I have a question about engines

"matt" (mattisnthererightnow) 09/22/2013 at 12:16 • Filed to: None

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Why do engines start popping under deceleration? I know that in turbocharged cars this could be some type of anti-lag, but what is it in N/A cars? Is it the same as backfire?

desertdog5051 > matt 09/22/2013 at 12:18

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Unburned fuel in the exhaust header.

Victorious Secret > matt 09/22/2013 at 12:19

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Overrun in the exhaust.

TheBloody, Oppositelock lives on in our shitposts. > matt 09/22/2013 at 12:22

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What they said.

GhostZ > matt 09/22/2013 at 12:24

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*Deceleration

From what I've gathered, when you slow down, the sudden reduction in speed on the engine can cause a sudden backup of hot exhaust gases within the exhaust manifold that, if the spark is advanced enough (on race fuel, the spark fires very late, so fuel is hotter and more ready to combust) it is possible for the combustion to not be fully done (especially if you're running rich) so that by the time the exhaust valve opens, the combustion causes lots of high pressure exhaust, air, and unburned fuel that can ignite with that hot, high pressure exhaust, sending the explosion down the tailpipe.

When a car isn't decelerating, this doesn't happen because the exhaust is being expelled just as quickly from the manifold as the fuel is being burned, or so I've gathered. It is possible to make a car backfire at pretty much any instant though if you have the right fuel settings.

Take it with a grain of salt though, I'm not a mechanic.

matt > GhostZ 09/22/2013 at 12:32

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Thank you for that and the spelling mistake is from my phone thanks for pointing it out.

Dogapult > matt 09/22/2013 at 14:27

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how do I make an older car STOP doing this? timing appears dead on, valves seem to be adjusted, plugs....well, 3 need to be changed because they wouldn't freakin' come out no matter what we did.....

Dualpoints > matt 09/22/2013 at 16:30

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When people hear a loud bang coming from a car they say "It's a backfire." But nowadays it seldom is. A backfire is uncontrolled combustion in the intake tract, not the exhaust. A backfire happens when raw gasoline or gas/air mixture explodes somewhere before the intake valves, like just after the carburetor or in the intake manifold. I think it's most likely to happen when first starting a car, before the intake airflow is well established and the intake tract is cold. Air cleaners used to have flame traps to keep backfires from becoming full blown underhood conflagrations. I have no idea if they do still, but I suspect that fuel injection has made backfires a thing of the past for new cars.