In a racing two-stroke, the exhaust starts out as a simple pipe, bells out at a certain rate, continues as a a much larger pipe, then narrows again to quit a small diameter, somewhat like this illustration [cid:part1.7eRQwO40.k4xp302Y@indexexchange.com] The diffuser cone should actually be a catenoid, with a diameter and length based on the RPM range you're after. I used to make them in three parts, a long shallow cone, a short steeper cone and a quite short cone taking me to the full size of the main "expansion" chamber. I fed the rpm in and calculated the endpoint of the first cone, rolled it out of cardboard and did the rest by eye with a pair of scissors and tape, so as to fit on the bike without burning the rider. I usually had to wiggle the pipes a lot on trail and motocross bikes, so I was always recalculating what RPM I would peak at if I made it a little shorter or longer. There actually were a bunch of calculations, and they were in the front pages of a workbook where I recorded my various experiments and how well they worked. Only one got used constantly, and got scratched into my "calculator". --dave On 6/2/23 10:32, D. Hugh Redelmeier via talk wrote: | From: Dave Collier-Brown via talk <talk@gtalug.org><mailto:talk@gtalug.org> | When I was a motorcycle mechanic I had a circular slide rule, with a | permanent mark at the coefficient for computing a catenoid, as I did a | lot of 2-stroke exhaust systems. Some hilariously wrong, some which got | me a reputation as a wizard. I know what a catenary curve is. The standard example is of a chain hanging between two posts (that's where the name comes from). Or a hydro wire. <https://en.wikipedia.org/wiki/Catenary><https://en.wikipedia.org/wiki/Catenary> I understand that a catenoid is a surface of revolution of a catenary curve: <https://en.wikipedia.org/wiki/Catenary><https://en.wikipedia.org/wiki/Catenary> I don't know what the coefficient for computing it would be. I don't even know what the inputs to this computation would be. How is this applied to 2-stroke engine exhaust systems? Is this related to back pressure or resonance? Maybe you are a wizard. [I hate 2-stroke engines because they pollute so much. I'm only exposed to them running lawn maintenance equipment.] --- Post to this mailing list talk@gtalug.org<mailto:talk@gtalug.org> Unsubscribe from this mailing list https://gtalug.org/mailman/listinfo/talk -- David Collier-Brown, | Always do right. This will gratify System Programmer and Author | some people and astonish the rest dave.collier-brown@indexexchange.com<mailto:dave.collier-brown@indexexchange.com> | -- Mark Twain CONFIDENTIALITY NOTICE AND DISCLAIMER : This telecommunication, including any and all attachments, contains confidential information intended only for the person(s) to whom it is addressed. Any dissemination, distribution, copying or disclosure is strictly prohibited and is not a waiver of confidentiality. If you have received this telecommunication in error, please notify the sender immediately by return electronic mail and delete the message from your inbox and deleted items folders. This telecommunication does not constitute an express or implied agreement to conduct transactions by electronic means, nor does it constitute a contract offer, a contract amendment or an acceptance of a contract offer. Contract terms contained in this telecommunication are subject to legal review and the completion of formal documentation and are not binding until same is confirmed in writing and has been signed by an authorized signatory.
