Exhaust Port Matching - rectangular flange to round pipe

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Exhaust port matching

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The "china girl" cylinders all seem to have rectangular exhaust ports, which when bolted up to a round exhaust pipe flange leaves a restrictive step in the exhaust path. In four-stroke practice this is a pretty bad thing, causing exhaust gas reversion (back-wave) right at the port. Seem like it would be even worse on a two-stroke, but I can't find anyone making port-matched exhaust header or even talking about this potentially big problem.

Here's one part I found for motorized scooters that looks like what I think is needed. Notice how the header piece has an adapter welded on that transitions from the square flange opening to the round pipe. Anyone tried this on a bike motor?
 
there are cylinders available around with a round exhaust port. ive seen them on mzmiami's site. cheap too. but I don't know how their transfer timing is...
 
The "china girl" cylinders all seem to have rectangular exhaust ports, which when bolted up to a round exhaust pipe flange leaves a restrictive step in the exhaust path. In four-stroke practice this is a pretty bad thing, causing exhaust gas reversion (back-wave) right at the port. Seem like it would be even worse on a two-stroke, but I can't find anyone making port-matched exhaust header or even talking about this potentially big problem.

Here's one part I found for motorized scooters that looks like what I think is needed. Notice how the header piece has an adapter welded on that transitions from the square flange opening to the round pipe. Anyone tried this on a bike motor?
I guess no photos because you're not logged in?
Adding a block between the cylinder and the flange would move the whole muffler forward and down which may or may not be desirable. On my build it would be undesirable.

I was porting the cylinder anyway, since it really needs to be ported to release the potential HP.


While widening the top of the exhaust port window (window meaning the opening to the bore) I also made the port transition to a rounder shape so it matches the exhaust header quite nicely.
View media item 61336

The exhaust header is widened at the flange with a smooth taper too, to aid in this.
 
i went to great lengths to match my exhaust headers, even had a neat little mandrel for smacking the pipe over to get it to shape...

doesnt help that the studs are too close together. gotta be delicate welding a new flange in place!
 
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