One thing to keep in mind is that going to the 250 would mean you need to use 93octane gas. If that's not an issue for you that's great. I have to travel a ways or treat to get 91-93, when 89 is available at my marina. Aluminum heads may lower the octane requirement enough to not be an issue.


btw Boorgie, I don't believe GM ever made a gen iv.

My bad MARK IV

Ramos, It sounds like someones been in those engines before. The SRP pistons are a giveaway. I would measure bore size to see where you are now. My guess is they aren't 4.250 is someone changed pistons. Prob 4.280 or 4.310.

As far as the heads. If you are going to add blowers, I would stick with rectangle port. For starters, probably 99% of the used blower packages will have rectangular port intake manifolds. Also, in a marine supercharged engine, the gains in low speed velocity aren't worth the tradeoff in the upper rpm range, with the roots blower.

You may hear people say heads aren't as important on a boosted engine, and their really isn't anything further from the truth. A restrictive intake port will kill power, and so will a restrictive exhaust port. For example, if you were building a N/A 454, I wouldn't recommend a 325CC rectangle port intake. But blown, you can get away with a larger intake port, as the blower can help make up for the velocity loss with the larger port. On the street, you may want the smaller runner, for a bit better part throttle response. In the boat, I don't think you'd notice a thing.

Things like port velocity, exhaust scavenging, and so on, are kind of covered up by the supercharger in a way. I have found, that when building a forced induction setup, is you don't want restriction anywhere. IE, too small carb, too small intake port, too small exhaust, and so on. You want the engine to be able to move a maximum amount of air, and a camshaft to compliment the build. Don't get me wrong, Im not suggesting bolting on a pair of monster port BMF 405cc heads on your 468's, but stay in the 300-325CC range and you'll be good.


So, with that being said, if indeed aftermarket heads were the route you wanted to go, I would skip the small ovals, and look into the smaller rectangle port Brodix. Like the 312 or even 320cc runners. And if you ever decide to build some 502/540 based shortblocks, they will work on those too. A lot depends on your goals. If 600HP is your number, the GM castings will get you there with the blower. If 700 is your number, you'll want to look into some Brodix, dart, etc.

Truth be told, your limiting factor is gonna be the heavy boat with bravo drives. I almost think in a way, your best route to take at this point, would be a rebuild on the bottom ends, some small rect port aluminum heads, a roller cam upgrade, keep the compression in the 8.5-8.75 range. Put them together right, and run the boat. You'll make decent power, a substantial increase over what you have, and then, down the road, if you feel the need, you can always bolt blowers on when you have some money to play with. Don't let the "it'll be a turd" with 8.5 or 8.75:1 compression scare you. You don't need 10:1 to have a strong running N/A marine engine. Mercury been doing it forever with under 9:1. And I wouldn't be a bit worried about a blower, aluminum heads, moderate boost like 5psi, with 8.75:1 static.

MT - Thanks for the write up. The notion I had stuck in my head was either full blown or higher compression N/A. I was concerned about it not "waking" up if I kept compression around stock but new heads, cams, carbs and better exhaust should prove otherwise. I think going N/A or now is in my best interest right now. The Brodix BB2 Plus seem like a good head to allow for future builds if i wanted. I'll keep the chambers at 119cc for now. If I ever decide to stick with N/A on this build then I can have the heads milled.

I'll be measureing the bores on the block tonight. The port engine should be down to bare block tomorrow night.

Also, what's the advantage of raised exhaust ports on the aftermarket heads?

So I got the port engine down to block with rotating assembly still installed so I can take all the prelim measurements tonight. This is definitely a stock mercruiser set up. The heads were stuck on there and I had to beat them with rubber mallet to get them loose. The stb engine the heads just came up easily. It also has the merc screwed on stamp with serial number just above the starter. The starter on the stb motor is about 2/3 the size and weight of the stock port motor. On parts lookup it seems there's a 12" and 8" starter that were used. I'll replace that monster with the 8" unless told otherwise.

I decided not to spend my loot on measuring tools yet and the let the experienced engine builder measure bore and bearings, ect. Everything goes to machine shop this Saturday. As for heads, I decided on the Brodix BB2-Plus with 312 intake runner as I don't want to limit myself too much for future builds. I already picked up two sets of crane gold roller rockers and manley inconel valves. The ball is rolling!

What pistons were in the port engine? When I tore down some of those they had GM part number stamped into the piston from mercury

Correct, the port had GM part numbers stamped on the face of the pistons. I forgot to write them down. But I believe it started with a 3 and had some 9's in there.

10/4. That's good news.

Awesome! Are you planning to have the machinist inspect your dimple rods prior to deciding which route you will take with the reciprocating assembly?

Yea, block, crank, rods with pitons attached are going to machine. He needs to press out the wrist pins anyways. I want to get back a yay or nay on parts and/or bearing sizes needed. If crank and rods check out it should be the same bearings that are in there, right? All bearings are smooth with some streaks in some and a little discoloration but nothing suggesting metal chunks flowing through. what I did find last night when removing the oil pan from the "stock" port engine was a small lock washer swimming in the oil at bottom of pan, scary.