330 to 340 mm works for me. As Believe It said, adjust to suit the boat / prop / conditions on the day for best compromise.

Has anyone ever thought of trying to make a variable length pipe for a Zenoah? IE shortens as the revs rise and lengthens to get some low down power through and out of the turns as the load goes up?

I kind of thought of something connected to throttle position but then think something related to engine revs might work better.

Any thoughts?

Glenn.