Porting for mid range RPM


New Member
Local time
10:42 AM
Sep 21, 2023
Hey guys, I was wondering whether it's worth modifying port lengths if you're not specifically looking for either high or low rpms

From my understanding, porting is all about shifting the powerband curve

your engine will always have a set amount of available power

with porting, you're robbing low to play high or robbing high to play low

is my understanding of this correct? And if I'm looking for mid range power, should I leave the port lengths alone on my engine? (which are ported for 6000rpm stock)
Porting is basically done to get more air flow into the cylinder by straightening the path. The tiny bit of material removed from the walls will increase the diameter and technically slow down the air speed. Changing the length of the manifold runner is what changes the resonate signal and the power curve. The intake does not deal with back pressure like the exhaust with it's expansion chamber. Porting is always a good thing if you know what you're doing and willing to uinvest the time. An incorrectly done porting job can actuall rob you of H.P.

To answer your question......... I'd advise to leave it alone. I would expect the designer chose the components and specs to be most usefull in the middle rpm range. That's where everyone will be using it most all of the time. Certainly not at low rpms and how often does anyone run max. rpms, unless it's a race engine. If that was the case that engine would be perty useless except at W.O.T.
My 2c about porting - In the case of my BT100, widening the exhaust port but not raising it any will increase mid range torque. Raising it 1mm gained about 0.8HP in the mid range but lost about 0.2HP down lower.

This same porting on my 66cc G4 jug didn't make any gains, just lost HP and torque.

I think you should degree your engine first to see where the stock intake/exhaust/transfer timings are at, compare these numbers with typical timings of a stock or lightly ported jug. Adjust your timings to match.
"Porting" has always referred to straightening or reshaping the actural runner, whereas any modification to the cylinder wall opening should be referred to as "port timing". In these discussions I don't feel they are interchangable.