Need help understanding intake porting.

[Nicholas]

What effect does lowering the intake port have on the performance of a saw? I understand it would increase the duration the intake port is open. It would allow for more air/fuel mix to fill the crankcase. By lowering the intake one is also advancing the intake timing. Does lowering the intake have a similar effect as raising the exhaust port. Raising the exhaust increases engine rpm, while sacrificing some torque. Lowering the exhaust increases torque, while sacrificing engine rpm.

Thanks,

Nick

 

[drkptt]

Yes, lowering the intake will increase the opening duration and improve high-speed performance. Too much is a bad thing, however.

As the speed increases there is less time for the crankcase to fill, so at high speeds the crankcase pressure at the moment the intake closes is lower than atmospheric. Thus the trapped air (by mass) is less and power is down.

Theoretically the optimum pressure of the charge trapped in the crankcase when the intake closes is atmospheric. If you port for this case at say 10,000 rpm you will lose performance at slower speeds. For example, at 8,000 rpm the intake is open for a longer time so the crankcase can fully fill while the piston is still moving down to close the port. As the piston drops it pushes the air backwards out of the intake. This takes a little power to do (increased pumping loss). In more extreme cases this backflow will push fuel/air charge into the airbox (spitback).