Difference of compression ratios

[azcourt]

I had a 13.5 to 1 comp 420 kit in my YZ 400. I am rebuilding it with a 426 rod, 426 piston 12.5 to 1 comp ratio. What does anyone think the lower compression ratio will do to the powerband? It had really good low and mid grunt as a 420, but not much on top (I like it that way).

 

[EricGorr]

Rich and I did some revisions in Thump's engine, switching from the displacement and compression ratios as you describe. He didn't notice a big difference, we did it because of the sensitivity to fuel. It probably made it run a bit cooler.