Yamaha outboard compression chart - Dec 03, 2016 · Compression I'd give the 175 HP version of the Yamaha Vmax SHO the edge over the Mercury 150 ProXS based on power and gears. 9HP 15HP 20 HP 25 30 HP 4-Stroke & 2 StrokeInsure your 2015 Yamaha F250XA for just 0/year*. On a two-stroke engine, compression is increased by milling the head (which decreases. DOWNLOAD Yamaha 40hp (40 hp) Repair Manual.