251convincer

I have a 1987 Checkmate with the original 454 Magnum engine. The boat was originally equipped with an alpha drive that I converted to a Bravo. I want to freshen up the motor this year but I'm not sure what my engine really is. The Mercruser decal on the valve cover states it is a 454 Magnum but lists the horsepower at 330. WOT is listed as 4200rpm - 4600rpm. I know the 330 engines had a cast crank, 2 bolt mains, smaller rod bolts and "peanut" heads. The Magnum is supposed to have the forged crank, 4 bolt mains, better rods and heads. Anyone know why the Mercrusier decal states Magnum but aslo 330HP? Only guess I have is that the Magnums may have been detuned in 1987 as they had an Alpha drive behind them. If this is the case, anyone know which internal parts I have? Also, any suggestions on minor build? I was hoping to leave the bottom end alone. Goal is 450HP.

Biggus

iTrader: (1)

The 1987 454 Magnum is a 330. (in '87, the 330 was a TRS package) 1988 and up 454 Magnum's were 365 hp and had all the good parts, forged crank, pistons, rectangle ports heads etc.

251convincer

Thanks for the reply. I was afraid it was just a rebadged 330HP. I guess I will do a mild build as recommended in other posts.

jamie 10017

You can easly build a 450 hp With cast internals and use the peanut port heads if you want. Granted oval port would be a better choice, Retangular ports for much higher RPM's but you will not have the torque down low that a oval or peanut port head with give you because of the port velocity. Do a little rechearch on BBC build with peanut port heads you will be suprized what they will do ....granted not the first choice but not as bad as everyone thinks, sometimes people jump on the band wagon because someone states that peanut port heads are crap, well that depends on what you are doing with them rpm range etc.

Pliant

Agreed but it does take some mild port work and larger valve's and spring's and..... Actually where does it end.,,,opp's a cam..then the intake...then the manifold's...ohh intake to.

MILD THUNDER

iTrader: (3)

Yes, you have the 330HP version. I am pretty sure though they use 4 bolt main blocks. The block itself isnt a bad piece.

On a budget build, you can get ahold of a good used uncut GM forged crank for around 200 bucks. They are out there. You can get a set of takeout GM 7/16 Dimple rods cheap ( i have a set resized with ARP waveloc bolts if you need). Speedpro makes a nice affordable forged piston, in a couple different dome sizes. You can put together a pretty stout shortblock reasonably that will take 5500RPM all day.

The cam and heads is where its at. IMO, forget working on the peanuts. By the time you spend the money rebuilding them, installing larger valves, doing some port work, it just isnt worth it. GM Rect ports are plentiful around here in good used condition cheap, or there are many decently priced aftermarket heads out there, which is the route i would go. A roller cam is a nice upgrade, but there are some decent flat tappet setups that work nicely for cheap. 450HP is a reasonable goal. But just not with 7.9 compression, and peanut port heads, at 5000RPM max.

dereknkathy

a friend of mine used to do alum small block heads for NASCAR race trucks. he refused to touch ANY iron head. just too much work. too much time spent holding a die grinder for so little result. the amount of port work to make round ports support 1 hp per inch is way beyond what is worth doing. anything from 781 ovals for 250 bucks off ebay up to top dollar aluminum aftermarkets would be a better way to go. no procomp chinesium though...

BUP

iTrader: (9)

You have just a plain old 454 CID from 1987 with alpha drive.They called it a 454 mag and came stock with an Alpha drive for that year and a few past years.

In 1988 the 454 mag changed and came with a Bravo or a TRS. Also the 454 mag for 1988 was rated 4600 to 5000 rpms with 365 hp at the crank and had the better internal parts along with rec port heads and 8.6:1 compression ratio.

1987 OEM stock Mercruiser 454 mag with alpha outdrive was 330 hp at the crank with 4200 to 4600 rpms.

Comp ratio was 8.5:1

rocker arms stamped steel

pistons cast alum

rods forge steel

heads cast iron with oval ports

intake manifold iron low rise

block cast with 2 bolt mains

This is the specs for a 1987 - 454 mag - alpha outdrive - rated at 4200 to 4600 rpms - stock OEM Mercruiser.

Also all OEM marine engines, drives, trans, transom assemblies go off of serial # for what you really have and to match up parts and Service manuals rather than year model alone. I do know what you have if it is stock OEM.

Pismo10

Put some hot heads on and go. Just keep the revs down and the cast will survive.

251convincer

Thanks again everyone for the information. I will crawl under the cover to get the serial number and post it to know what I have for sure. Boat hasn't had any use over the last few years as my kids were little. I'm hoping this is the year to resurect it