Exhaust Port Matching - rectangular flange to round pipe

[Exhaust port matching]

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?

 

[Archiwax]

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...

 

[FurryOnTheInside]

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.

Casting flash in exhaust port

• FurryOnTheInside
• Sep 14, 2017

It is common to find the exhaust port window is obscured on the right side by a big chunk of metal.

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

Modified exhaust port

• FurryOnTheInside
• Aug 23, 2017

Opened the outside of the exhaust port a little rounder, and widened exhaust window a little at...

The exhaust header is widened at the flange with a smooth taper too, to aid in this.

Happy Time exhaust flange

• FurryOnTheInside
• Aug 30, 2017

Opened up the end of the exhaust and made it slightly more oval, and smoothed it.
Ears bent...

 

[HeadSmess]

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!