Merc 7.4l 454 Gen 4 to 5 interchangeable?

[thormx11]

I recently purchased a 1997 Maxum with a bad 7.4l/bravo 3. I am exploring my options for repower and came across what looks to be a good deal on a rebuilt gen 5 long block. I believe my current engine in the boat is a 1988-1991 gen 4 (serial # C602211)

I am already planning on needing to replace the exhaust, intake, carb, and oil pan as I know these are probably bad as well on my current engine. Will the accessories (alternator, P/S pump, etc) swap over to the gen 5 or would I need to change all the accessories as well?

 

[tpenfield]

That serial number comes up as a 1989 7.4L/Bravo . It is interesting that a 1997 boat would have an older engine. I assume it has been repowered previously with a used engine.

The Mark IV 7.4L will have the stamped steel valve covers whereas the Gen 5/6 will have the cast valve covers. I'm not sure if the bolt patterns will be different. I think the intake manifold is different, so I'm wondering if the Gen 5 long block will come with the intake manifold . The Gen 5 won't have a mechanical fuel pump, so you will need to go electric or the fuel pump that is driven off of the sea water pump (think $$$$)

@Scott Danforth probably knows more of the specific differences . . . some are internal to the engine and won't really matter. Your concern would be bringing the external parts over from the Mark IV engine.

 

[alldodge]

Going for another block, maybe think about the 502

 

[thormx11]

I was planning on needing to replace the exhaust, intake, and oil pan anyway so I could buy those for the gen V. Also the boat already has an electric fuel pump with a block off plate over the mechanical fuel pump hole on the engine.

 

[matt167]

Keep in mind, if your exhaust risers/ other parts are still good, then you could very easy grab a TBI 454 out of a wrecking yard and swap over parts. It's the same engine, and all you have to do is add a marine cam, brass core plugs and head gaskets... Chevy 2500 and 3500 trucks and vans are where to look. If it's TBI it should be Mark IV.

There are a handful of differences to the Gen 5 engine

 

[Scott Danforth]

all Chevrolet motors have the same bolt pattern for the flywheel housing.

post the engine casting number (behind the starboard cylinder head, on top of the flywheel housing boss)

 

[thormx11]

I will check it tomorrow when I get to the shop. yesterday I took a quick look for numbers (mostly just the serial number) and grabbed a picture of a number where you are describing but it is on the port side. It is hard to read but I believe it is 615445. I will look at the otherside tomorrow for numbers.

 

[Scott Danforth]

Casting number will start with 101xxxxxx or 125xxxxxx

 

[Scott Danforth]

I will check it tomorrow when I get to the shop. yesterday I took a quick look for numbers (mostly just the serial number) and grabbed a picture of a number where you are describing but it is on the port side. It is hard to read but I believe it is 615445. I will look at the otherside tomorrow for numbers.

the casting numbers are always in the once spot - behind the starboard side head deck, on top of the flywheel housing embossment. the engine serial number from GM is on the port side (not needed).

 

[thormx11]

I looked this morning and did not see a number like you described. I think there was a smaller 3-4 digit number starting with the letter M. I am heading back to put the engine on a stand and take a better look.

 

[alldodge]

Can look here as to where the numbers are and how to look them up
https://www.mortec.com/location.htm

 

[thormx11]

According to that link it looks like the casting number should be on the port (drivers side of vehicle.) That is the first number I came up with, the only number in that location on the starboard side is M258. The closest number I could match up to the number on the port side of my block is "14015445...454.......78-90...2 or 4 bolt"

I believe there is quite a bit of flaking paint/rust covering up some, I may have to take a wire wheel to the numbers and try to come up with more.

 

[thormx11]

I found a decent deal on a rebuilt gen 4 (about $2500 shipped) but after the core charge it wouldn't be such a good deal. I would assume with an internally cracked block they wouldn't accept it as a good core.

The gen 5 longblock was only $2200 shipped, but it sounds like it may be too expensive to swap everything else over.

 

[Scott Danforth]

I looked this morning and did not see a number like you described. I think there was a smaller 3-4 digit number starting with the letter M. I am heading back to put the engine on a stand and take a better look.

then you didnt look in the right location.

https://www.mortec.com/location.htm

however the 14015445 is the casting number, making it a late Mk IV block

i would skip the Gen V and go straight to the Gen VI myself.

 

[thormx11]

Would the accessories swap over to gen 6? I had found one with the oil pan, timing cover, and valve covers included but would need to swap the alt, p/s pump, mounts, etc.

 

[Scott Danforth]

yes.

as a side note, your 1997 boat should have had a gen 6 motor in it to begin with.

 

[thormx11]

I will look around for a while for different engines and see what will be cheapest/easiest way to get on the water come spring. I have found a complete ready to drop in used gen 4 (I believe) for about 3k. I'd like to find a gen 6 for a similar deal but that probably won't happen. I may go with the gen 4 to get me on the water quickly and maybe after a year or two I can add some more HP with bolt-ons.

 

[Scott Danforth]

You need to build a motor for HP. There is a limit to what you can "bolt on"

 

[thormx11]

I should have stated the boat I have too. It is a 1997 Maxum 2800 with a single 454 & bravo 3. I believe torque would be more important than HP for this wouldnt it? I am not worried about going very fast, but would like to be able to get on plane easily with a few people on board. I have seen cruisers around this size that will struggle to get on plane with more than a few people on board.

Also, I have heard of people putting a 383 in these boats in place of the 454 and it working better than the 454.

But on a side note, I came across a video of one of the hot rod shows taking a 454 from an 80s motorhome with 330 hp and 475 torque and using a top-end kit from jegs/summit and ending up with 560 hp and 535 torque. I know the numbers would probably vary a little bit going to a marine application, but that would still be more than plenty for my boat I think. And I dont think I would necessarily have to do it all at once, I could probably get an intake/carb one year and upgrade the heads another year.

But at this point I am still just weighing my options and trying to figure out the most cost efficient setup to go with.

 

[Scott Danforth]

You cant take a 330hp boat V8 and get 560hp without a supercharger unless you change crank, cam, pistons, heads and intake. You need to do some reading on cam overlap and reversion

You can take a 330hp gen 6 motor (oval port heads with 360-370hp at the crank), change cam, modify the carb and get 425hp at the crank (385 at prop)

You can take a 385 hp gen 6 motor (rectangular port heads) and change cam, port the heads and change pistons to get 500hp to 525hp at the crank