wexrocks

I have a 1990 454 Magnum and just have a few questions, some things on this motor don't add up to other info I've read, maybe it's differences in years?

I figured the Magnum would be 4 bolt, it's 2.

I read that it should have an aluminum intake, it does not.

it has large oval heads, not rectangular.

are these normal for a '90 Magnum?

thanks for any input!

JaayTeee

The only 454 Magnums that had those characteristics
were the 86'-87's that were coupled to the alpha drive.

After the Bravo drive came out, they called that engine
( 454 mag alpha) a 7.4, they were rated at 330 crankshft hp.
(300 ps hp)

The 454 magnum ( bravo drive) had the 4 bolt, rec port heads, alum. intake, 365 hp rating.

wexrocks

can I get the right info from casting numbers?

it is mated to a Bravo, in a 1990 boat

wexrocks

now that I'm thinking about it, the previous owner said it was rebuilt, which is apparent from the .020 over pistons, but I'm thinking whoever did it used a 330 motor.

wantsa311

I rebuilt the 454 Magnums in my 1988 Formula SR-1. Both were never opened previously, had 4 bolt mains, and rectangular heads.

JaayTeee

Does it still has the merc serial # tag on the block ?

That will tell what it originally started out life as.

wexrocks

it has the serial number on the valve cover, and on the spark arrestor cover, hard to say how accurate either of these are, considering they may have used both from the original 365 motor that was in the boat. I was hoping the casting numbers of the block would help. the block is stamped HI PERF PASS, but the heads say PASS and 8T. I'm guessing the block casting numbers would be whatever GM made, but what they ended up being for Mercruiser could be completely different...?

Griff

Sounds to me like one of the prior owners bought a complete engine that was not a mag engine and used all his merc stuff to make it look like a mag engine. Even the old 330hp/alpha combos used 4 bolt main blocks.

wexrocks

I am guessing the 330 "Magnum"s that had the Alpha were 4 bolt? I know I've seen alot of posts about 330hp 2 bolts, I'm guessing that was in the "standard" 330hp? it has the big oval heads, not the peanuts, so that's good, and everything else that would make a difference between the 330hp and the 365hp was all going to be upgraded anyway (cam, intake, carb, etc.) so it doesn't really matter I suppose. I'm not planning on going beyond somewhere in the 425-450hp range anyway, so the 2 bolt and 330hp crank doesn't hurt my feelings. I was just curious, as nothing made sense when I took it apart.

Scarab28

Can you get the serial numbers off the Block and heads. I'm betting they match an older year (86, 87)