the resonances of the intake and exhaust match the engine speed. More RPM means more power so race cams open valves really early, close really late and so there is a large period where both intake and exhaust are open at the same time - overlap. The resonances are also built to exist at much higher engine speeds for that reason - shorter intake tubes, shorter exhaust header pipes.

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Could also be the anti lag. Some cars dump fuel into the exhaust and ignite it there to keep the exhaust gas hot and the turbo spooled up so that the turbo is ready when you get back on the power


I thought about that. Also the CART series “Pop-Off” valves which limited turbo boost to help equalize engine power between the teams. I attended several races of the now defunct series in the 1980s/90s and remember the sound being not as loud or sharp as valve overlap backfires.

Perhaps. I didn’t look for plates, but the lack of license plates wouldn’t prove anything. Displaying license plates is optional for car owners in these parts of California. 10-15% of the cars on the road have no identifying license plates.

It’s just a little excess fuel that made it into the exhaust system cooking off on the overrun. My Mustang does it sometimes if I let off the throttle at higher RPMs. It started doing it after it was tuned. I like it.

So in California no plates = no problem, but if you change out the muffler, it’s an instant $1000 fine?


The old Harley Davidsons could be made to do this. The right hand grip was the throttle and the left hand is the distributor retard to aid in starting.

At highway speeds one could roll the left hand grip forward and back off the right hand throttle. The result is a 4 foot blue/white flame out of the exhaust with a bang to boot.

On turbo charged engines, it would be the pop off valve that prevents the intake duct between the turbo and and the throttle body from over pressurizing when the throttle is closed.

Apparently, by way of the evidence rolling down the roads. It’s more ironic that that, believe it or not. I got stopped one night for not having a working license plate light bulb. Seriously! Nobody’s getting stopped for no license plates at all, but if your license plate light bulb is burned out you’re a marked criminal in Calif … lol …