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


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>

| 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>

I understand that a catenoid is a surface of revolution of a catenary
curve:
<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.]
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-- 
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
dave.collier-brown@indexexchange.com |              -- Mark Twain

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