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.