I've heard the 25 XD Merc from the 80's is the motor you can turn into the 25 XS. Trying to figure out what all can be done to these motors to slightly improve the horsepower. I know 400 CC is the limiting factor on these engines. Once again I'm not going to be racing and this motor is going to be on a 1436 flat bottom Monark boat. I built this engine last year from about 5 - 25 mercurys we had laying around. It's like a Johnny Cash Cadillac! 25 XD powerhead and the rest is 98-2004 parts. Took a little grinding and machining but I got a new/old combo that's running good. But wouldn't mind for a little better as always! Thanks