Single Cylinder 2 stroke bike with twin exhaust, will it need two chambered exhausts?

Question

I am asking specifically about Yezdi (not in production anymore) 250/350cc motorcycles. Yezdi RoadKing was a 250cc single cylinder two stroke with twin exhaust. If upgrading to a calculated chambered exhaust, will it need two chambered exhausts or just one? I was told that for two strokes, only a single exhaust is needed for a single cylinder (should there be multiple cylinders) and that separate exhaust pipes will be required for each cylinders (ie, exhaust gases from more than one cylinder can not go into one exhaust pipe. Hence the number of cylinders and exhaust pipes will be equal).

Answer 1

Two stroke engines, by their very nature, require some of the 'plumbing' you note in your question. Specifically the expansion chamber. Unlike 4-stroke engines, two-stroke engines require the expansion chamber and stinger (reduced diameter over distance) at the end of pipe. The energy in the exhaust wave expands and wave front slows in the expansion chamber and then accelerates as the diameter decreases to the stinger and bounces back toward the front of the pipe to stuff escaping hydrocarbons (unburned fuel) back into the cylinder head.

A Jawa engine is probably installed in your Yezdi Roadking.

Here's the weird part. You have two exhaust ports, if I am correct, coming off the head. You could custom build a pipe with expansion chambers that join together, perhaps. Seems a bit painful. There are aftermarket pipes that are properly calculated and join into a single expansion chamber.

For aesthetics, performance and ease of procurement I would go with two new and independent systems with calculated expansion chambers. They won't 'conflict' with one another and would IMHO look much better than a bastardized expansion chamber thing, which in my mind would be created by trying to combine into a single pipe with expansion chambers.

Answer 2

Considering that you have a single cylinder pushing out exhaust into dual pipes, you wouldn't need to worry so much about equalization efforts. By its very nature, the exhaust is going to find the path of least resistance and therefor will flow to which side it can flow through easier until things are equalized. It appears from what I've seen that the Yezdi had the dual exhaust for aesthetic purposes (id: they made it this way to equalize the look not to equalize the performance). If there were two cylinders, this would be of much more importance. I'm not saying you wouldn't get better performance from a better exhaust system. I'm saying equalization is not a huge factor.