Talk to me about Exhausts

[Doom Patrol]

Without wishing to go into the pros and cons of the latest fancy new expansion pipes is it possible to explain in sort of laymans terms how they generate power? As a general rule of thumb is it true to say that the less restrictive they are the more they push the power up the rev range? Hence the corresponding increase in performance?

I've been thinking about this lately and the desirability of this would seem to depend entirely on what sort of performance you want from your engine. It's a train of thought that began with a brief ride on a scooter fitted with a Casa 186 kit and a standard Tutto Lambretta exhaust. If the above theory holds true then obviously this kit is never going to rev its nuts off. But didn't it pull. So what exactly is happening here? Is the exhaust holding it back, or is it just producing its power lower down in which case could a standard exhaust be said to be as good as a Clubman, for instance?

[GP Kevo]

Without wishing to go into the pros and cons of the latest fancy new expansion pipes is it possible to explain in sort of laymans terms how they generate power? As a general rule of thumb is it true to say that the less restrictive they are the more they push the power up the rev range? Hence the corresponding increase in performance?

No, that's more true of 4 stroke engines.

On a 2 stroke, an expansion exaust changes in diameter and angled cones to achieve these at specific places along the length of the exhaust. The cones reflect sound pressure waves back toward the exhaust port back where they come from, and the timing of these sound pressure waves (or pulses) is critical in order to stuff unburned, fresh mixture that is escaping from the combustion chamber out the exhaust port. That timing is achieved by the angle of the cones, where they are along the length of the exhaust, and the total tuned length of the exhaust (from exhaust port to end of stinger, if I'm not mistaken). An exhauset designer can decide at what rpms the exhaust works best. My knowledge on this subject is very basic and others here can explain this in far more detail and with much more accuracy. There are some books that explain this subject very well and some of them are available here http://scooterotica.org/forum/viewtopic.php?f=5&t=8

drunkenmonkey has much better description of basic 2 stroke operation and tuning that he has compiled here: http://scooterotica.org/forum/viewtopic.php?f=5&t=10

Go here for a cogent explanation with drawings and this animated gif:
http://www.vf750fd.com/blurbs/stroke.html

Queen's University Belfast did much of the research on 2 stroke tuning and especially expansion chamber theory.

[Doom Patrol]

Thanks for that Kevo. That animation is mesmerising. You're right. It doesn't work quite how I had anticipated. People always state that a Clubman will allow you to rev on beyond a standard exhaust and create more power and that was my interpretation of it. So, going back to the differences between a standard exhaust and a Clubman, for instance. Is it right to say that a standard one will produce more torque?

[GP Kevo]

Well, there are clubmans and then there are clubmans. Some are very good. You'll notice that there is an expansion cone welded into them, and then there is the box, then where the tailpipe exits the standard shape of the exhaust will reflect pressure waves back but not as efficiently as a good expansion exhaust does. There are also clubmans with rear cones welded in and these are more like expansion exhausts, really. Most clubmans produce good low down torque and they rev on more than a standard exhaust and they make more power. On a 2 stroke engine, the exhaust affects the performance more than any other single component, i.e. the same engine will be like a completely different engine with a different exhaust. I found that out going from a ScootRS exhaust to a JL 4 on my TS1 Humiliator. The difference was like night and day.

You have to ask yourself what do you want your bike to do and how you prefer to ride it.