It has been discussed in depth, in detail, and many times here and other boating forums I frequent.
Majority of opinions hold that the bottom end of that motor is not strong enough to support additional rpms and horsepower. Starting with a Mag motor gives everything MUCH better chances of survival. Not that it can't or hasn't been done, but MANY end with broken bottom ends. I had a mid 90s 7.4 myself and had same thoughts a few years back. I got talked out of it REAL quickly. In stock form they will provide alot of reliable service, but crank them up much beyond stock and the weak spots will show themselves.

 

i agree...and the peanut heads make more low end torque than rectangle port heads but die in the top end

 

Sad but true. You could probably get away ok at your 450 hp but youd want to get the closed chamber oval marine edelbrocks, maybe put in some small dome pistons to get you to say 9.5:1 compression and upgrade the rod bolts to arp waveloc. And a nice cam that will not revert in your exhaust system. I'd not fear that breaking just keep it under 5200rpm and make sure the rotating assembly is properly balanced.

 

.060 forged pistons 9.2.1 cr. good rod bolts, a ported set of 049/781 cast heads performer rpm int, jetted q-jet carb or holley 850cfm. hyd cam flat tappet with about 236/244 @.050 and a good log exhaust 480 /500 hp. using your block, crank, rods done many over the years, best bang for the buck. just make sure to have zinc additive for the cam, power under 5500rpm. Tom. www.accelerationmarine.com.

 

or get a roller cam for freer reving and less friction..steeper ramp angles..more power

 

I was looking at the Edelbrock Oval Port Cylinder Heads, Performer RPM, Marine, Aluminum, 110cc Chamber, 290cc Intake, (61459), Performer RPM AirGap Intake, Edelbrock Marine 750cfm carb. (1410), and Comp Cam Xtreme Marine 11-232-3 Camshaft, Hydraulic Flat Tappet, Advertised Duration 262/268, Lift .505/.515. The Block has 4-bolt mains and the machine shop replaced all rod and main bolts with ARP's. I am using Speed-Pro forged flat top pistons. I wanted to stay around 9:0:1 compression so I could still run pump gas. Plus, I put a steal forged crank in it. Also, the block it had to be bored 0.030 over and decked 0.015.

 

The 110CC chambers sound kinda small but that will bump your compression without having to do alot of other work. Run a compression calculator on your numbers and you will see if you are at pump gas or not. Worst case scenario you can open the chamber cc's to lower compression if it is too high.

With the forged crank and all you mentioned it sounds like you will be close to where you want to be in power and just fine for the structural integrity to be safe.


I'm expecting over 500HP but I stroked my 454.

 

with the alum heads i would try to shoot for 9.8.1 cr. or about. havent used that cam so i cant give an opinion but min duration should be 230in 240ex. @.050. with about 540 in 560 ex lift with 110/112 cl. use the performer rpm port matched to your heads. if able try to dyno the combination to verify timing and jetting info and peak hp and torque so that you can prop it correctly. we have a superflow x-test dyno and are located on the ohio river with a private boat ramp to verify the results. the web site is down now but should be operating soon. hope this helps.Tom Pritt 304.615.7148. www.accelerationmarine.com.

 

question for maxcat, I'm not sure about using the performer rpm cam. I know it's port matched to the heads but it doesn't say that it can be used in a marine application. I have heard horror story's about using the wrong cams in boat engines.....like sucking water back through exhaust into engine.

 

I built a 330 with the oval port Eddys and left the stock cam in it. First had to pocket port the heads to acheve there advertized numbers. Think I got them to 290 CFM. Could have taken them alot further. Dont rember what lift I fklowed them to thinking it was .500 to get that.

Tried 3 intakes a performer RPM (that came with the boat) Was dumb enough to purchase a new Air Gap and had a Torqer II. Tried each manifold a three different carbs on each intake. A 750, an 830 and a 850. Everything was out of the box except pock porting the heads.

Swapping each of the three carbs on each of the intakes
Made no difference.
Manifolds were a different story the Air Gap and the RPM ran the same other than the air gap coast me a bunch of mony for hype like 260 some. The Torqer felt like there was a 50HP or better increase over the dual planes. The boat just jumped out of the water with no bow rise right onto plane.
If I had the chance and realy wanted a dual plane foir some reason would go for a Wiend Stelth over the Eddys possibly.
Bet you go for the hype.
One thing ids also your hull design, weight and length

Look atthe Crane stuff they developed all the Marine cams for Mercruser so they have a lot of R&D on what works. If you want to stay flat tappet Schieffers makes a good retro oil 65ish a case and Amsoil does also about 100+ a case.

 

Yes its called reversion, Thats why I recommended the 230/240 @.050 cam on a 110/112 cl. It would be nice if you could find a gill system with a turbulator plate between the manifold and the riser or an eddy marine or equivalent {dry type} log manifold with a center exhaust riser. I have in the past used the factory 4"center riser with little problems. But I would hope you have already upgraded to one of the lighter aluminum exhaust manifolds by now. Most of these 4.00stk engines with this type of cam peak at about 53/5500 Rpm. and with a 1.5 gear in the drive and a 25 P Prop will idle at about 650/700 in gear. having a higher initial timing without exceeding 34deg helps also. this is with the typical open chamber 049/781 head i talked about earlier.

 

Fyi on the gm blocks and the 3/8 rod bolted rods. Before there was a 502 that mercury marine had we were sucessfully running 2 Bolt GM blocks .060 with 9.2.1 cr 3/8 rods with sps/arp bolts,4.25 Stk cranks and the pocket ported 049/781 heads, 2.25 in/ 1.88ex valves. with a solid roller cam and a ported holley strip dominator intake and a 1050 holley dominator {annular boosters} We were always getting over 600hp and 580lbs of torque. this was with pump gas and 34deg timing and cool water, Stellings headers. I have yet lost any of these engines. they were 496 cid. engine tune up is very important. a note these engines were making peak HP @ 6000 RPM.

 

Sounds like everything I was concerned about has been covered.
4 bolt block, forged crank, forged pistons, rod bolts, GOOD to GO!
When I first read building a 330 horse motor, I was thinking all stock bottom end...sorry.
Good Luck with the build, ENJOY !!!

 

Thanks to all!!! I appreciate all the comments and input. I only want to spend the money once on this engine!!!!!

 

Alot of "talent" here to give you good advice.