MOPED ARMY

do the math yourself. then you can change things kinda easier. just use jennings stuff, it's pretty easy.

I looked at the diffuser angle with my protractor, it's a three stage, gut it goes out at approximately a 7 degree angle, this is what is suggested for a pipe with a pretty wide power band. I guess its worth a shot. It seems like all the math is right, it just doesn't look like anything I have ever seen before.

Yeah the thing is most moped pipes just dont have a wide power band. And when you try to get a wide power band like that, you are sacrificing top speed.

Personally, I would like a pipe with a wider power band. As long as I get a couple mph on top end Im happy. I dont want my motor to rev that much, and when an expansion pipe is giving you a higher top speed, it is because it is making it rev higher. And higher revs generally means less reliability.

I'm not really sure what to think now, because when I look at the puch exhausts that are listed in the wiki they arn't even close to the proportions that bell or jennings describe. take the tecnigas next for example, the header, which is supposed to be no more that 11 times the width of the exhaust port long is definitely longer than that. the diffuser also opens up at way more than 10 degrees which is supposed to be the max and I'm pretty sure that its shorter than it's supposed to be following the formula for tuned length, exhaust port duration x wave speed (1700fps) / rpms, but that is debateable.

One thing you have to realize when useing a program is. That it is designed for a much larger engine, usually attached to a dirtbike. With the larger engine there isn't enough space to make an optimal pipe. Thats why you end up with a very skinny pipe after starting with a very small exhaust port. The idea about looking at the wiki is a good one. Another option is to look at what kart racers are running on the one speed 100cc yamaha motors. That would be close to what you should be comparing your motor to. These motors need a very wide powerband and extreme top end. Quite a task.

I read the Expansion chamber design section in Jenner's book. I don't think that it seems any more modern than the program that I designed the chamber on automatically. It says right in there that you should expand the pipe at about 8 degrees for maximum power recovery, even though most of those pipes in the puch section expand at way more than that. I don't know what to do.

Will someone please give be some measurements then? I'm afraid to do the picture thing because being off by an inch in length will move the RPM peak up like 500 RPMS, so the little mistakes can really add up.

the jennings stuff is wayyyy out dated, but using the Blair formulae, which i think bell also uses, you should take into account exhaust port area, engine displacement, exhaust temp, 'optimized' power band, distance from port to flange, exhaust port duration... i think thats everything, maybe crankcase compression ratio as well.

jennings uses a primitive 'fudge factor' approach which just estimates a pipe that will probably work reasonably well. the work done by blair uses thermodynamic simulation and should yield better results.