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Exhaust pressure on over-run (bernoulli's principle?)

  1. Aug 24, 2010 #1
    Hi all,

    I have a car with a straight through exhaust, consisting of a 4 into 1 manifold, a single pipe, then a single silencer. Between the manifold and silencer is a join.
    When i am driving on over run (closed throttle, decellerating) i know this joint sucks air in, as the exhaust makes a popping noise as unburnt fuel meets the air leaking in.

    How do i explain this scientifically? Surely there must be positive pressure in the exhaust due to the engine pumping air into it? I presume this is something to do with bernoulli's principle?


  2. jcsd
  3. Aug 24, 2010 #2


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    Here's a situation to consider that I'm sure we've all (unfortunately) seen.

    Someone is in a moving car, smoking with the window cracked. Where does the smoke go?

    You're correct in attempting to relate this to Bernoulli's equation, which is a form of conservation of energy. The equation states that energy in a fluid flow can be transferred between kinetic (velocity), internal (pressure), and potential (elevation). That is, if a fluid accelerates and energy is transferred to kinetic energy, then either the pressure must decrease, the elevation must decrease, or both.

    In this case, however, during combustion, energy is added to the flow by combustion, then removed by the work the fluid does on the piston. However, I would guess that the pressure is still lower than ambient.
  4. Aug 24, 2010 #3

    Ranger Mike

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    hole or crack in exhaust pipe before the muffler will tick tick tick all the time..popping sound is mist likely a lean mixture condition caused by vacuum leak some where on intake manifold gasket, missing vacuum hose, carb or throttle body gasket leak...could be warped exhaust valve too?? it is not a situation of cold air being sucked into exhaust and causing unburnt fuel to spark...there is such a thing as exhaust reversion of sound wave hitting atmosphere and reversing but wont cause popping,,check for vac leak by using choke cleaner and spray intake maniforld, carb gasket ..if engine speeds up...you found it
  5. Aug 24, 2010 #4
    i've recently had the CO level checked so i know it's not running lean. I guess i am gonig to have to seal the exhaust and see if it makes a difference, if only to confirm or contradict my theory. The popping noise itself is not a problem to me, hence why i have never fixed it - i am just trying to understand the theory here!
  6. Aug 24, 2010 #5

    Ranger Mike

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    some times you can have a over rich fuel condition too
    when yo u accelerate the engine uses the mix up, when you slam the throttle closed cutting off the air unused gas spills over into the exhaust and ignites due to hot spot in exhaust port, ex valve header etc..it pops when it ignites and since you got a non closed condition..leak in the exhaust system..you hear the pop....gasoline has autoignition point of 475 degree F and the ex valve and ports are a lot hotter than this..our race car header has 400 degree F heat on outside of primarys on the headers so inside at the port is about 600 degree F
    Last edited: Aug 24, 2010
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