another custom exhaust

Michael L. /

Well I got bored again, so seeing as I'm not entirely happy with the way my previous exhaust project turned out, I'm going to build another one. I went to the Harold Washington branch of the Chicago Public Library and picked up a copy of Gordon P. Blair's 'The Basic Design of Two-Stroke Engines.' This book rocks.. it's got everything I've ever wanted to know about two-stroke engines.. for my last pipe I used that JavaPipe thing, but I think that guy designed it for RC airplane and car engines.. I'm going to try designing one using the equations from Blair's book and doing some more precise measurements of my moped's engine than I previously did. Hopefully it should turn out better (and look better now that I weld MUCH better than I used to). I'm thinking about making a small website on this project.. I got a question though. What I seem to understand is that a larger middle pipe diameter (the largest part of the exhaust) is better for narrow power bands, whereas a smaller one is better for broader power bands. I need a broad band since I'm only 1-speed. I was wondering if any of you people with bi-turbos could tell me the middle diameter was..

Re: another custom exhaust

Just so you know... I thought your target RPM on your first one was way too low... you said it was 6500 rpm ??

You should be more like 8000 or 8500 with a small bore like that. (even that is fairly conservative).

Oh... and did you know some go-kart race motors use a sliding mid-section for better power at both low and top?

Do a search for the Yamaha KT-100 motor.

Re: another custom exhaust

Michael L. /

yeah the "trombone slide" exhaust was mentioned in that book, and I was actually thinking of that before but decided against it because it would be too much of a pain. I'm kindof limited with the exhaust I can build with this moped because on variable speed Peugeots, the entire engine (and exhaust with it) tilts forward as the front pully enlarges.

And RPM is the thing I'm not sure of..What rpm should I optimize this for? The reason I chose such a low one for the last pipe was that I was trying to get some better low end power out of it. Right now its top RPM is about 5000-6000 or so. Should I try as high as 8000? Higher? or should I stick with something around 5000.

Re: another custom exhaust

I say design it for 8000+ rpm to start with... but!!... maybe you could make a temporary slip joint in the center section and try 3 or 4 diff lengths and try all of them till you decide which you like best... then cut and weld it up permanent.?

Most of the books are for high output motorcycle and snowmobile motors where they don't need good low end... and RPM's are the maximum safe RPM (for the rod and crank and piston)... if you did that with modern 2 stroke tech...you could easily rev it to 11,000 rpm.

But.. you don't have a modern design, or a multi speed gearbox... so if you did your first for 6500 rpm... I would try 8000 to 8500 rpm... then mill the head .025" to get back some low end.

Re: another custom exhaust

Do you know if the standard exhaust on a Puch Maxi can be altered to increase performance? Not sure I would want to weld, but cutting and brazing are within my shop capabilities.

Re: another custom exhaust

Michael L. /

i dont know an answer to your question, but i dont think brazing would work too well considering the temperatures involved (somewhere around 500C). braze welds are usually only strong up to a couple hundred degrees max unless you use silver alloys as filler metal (which is WAY expensive). if you can braze (i assume oxy-acetylene) you can probably figure out how to weld. use the same tip you'd use for brazing and goto a weld shop or a home depot or something and get a few copper coated steel filler sticks. just melt the metal and dip the filler in whenever the weld pool needs it.

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