Whats the difference between the 7.4L and 454 Magnum engines?
(Apart from the increased power output).

Is it possible to upgrade the 7.4 to the Magnum power output?

Thanks.

Different crank, heads, cam,,not really cost effective to upgrade the existing motor.I personally would be inclined to sell the 7.4 and buy a G.M. crate 454ho or maybe a 502.

Theres quite a pile of us running 450 hp or so out of 7.4's. It requires precision work on the bottom end, better rod bolts, better pistons, heads, cam and valve train but is very do-able.

I have a 7.4 and have considering doing what you did. I think 450 is a very realistic target and 500 is possible with the right intake, cam, and heads.

However, I'm leaning towards selling it outright the dropping in a crate 502. It would be just a little more $$ up front, but having a 502 should really help the resale value vs the hopped up 7.4. If resale is not an issue, rebuilding the 7.4 is the cheapest route to 450-500hp.

Your right,,,its very do-able,,,but is it cost effective and what of longitivity, two bolt main vs four bolt main, the price of better flowing heads? With what you could sell the 7.4 for you should be able to upgrade to more motor for pretty much the same money.

Just my 2 cents worth

Am I right in thinking that my Gen V 7.4 is a 4 bolt main block???

4 bolt, 2 bolt, doesn't really matter below 500 horses. Weak link on the 7.4 is the rod bolts.

7.4 has a 2 bolt main, and as far as the rest,,,like I said, thats my opinion, but if the 7.4 was that strong with a 2 bolt main and cast crank,,,why did Mercruiser go to a 4 bolt main and steel crank in the 454 magnum,,,its not like they wouldnt cut a corner if they thought they could get by, prime example was the use of the alpha drive behind a big block in 1987, they found out real quick that that wasn't the ticket.

Like I said,,,Just my 2 cents worth,,,,nothing more.

Remember, thats not your fathers cast crank. This is a modern cast crank with the lastest in metalergy and machining techniques. Is it as good as a "good" forged crank? No, but if the engine as been line bored and your engine assembly is competent, that crank is good to 500HP as long as you keep RPM below about 5200. Like someone else said change the rod bolts to ARP's, and I guarantee the engine is a 4 bolt main. Even if it wasn't its fine to a least 500HP. These BBC's have a good bottom end.

We pulled a 7.4 carbed motor and dropped a GM crate ZZ502/502 in my 23 Nova. Been in for 2 seasons now and so far it's been great. Boat goes like a raped ape and the engine is verydependable. Minor oil temp issues though {whole other story}.

If you decide to go the crate engine route, you'll need a marine ignition, starter, carb or carb work, exhaust system, water circulator/pump, and a few other things, including a decent oil cooler. You'll need to remove or plug the pressure relieve valve at the side of the oil filter pad also. And remove the valve in the center.

Good thing is if you already have a 7.4, you can switch that mentioned above from the 7.4 over to the crate motor. I did the crate motor thing for a couple of reasons....It was relatively quick, and the engine is/was brand spank'n new, and full of good parts.

Good luck!