Well prepared 915 heads to about 76-78cc. Zero deck with 12 or 13 cc dish pistons (Diamond, ICON... (Diamonds are lightest). Put a 750 gram rod in it. Solid lifter cam designed for manifolds. 2 1/2" mandrel bent exhaust with DynoMax UltraFlo mufflers. Get the tune right as rain, and you should be at a solid 475 hp.