My gut feeling is that much/most of the “work” is probably done in the first 45° or so of crank rotation. So at high revs, there is probably more to gain by keeping the port open “long enough” than is lost by reducing the number of degrees of crank rotation where work can be done.
Flame speed of around 90’/sec (1,080″/sec) means that it takes less than a millisecond to burn all the mixture in the combustion chamber, and probably 1/2 a millisecond to burn most/all the mixture in the dome.
At 12,000 rpm (a convenient number to assign for peak horsepower, it takes about 5 milliseconds for a complete turn of the crank. Ignition probably happens around 22°-30° BTDC (depending on engine/tuning), so it’s a safe bet that the mixture has all burned by 45° or 50° ATDC. (and “most” of the mixture has likely been burned by 10° ATDC).
So in a nutshell, as the revs go up, the *time* between the port events (transfer opening to exhaust closing, which is probably the most important) gets shorter. Raising the exhaust brings that into a better state of tune.
Looking at this just as a “port height” thing is probably not ideal though, since the exhaust pipe, crankcase volume, and many other things enter into it. I’ve seen engines with low exhaust ports rev insanely high, and I’ve seen engines with high exhaust ports put out monster peak torque numbers at “typical” revs. Yes… very generally speaking, (in karting) we’ll see higher exhaust (and often more blowdown) in a high-revving engine (one that puts out peak horsepower at higher revs), though there are many ways to get there.
This reply was modified 1 year, 2 months ago by Pete Muller.