Help me pick a cam
 

I've got a Jasper Marine 350 motor in my 21' Scarab. The motor is stock and I'd like to upgrade the cam to one of the Comp Cams extreme marine cams. What size would work best?

The cam choice will depend on your exhaust set up

It's 4" pipe thru-hull.

Specifically, what exhaust manifolds do you have? Are they stock or aftermarket.

The Manifolds are stock. As far as the cam goes, I looked through my Comp catalog and they make two different Xtreme marine cams that might work. The first is part # XM262H and the second is XM256H. Comp says that the 262 is the biggest cam that should be used for an inboard/outboard application. This seems a little small to me, alot of guys on this board seem to be talking about the 280H cams automotive cams which have a bunch more lift and duration. What do you think I should use? Also, will upgrading cams really sucks down a bunch more gas?

You cannot install any larger camshaft before you get aftermarket exhaust manifolds and jacketed risers that move the water mixing point down the exhaust path. In addition, I would not install just a cam, you should also upgrade the intake manifold and carburetor to realize the improved flow. It is very easy to overcam a marine engine, be conservative with your selection.

It's got an Edlebrock Performer Intake and a Holley 750 carb. Are those good choices? I always read posts on here stating that upgrading the exhaust manifolds really doesn't make to much difference. Is that right? Thanks for the help.

Upgrading the exhaust on a stock Mercuiser engine shows limited improvement. Upgrading the exhaust is the first step when considering performance modifications. It allows more duration in the camshaft without reversion issues and better flow. I personnaly prefer an air gap style of intake, single plane over dual plane because of fuel distribution issues, but that also depends on power output.

I thought that the single plane intake kills your low end torque. Would I notice any improvment switching my Performer out for an edelbrock airgap performer rpm?

I think it would be a much better choice because it will keep the fuel/air charge cooler and not expose it to the heat of the engine.

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.

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.

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.

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.

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.

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.

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"

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 ]