imq707s

Thanks for the info Bruce, but I've got a few more questions. What would happen if I upgraded the cam to one of those mild 262 extreme marine cams and kept my exhaust manifolds stock? Would I loose power, or just not notice any gains? If I did decide to upgrade to aftermarket manifolds, which brand is best for the money? Can anything be done to the stock manifolds to make them frow better? Thanks again for all the help, I'm learning a lot.

Tinkerer

BRUCE --- Installing good aftermarket through hull exhaust will usually get you at least 200 rpm and 2 mph. I have seen at least that many times. I always recomend doing the exhaust first because it allways has good results and everything else done after that will work better also.

Bruce

The exhaust upgrade is not about just flow improvements. The largest benefit of aftermarket exhaust is the separation on water in the exhaust stream. The stock manifolds mix right after the elbow, if you add duration to a camshaft the reversion effects are increased. You need to keep the exhaust gas separate from the water to guard against reversion effects. Aftermarket exhaust does this. Marine engines at idle are at risk of reversion, camshaft selection is critical to the amount of reversion. Tinkerboater, I was responding to his comment that aftermarket exhaust has little effect. I recommended it be the first thing he does, he was thinking it wasn't necessary. I usually see minimal gains with only the installation of exhaust on a stock engine. Most people think they are going faster because it is louder, so that's good.

Tinkerer

I consider 200-300 rpm increase and at least 2 mph a pretty good improvement when it comes to boats. I do understand the entire reason for aftermarket exhaust. I have built many engines in the last 24 years.

Bruce

Are you going from thru-hub only, to thru-transom with the aftermarket exhaust? I haven't seen the improvement your seeing when the boat has thru-hull with stock manifolds, then installing aftermarket manifolds.

Crazyhorse

I'm with Bruce on this one. Exhaust by itself won't add much to the existing performance, but when combined with a good cam selection (and possibly a prop change), the boat can really come to life. I understand the induction system has been upgraded already.

boater31

i also agree about the exhaust not doing much alone but i did mine backwards first time. i had a 454mag in a boat and it ran about 62. changed the cam-intake-carb and still ran 62. also the boat took all the throttle to get on plane. then i changed to the stainless marine exhaust system and went from 21 to 23p prop and the boat ran 69mph. also only took about 1/2 throttle to get on plane. so i saw a drastic improvement but only because i had the cam-intake-carb on b4 i put the exhaust on. like i was told " a motor is nothing more than a big air pump and it can only take in as much as it can get out"

Tinkerer

About the only stock engine you won't see 200 rpm increase on are the newer fuel injected engines because the computer won't flow the increased fuel needed. You have to reprogram the computer to get the full effect. Or if you are allready up against the rev limiter.

[ 02-06-2002: Message edited by: Tinkerboater ]