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Old 11-05-2014, 03:36 PM
TomSimon TomSimon is offline
Motorized Bicycle Newbie
Join Date: Oct 2014
Location: Brentwood
Posts: 4
Default Re: Expansion chamber header pipe angle question

Back when I was racing 100cc Karts, it was pretty common knowledge (and worked in practice) to change the expansion chamber's straight section length, to tune the engine for peak power at optimum rpm range, for a given track, altitude, and to a lesser extent, weather conditions.

If fact, all the fast guys carried various lengths of straight section in our spare parts boxes for use on race day. I found that a certain pipe (expansion chamber) would have an effective rpm range in which it gave that particular engine a boost. The rpm range in which it worked, or boosted, was pretty constant. It's been 20 years, but I remember that range being about 3,000 - 4,000 rpm on the 100cc 2-stroke Yamaha engines I used.

That length between the piston and beginning of the first expansion cone made all the difference in the world. The difference was not peak power, rather, when peak power happened withing the rpm range. We used centrifugal clutches, and you needed to adjust the rpm engagement of the clutch to work in concert with the pipe's rpm range... it was a lot of trial and error, tuning, note taking, stopwatch anf lap times, and 'seat-of-the-pants' engineering.

All things being equal, if you increased the length of the straight header section, it had the effect of lowering the place in the rpm range where the expansion chamber would 'kick in'. To compliment this effect, you would tighten up the clutch springs, lowering the clutch engagement rpm as well. Conversely, if you shortened the straight section, the 'pipe' kicked in at a higher rpm.

I appreciate having to add pipe to make it fit, but know that by doing so, you are changing the engine's 'optimum rpm power range'. You could make it so long, that it would kick in at 16,000rpm, an rpm your engine may never reach.

I'm not expert on the subject of exhaust tuning, just a student, trying to pass on a little experience... good luck :-)
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