Pro 80

[steveohara]

The measurement is from the piston surface to the start (small end) of the scavenge cone. When dealing with curved headers and elbows like you have on your pipe you measure along the center line. The best way to measure the curved sections is to measure along the outside of the bend and the inside of the bend, add the two measurements and then divide by 2. For that type of pipe and the engine on your kart a good starting point is 10"
Steve O'Hara