quote:Originally posted by 4V & Proud:
reversion is a product of valve overlap, when the exhaust valve is closing and the intake valve begins to open there is a pathway from the top of the air inlet to the tail pipe
That only happens at idle on engines with lots of overlap. It is the cause of the rough idle in those engines.
At anything past idle precisely the opposite occurs. Low header pressure allows the cylinder to exhaust quickly, the momentum of the outflowing gas lowers cylinder pressure to below intake manifold pressure and the inflowing intake charge scavenges residual combustion gases out of the cylinder. Sometimes fresh intake charge can even go out into the header pipes.
Scavenging out residual combustion gases is exactly what overlap is for and is why it makes the engine more powerful.
What that article says on reversion is twaddle. The author doesn't know what he is talking about.