Originally Posted by CosRush
My understanding is that the Turbo cam profile
does not need as much lift and overlap as its N/A counterpart due to the increased cylinder filling caused by the induction being above atmospheric pressure.
Therefore for any given valve opening in basic comparison, the Turbo will flow more than the N/A.
But there must be alot more to it than that.........
It's not doesnt need... it can not have.
Why?
Because at low speed, when gas speed is low, if we open the valves for a great deal of time we'll have gas reversion (gas going out of cylinder through the inlet, going into the cylinder through the exhaust).
This problem in the turbo'ed engines is the low CR's used. If we have a lot of gas reversion and low CR, we'll have very low power at low engine speed. that's why competition engines (normally) have a very high idle (2000-3000 rpm instead of 800-900 of normal engine).
On a NA engine, with an "aggressive cam", we normally also have very high CR (11-13:1). this makes the NA power band wider, and lower idle
That's why we now have i-vtec's, vvti's and the like. we can vary the cam duration and lift with the rpm to ensure minimum reversions, maximum cylinder fill and consequently torque and performance through the entire rpm range, low idle and emissions, much wider power band.