I have a pair of 454/420HP motors(gen IV)

Is it possible to buy a new assembled 502 or 540 shortblock, and re-use my 990 rect port heads? Or is it cheaper to stroke my current blocks? I would like about 550 reliable HP, without spending big money.

I'm guessing it would be cheaper to stroke the motor, especially if you're going to do the work yourself. At least you'll know it's put together correctly. I've heard too many horror stories about non Merc mechanics putting engines together.

You can stroke your stock blocks for more cubes but you will not be able to reach the 540 CID in the normal way. Common 540's are 4.5 bore by 4.25 stroke. Your blocks can not be bored to 4.5, and putting a longer stroke will require extensive clearancing in the lower end. No cheap way to accomplish your goal.

Larry

62 bel-Air 409

260 Eagle XP HP500EFI

2002 Corvette M6 Coupe

What about bore/stroke the Mark IVs to 496ci? Would $3500 be a reasonable price for an assembled short block with at least "good" rotating assembly parts (not top of the line, not bottom of the line parts). I don't think that price included camshaft.
Would it be much cheaper to have the work done on your own cores?

Dyno Flo has new complete 540 short blocks for $3800.

This guy is one of the biggest around here. His prices are fair and service is great. They dont do much marine engines, but you cen get whatever you want from them.


http://www.shafiroff.com/shortblocks/468_shortblock.asp

If you take the Mark-4 454cid engines you have now and use a 4.25" stroke crank and bore the cylinders .030" from stock bore size it will give you 489cid or if you go .060" will give you 496cid. This should be enough to accomplish your goal for the 550hp you're looking for in a marine application @5500-5600rpm or so using a hyraulic roller cam, and pocket ported heads although a 540cid engine will accomplish this much more easily than a 496. If your Mark-4 blocks aren't the SIMESE (spelling???) cylinder wall blocks then you won't want to bore them more than .060".

I notice one of your considerations is cost. Remember, if you go with a new Gen VI you will also need alot of additional stuff that you can't reuse from your MK IVs. Flywheel, balancer, timing covers, oil pans. You'll have to use a electric or belt driven fuel pump. I'm sure someone with more knowledge will let you know for sure, but make sure your heads will work, I've heard yes and no. Even if you can use the heads, after you replace every other component will you want to reuse them as there are better choices. I had to make this same decision and I chose to go with the Gen VI 540's, if I had to do it again I probably would have stayed with the Mk IV. If I absolutely had to have 540's I think I would have gone with an aftermarket MK IV compatable block or found some Bow Tie blocks that could support the 4.5 bore.

Stay with the Gen IV style block. You can find many aftermarket shops that offer a 540 shortblock for around that $3500-$4000 price range. The #990 casting heads have a 119 cc combustion chamber, unless they were milled previously. Just a small amout of work makes them come alive. Don't go crazy with compression either. Stay with under 10-1, and 93 oct will be fine. The biggest cost I see that you'll have is the exhaust. Don't you run stock mercruiser cast iron manifolds now??? Correct me if I'm wrong, but you'll never make that kind of power with a mild setup unless you get a good flowing exhaust.

Frank

You are right Frank. My boat originally came with 400HP mercs with 3" cast iron logs. I ditched those last spring and went with the Cast Iron "420HP" manifolds with the Gil tail pipes. An improvement over the 3" for sure, and I would think still better than the 4" standard merc iron manifolds. The guy I got them from ran them on a pair of 550HP 496's. He then swapped them for a set of CMI headers, and he noticed a slight increase in the midrange, but not much on top. The drawback is the wet joint that is prone to leaks. Mine have been ok, but none the less, a pair of aluminum manifolds would be nice. Not too mention the weigh savings, these suckers are HEAVY!

My boat tops out around 58-60 right now, Id like to see 65 or so. Who built your motors Frank? They sound sweet, and I know that thing runs!