Don't know, I only make ignitions.
Our race twin 289 is such a tractor it just takes a 28 pitch prop to go fast, but I do set the compression distance at .022 and intake duration at 140 degrees and clean up the roughness is the ports....
FastRat Porting will know, maybe he will see this..
.055 is weedeater territory, maybe DA does it this way for torque at lower rpm ?
I made a new set of really high compression heads for my first Zenoah 445 twin, it went slower..[8|
Welded up some higher compression piston tops for the 289 racer, the compression was too high and the centers of the glow plugs melted out....
Made a high compression head for a 125 Maloney, same result, lost 1000 rpm...
Only good result I ever had was with a GT80, put in another transfer port, good for 200 rpm, hardly worth the trouble...