I have been asked a very intelligent question about how the exact timing dimensions are found for the exhaust, transfers & boost ports by a manufacturer of hundereds of engines. I can assure you that a "degree wheel" is NEVER used to determine or check any timing point in any manufactured or custom built engine's cylinder!
Jim Allen
What #'s should we be shooting for Ex,Boost,Side's,and finger port's. Does it differ from rig to mono...21's to 101's. What is simple,Mild and WILD.
There is no magic number for ideal port durations as you need to look at the entire picture of intake tract length, intake duration, the volume and shape of the internal flow paths in the engine, the width of the ports, the compression ratio and the tuned pipe shape and length.
I think you'll find that with modern engines that flow fairly well and run a tuned pipe that an inlet/exhaust duration of 120/180 to 126/186 is going to make pretty solid power across the board, but reasonable to tune and can rev as much as you want with an appropriate pipe and it doesn't matter if you're talking 3.5cc or 500 cc.
Some people like to run much higher durations, and those engines can really scream, but they can also struggle to pull a prop off the beach, can be hard to tune, can be hard to drive and can be less likely to finish a race. IMHO there is more power to be made or lost in the needle valve, pipe and combustion chamber than going to radical port timing and a solid tractable engine that finishes races all season in various water and atmospheric conditions is faster than the wonder engine that makes crazy power one race a season.