Quote:
Originally Posted by JonL
A little suction like that probably isn't abnormal. The exhaust valves stay open a little bit past top dead center so that when the engine is running the inertia of the exhaust gas leaving the cylinder continues to scavenge the residual exhaust even as the piston starts its down stroke. At cranking speed the gas velocity isn't high enough to create that scavenging effect, so there will be some slight suction in the exhaust system.
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That is a very imaginative description... but I just can't go with it.
The idea that the slowing down of the exhaust gas due to the cranking speed being slow making a vacuum effect as far back as the end of the exhaust pipe just does not fit my intuition of the physics.
First, those two , or even just one, mufflers would sure mix up the pulses.... a dampening effect which would preclude an actual backwards movement being available at the end of the exhaust pipe... I can see a lessening of the exhaust speed...but not a reversal needed to give this effect... remember that other pistons are sending gas into that confined stream also... even if one could do that the combo of several feeding that tube would surely.
Also, this supposed valve opening which occurs after top dead center... ignores the fact that we are dealing with the two strokes of the crank in that bore which have both valves closed ... Compression AND Power.. so that does not fit either..
I agree with the concept of inertia creating a scavenging effect... but given the whole situation I think slow cranking or idle would only take that away.... as compared to going into the other side of the realm... (of vacuum. )
I think your mental concept of the valve overlap comes from gas engines....where a hot cam can cause that kind of effect at certain rpms...
Nice try though....