swiss are you the swiss cheese exhaust guru?
I've messed around with exhaust tuning for a few years. As someone said one time in the past, design the perfect exhaust system and the hardest thing to do will be to make it fit!
You always need to include the port length as part of the tuned length. Using an Anti-Reversion design (A-R)(like a Power-bomb, Mega-bomb, Yamaha chamber etc..) will change how a system tunes depending on where in the tuned length the A-R design is used. There is no "perfect" formula for tuning exhaust, variables like port restrictions, pipe bends and those stupid things called "mufflers" always throw undesired problems into the results. The numbers will get you close and then some track or dyno testing will put it where you want it!
The pipe in the sketch will not peak anywhere close to 4,000rpms because the primary pipe length is only 26" + about 3" for the port length or roughly 29" long. The primary pipe length is what tunes for the peak power rpm. The megaphone does not tune in the same manner that the straight pipe does. The straight pipe tunes at its exit/end which is the 29" length. The megaphone actually tunes along its entire length with reverse waves being generated constantly as the sound wave expands. So it in effect tunes for the length/time that the sound wave travels along it. Megaphonitis typically tunes like a straight pipe but much more intense. It is when you use the megaphone without the reverse cone. Nice peak in the powerband but ONLY at a very narrow rpm range. The reverse cone works with the megaphone to spread the tuning BELOW the peak rpm tuned length of the primary pipe. If your primary length is 7,000rpms then the added megaphone will tune below that 7,000 point. The width of the tuning effect of the megaphone is determined by the length of the megaphone. The intensity of the megaphone effect is determined by the angle of the meg. cone. The larger the angle the stronger the "boost", within limits. Too strong an angle and the flow exhaust flow is separated from the meg. wall and creates turbulence which impedes the exhaust gas flow (effectively reducing the area of the cone).
Longer primary pipes, like we see on the typical MX bike these days are tuned for a "lesser" harmonic. They are still tuned but not for as strong a peak power effect from the pipe. They mostly typically use a straight pipe design which dumps into the muffler so their tuning is restricted to basically one rpm +/- 1,000rpms. Different Engineers, designers and Scientists have varying ideas on the actual width of the "powerband" effects of straight pipe tuning, typically varying from 1,000rpms to about 2,500rpms. The different attached cans and chambers that we see in today's performance exhaust systems help to widen the powerband by eliminating Reversion causing problems in the combustion chamber. Basically A-R designs similar to a "Helmholtz" effect.
It is hard to use "rule of the thumb" tuning for race engines that vary in designs from low rpm Torque power to high rpm HP power. Some of the differences in formulas for pipe designs are because of the differing ideas of either tuning for the Torque peak or the Horsepower peak. The Torque peak of course is at a much lower rpm than the horsepower peak. By tuning for the T-P it will give a strong mid-range but tend to run out of strong power as the rpms climb. Tuning for the H-P will give a nice top end rpm boost if the engine is put together well enough to handle it. I have seen many dyno run results where the engine peaks and then falls off as it nears max-rpm/redline. Some engines have better high rpm breathing and will still be building HP as it hits redline. Simple dyno runs can help you determine where to place the tuning rpm for your exhaust system. Rules of thumb only work for "spec" engines that are all built the same and typically the "rules" are based on trial and error used to determine the "correct" dimensions for that specific engine combination! Change the cam or the carb and everything else changes as well!
Sorry to have rambled on mates! SlideRulz, your dad had some good notes, probably worked out either by trial and error on the track or on the dyno if he had access.
Swiss
