Exhaust Step

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Dec 16, 2003
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#1
I have noticed a step or mismatch that exists between a cylinder exhaust opening and exhaust flange. The exhaust flange opening will typically be round and the mating opening in the cylinder will be "D" shaped. It seems to be rather deliberate and I have witnessed it on a CR125, TM125 and a RS125. I found a reference to this "step" here. http://www.bonavolta.ch/hobby/en/moto/rs125r.htm ---1/3rd down the page.---

Q: What is the purpose of this step?

Thanks,
Koryo
 

EricGorr

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Aug 24, 2000
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#2
I believe that "step" has two purposes. 1) For out-flow to maintain a range of exhaust gas velocity. 2) For back-flow to shape the plugging pulse or perhaps give some reversion benefit.

I know from practise that if the shape of the exhaust port at the manifold junction is either grinded larger or welded smaller it has a dramatic effect on the powerband. Enlarging the area helps the overrev but sacrifices low end throttle response.

In the past I have added a weldment to the exhaust spigot of the cylinder on TM250 motorcycles that were intended for enduro use. In fact Phil McDowell tested that cylinder and actually modeled it's effect on performance with a program called Motra.

There was an interesting SAE tech paper penned by the two-stroke gurus at the Queen's University in Belfast where they measured the flow coefficent of exhaust spigot to pipe junction in both directions. Over the years there have been many design iterations ranging from the classis "waterfall" exhaust port to the lastest rage of oval exhaust port ducts with minor flat edges on the top as used on late model Japanese and Italian 125cc singles.
 

Elly

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May 17, 2004
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#3
I've been wondering this step thing lately too. Looked new CR125 and there's the step too.

Now my question: Which is better option, leave the step or make a flange that smoothly shapes the oval port side to round pipe side?