Ok I need some input. I'm in the process of rebuilding my 12A. I decided to experiment on the old unusable housings. I didn't have a template or anything just free hand. I just tried to enlarge and retain the stock oval opening. The upper inside of the port is tapered upward. Basically just seeing if I could port at all. All opinions welcome!

 

Craftsmanship looks pretty good to me.

Exhaust porting is a black art. Paul Yaw uses a special rotary engine modeled flow bench to test his exhaust ports. It's amazing how the port shape and chamfer radii enhance or disrupt the flow.

http://www.yawpower.com/Flow%20Testing.html

I really want a flow bench now!

 

Thx for the complement! That site is where I learned about keeping the 2mm bevel around the port.

 

Thats pretty clean, but you could prolly go farther up.

 

Well with this design, I'm trying to keep the engine somewhat emissions compliant. I may try something differant on the other housing.