856 ACME For the best holeshot and heavy loaded boats. A 644 ACME is the next step down witch is good for medium weighted boats. You need to make sure you pay attention when full throttle though because you will hit the rev limiter.
They say the 644 will hit also but if you think about it your rarely at wide open throttle. When you throw the throttle down your bringing up right away. We have been putting the 644 on alot of our boats and haven't had any issues.