82' 350 merc. rebuild questions

[cobia-boat]

I'm rebuilding my 1982 merc. 260's (chevy 350s) For my 30' wellcraft.
I have the motors out and torn down, and would like to do some performance mods and need suggestions. I would like to get closer to 300 hp out of them.
1. intake replacement? what kind?
2. Cam replacement?
3. gear drive?
4. Change from points to electronic ignition?
5. more compression?
I don't know a lot about motors myself, and my buddy a chevy mechanic has been doing all the work, but has never done a marine rebuild. As rpm's are completely different.

I don't want to break the bank, as i'm rebuilding two motors but plan on spending 3 grand or so. any if=nfo would help.

 

[WillyBWright]

Re: 82' 350 merc. rebuild questions

Larger bore marine carbs on highrise intake manifolds, performance exhaust manifolds, stroker kits, and 3" thru-hull exhaust. Those would be the most effective mods. Also very expensive. But 320Hp is easy with those mods.

Cheap mods would include exhaust tubes instead of exhaust bellows and different flame arrestors with larger intake surface area. You might also have him Dremmel the manifold passages to match the gaskets, and smooth the surfaces for an cleaner flow. Those could add 10Hp per motor combined. I believe the 260s already have tubes instead of bellows, but that isn't carved in stone.

Don't go to higher compression. Then you'd have to use Premium fuel and that can be hard to find on the water. Electronic ignition over points doesn't really add much, except that it does maintain performance since points wear and that affects timing. You need to stick with a marine cam rather than go to a hi-perf cam. They're pretty beefy to begin with. They do make hi perf marine cams, but you'd want to include that with the expensive mods mentioned above rather than as a stand-alone improvement.

Don't be afraid of a mechanic unfamiliar with marine engines as a rebuilder unless he starts venturing into auto replacement parts. They're not all that special compared to any other production premium quality GM product. They fit together just the same.

If you'd happen to need to replace the heads anyhow, look for marine Vortec heads. That would be a significant improvement of maybe 20Hp per motor.

You don't want to exceed 300Hp for those drives by much. You'd need to upgrade to SS drives and that'd blow your wad alone.

 

[180shabah]

Re: 82' 350 merc. rebuild questions

The best bang for the buck will be the Vortec heads. They are worth more than just 20hp(there worth more than 20hp on the V6's) and when combined with a good Intake manifold(which must be replaced if use the vortecs) will give you the power you are looking for without spending big money on hi-po exhaust and internals.

I agree with Willy, you don't want to push these drives too hard though - they just don't like it. Although with brand new RPL drives available for $1200 replacing them every five years or so may not seem so bad. Still alot cheaper than the payents on a new boat.

 

[FreeBeeTony]

Re: 82' 350 merc. rebuild questions

I agree..........go with the Vortec heads and new manifold
From what I have read (and been through), this is the best hp/$..

Might also look at replacing the carb if it needs also.

 

[paulie0735]

Re: 82' 350 merc. rebuild questions

Is that $1,500 per engine.......? Won’t buy you a lot and will require your base engines to be reasonably healthy to begin with (crank, rods, block, cooling components etc all re-usable or serviceable) I’ll assume they are marine engines and your buddy has the basic engine re-build stuff sorted out already. You need the engines built with a nice flat torque curve, they need to work well from 1,500 to 4,000 with a peak rpm around 5,000. A cam with around 270 deg duration and 400 – 420 thou lift should be fine. The usual array of rebuild parts, I wouldn’t bother with a gear drive, any quality roller chain and sprocket kit will be fine. Stick with dual plane manifolds, most people have a preference for the brand that they have on their boat, its kind of their way of justifying the initial purchase, but under all the chat is the truth and that is…. ‘All the quality brands work about the same’. Get your heads assessed by a reputable head shop, if they are ok just have them serviced (valves faced, seats cut, guides replaced etc) If their ‘not worth repairing’ than the vortec’s are worth fitting, (that could blow your budget though) they also require a unique inlet manifold so get the heads looked at before you go shopping for the manifolds. If your carbs are the originals they should be fine just get them professionally rebuilt. Electronic High Energy distributors would be nice but need to be marine spec so they tend to be a bit pricey, there are a few cheap ones around that work fine in a car but I wouldn’t use them in a boat! Otherwise an electronic conversion is worthwhile. The standard marine spark curve should be fine (again assuming they are the original marine distributors) have them tested and matched to be sure. I wouldn’t bother with headers, they are a significant cost and the originals will handle a warmed over 350 with no problems. Converting to thru transom is a worthwhile upgrade though. I would also get all the cast iron plumbing parts including the exhaust assemblies hot tanked and sand blasted and also get the gasket surfaces re-faced, any good head shop can do that for you at a minimal cost. Naturally we could all go on forever about doing this and fitting that, but I think I just spent your $3k and for that you should have 2 healthy engines that will give you many years of trouble free boating…….. The horse power will be what ever it is!! Don’t make the common mistake of getting hung up on ‘peak’ horse power numbers. In reality, (take it from an old ‘hot-boater’) on the water the numbers don’t mean sh1t, its how it feels through those throttles that really counts. That should wake up that old Wellcraft 30 nicely. Good luck.