Lynn, 

Interesting point.  I had never really considered that aspect of exhaust design.  

What ID tube works best for a p-port?  

Mark

 

I used what the engine builder (Darryl Drummond) started us out on. That was 1 7/8" ID for the bridge ported 12-A. Same for the Periphery ported 12-A. Good up to 10,000 RPM. Probably fine for a 13-B at 6,500 RPM. In no case would I go over 2" ID. The scavenging effect comes from a high velocity slug moving away from the port. The larger the diameter of the tube the lower that velocity. Measure the port opening and use that as the inside diameter. You can go a bit larger, but not smaller.

 

Once collected you can go larger to exit to: Maybe even that homogenizing muffler.

 

Lynn E. Hanover