Overlap and Reversion?

Who can explain these two phenomena in layman's terms? Are they related, what's ideal for what, ect. Have a good day fellas. I'm just sick of seeing that Tecumseh work on the front of this thread. Lol.
 
Overlap is the distance in crank rotation that both valves are open at the same time.
This is used to have the velocity of the gasses leaving the chamber thru the exhaust help get the intake flow moving.

Reversion is simply the reversal of flow.

The exhaust pulses set up pressure waves which travel in both directions.
If the exhaust valve is open when a wave arrives at the chamber, the flow reverses back up the intake tract.

Usually see this as fuel spray out of the carb mouth.

The velocity of the waves in the exhaust is more temp related, than rpm related.

It's a speed related phenomenon. Less rpm, more time for this to happen. Same with shorter pipe.

Cams with more overlap are usually meant for higher rpm.
 
That explained it pretty well right there.

With that in mind, you can change the overlap some by adjusting the valve lash.
More lash will decrease the overlap by opening the valves later and closing them sooner. This will move the power band down in rpm. It's become a great tuning device on spec engines like the LO206 where you can't change the cam itself.


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Brian Carlson
Carlson Racing Engines
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