What kind of duration numbers do folks like to see when it comes to case pressure?
Depends on size of engine and port shape. Wider ports need less duration.
For 90cc saws I like Intake in the 158-164 area, Exhaust around 156-160, Transfers somewhere near 112-120.
On the other end. For 25cc I like less intake (136-142), less exhaust (144-150) and similar transfer (110-116).
Some of the new Stratos have strange transfer durations and it’s important to note the cold air duration vs fuel mix duration on the intake side. The fresh air may open at the port at very different time than it feeds the transfers. For example I just timed a 550mk2 at fuel mix duration 148, Fresh air at intake 154, piston to transfer 140.
Figuring out what I want with those is specific to my changes and the individual model. Mostly I just leave them alone.
I feel Intake duration is the most significant for case pressure. Width doesn’t matter as much as timing because the real issue is piston position when the intake closes. The earlier the case is filled and the intake closes, the better. I like short intake duration and a wide port.
Transfer height effects pressure, but will still move nearly the same volume, maybe more at longer durations as rpms increase. In general, longer transfer durations seem beneficial regardless of effect on case pressure.
For exhaust a low roof (shorter duration) adds force to the down stroke and add a slight bump in pressure right before the transfers crack. That translates to higher transfer velocity, improved scavenging, and saw with some extra torque. I like shorter exhaust durations with very wide ports. But if there is too little duration the saw will just be a slow grinder. I try to set my exhaust at a happy balance. Usually with slightly more duration than stock.
Every model has its peculiarities. I probably go against my own habits 30% of the time.
…
Not sure why I wrote all that. I get carried away sometimes.
