Hey Guys,
New member here with a couple of questions. I have a 1980 Sea Ray Sundancer 260. Merc 260 with a Alpha One Gen One. Like many other DA owners with a SBC I think this boat should have come with a BBC straight from the factory due to its size and weight. There are a lot of guys on the Club Sea Ray site that have thrown BBC's in their boats and were really happy with the boats performance with the upgrade. Anyway I came across a steal the other day on a 1987 Mercruiser 454 Mag that I couldn't pass up. The motor is completely disassembled but is all there and I am going to build it up as a project over the next couple of seasons. My first question is about the motor itself. Its rated at 330 HP but its a Mag version. This dosen't make sense to me because all the info I can find on 454 MAGs are the 385 HP versions. I am figuring that Merc downgraded the HP so they could mate a BB with the Alpha drive but was wondering what specifically they did to decrease the HP. My first thought was using the peanut port heads but my engine has the large oval heads, so I am thinking it has to be in the valves. Anyone confirm this? Since the motor is a MAG version that means it has all forged components so technically it could handle a lot more HP. If its bored the same as the 385 HP version then getting it back to 385 shouldn't be that difficult.

I am also looking for ideas on what to build the motor up to. I know the block is the same as a 502 Mag, so if I bore it .060 over and put all 502 components into it with a head job, would it essentially be a 502 MAG? If so thats probably what I will do. I know another guy with the same boat that put a 502 with excellent results. Would the change from a 454 MAG to a 502 be a super complicated upgrade?

Thanks in advance for your answers and suggestions.

Billy

I think there is some confussion some where along the line.I don't think there was ever a 454 mag that was 330 HP.The 330 has cast pistons,crank and oval port heads.The 454 mag and 502 have forged pistons,crank and rectangle port heads.They all have the same stroke of 4 inches.The block you have is most likely a mark lV and the most you could bore it is .125" over which would give you 482 cubic inches.You would have to bore it .210" over to get to 502 cubic inches.If you want that many inches with that block you could put in a 4.25" stroke crank and .100"over pistons(piston must match crank) and you would have 505 cubic inches.502's are mark V & Vl blocks. Their bore is 4.460 where as a 454 is 4.250".

I dont no why but my 87 sea ray w/454 are classed as mags 330 hp

They had 330's from 82'-87' and the 454 mag Alpha from 86'-88'. Don't know what really went on with the naming but the good ones have forged crank,rods,and pistons,rectangular port heads, and an aluminum intake.Can't believe they would have all those good parts for 330 HP.

I would have thought the same thing, but check out the picture. Its a 454 Mag 330 HP printed from Mercruiser! Its odd though.

454/330 was called a 454 Magnum in 1987 when combined with the alpha drive. Cast internals, oval heads, etc.

454/365HP was also called 454 magnum. This was a bravo drive package from 1988 and on. Forged internals, rect port heads, all the good stuff.

Sounds like what you have is the 330HP motor with cast internals. You can build that 330HP to 400-425HP, but I highly doubt your alpha would last long with the boat you have.

Thanks for the info, at least that's one mystery solved. If I can turn this motor into a 502 then that's the route I will go. Obviously I will swap out to a Bravo Drive and Transom Assembly to handle the extra HP and Torque. The engine is completely disassembled and will need all new internals anyway so I will put all forged components back in. Maybe a full rebuild kit for a 502 would be the easiest way to go? That would give me the forged pistons and all the forged connecting rods etc. Then I could take it to a Machine Shop and have them bore it out to the 502 pistons and put the lower end together. Also how can I tell if the crank I have is forged or cast? Its from another 454 MAG and Im hoping its already forged so I can just get it turned rather then buying a new forged one.

Will I have to get new rectangular heads or can I port the ones I have (large oval) to a larger size and install the 502 valves?


Thanks again,
Billy

Theres alot more to it. If you have a forged crank, and wish to use that, then you wont be able to stroke it for more cubic inches. You can bore the block to make it say a 468CI.

You can buy a stroker kit that includes the new crank, rods, pistons, for roughly 1900 bucks for decent forged stuff. That will put you at 496CI. 502 and 454 are different blocks.

You should be able to make 500HP out of a 454, or 550HP or so out of the 496 reliably. I think you would be better of getting different heads, what you will have in machine work, porting, valves, etc, you'd be better off going to a better head. I had 1200 bucks into a set of heads with all new valves(inconels arent cheap) guides, springs, machining, etc.

Okay now I understand what your saying, sorry I'm new at big engines, mostly have Motorcycle engine experience. I was under the impression that the 454 and 502 were the same block but I guess not.

So new plan, stroker kit and bore combined = 496 CI = 500 HP ? What kind of torque you think that will put out? Sounds good. Any recommendations on the stroker kit or heads I should get? Probably a good Idea to have the stroker kit in hand before I go to the machine shop.