454 information

[Westerndiesel25]

I have a 87 welcraft nova 23 xl with a mercruiser 454 magnum, it has a cracked block and I'm inquiring about more information on the differences on anything that was done differently from gm

 

[Scott Danforth]

The block is just a GM block

Prep for marine use:
Brass core plugs
Marine cam
Marine head gaskets
Marine water circulation pump

Your motor is a 1970-1990 Mark IV big block

I personally would look for a 1996 or later Gen VI

 

[alldodge]

Welcome
Can replace your 454 with a 502, but might need to change up some brackets and belts

 

[QBhoy]

A close friend of mine, not long ago, traded in his tired old 7.4 carb, for a late 90’s 7.4 mpi. Best thing he ever did, he reckons.
Should be mentioned however, that these early gen mpi aren’t to be confused with the later mpi models, from 2002 and the decade to follow, where the short block v8’s and smaller v6 variants, produce significantly more power and efficiency over the carb.
These base 7.4 mpi are about the same power as the base carb. The mpi magnum has more power perhaps. They are a little better on fuel he reckons and certainly start on the first sign of the key turned.
Anyway. Best of luck however you go.

 

[Scott Danforth]

the standard 7.4's are oval port motors
the magnum is a rectangular port motor with a different cam

 

[QBhoy]

the standard 7.4's are oval port motors
the magnum is a rectangular port motor with a different cam

Good info Scott. Thanks for that info. I was mildly aware that the intake was different between the mpi 310/330hp base model and the magnum. Just because of previously looking into K&N filters for each of them years ago. Didn’t know about the cam before you saying so.
What is the difference between the carb 7.4 in its base form…and the more potent magnum carb version ? How is the extra power made there ? Always something I’ve wondered about on some of these GM engines. Especially ones that are the same cubic capacity and the same rpm range between them. I could understand power gains over an identical engine, achieved by a higher rpm range. Or perhaps one having higher compression ratio, by build or forced induction etc etc.
One particular example that I just don’t get at all, is the difference between factory produced (not later repower or quicksilver) 4.3 4 barrel carb and the same era mpi 4.3. Or even a 4.3 efi and a 4.3 mpi. These all have the same 4400-4800 rpm range and identical compression vortec spec. How does the mpi make so much more power ? The other era 5.0 and 5.7 mpi making more…I get. They have more rpms on tap, over the carb. Anyway. Just a rant and wonder from me

 

[Scott Danforth]

How is the extra power made there ?

I can give you a naturally aspirated boat motor at 7.4hp running a carb close to 500hp or 250 hp.

I can take that same 7.4 liter displacement in a non-marine application and get well over 800hp with a carb or EFI

has nothing to do with how big the cake is, it depends on the ingredients and how it is baked.

once you understand engine architecture, its not a mystery.

 

[alldodge]

What is the difference between the carb 7.4 in its base form…and the more potent magnum carb version ?

Talking Merc only, the 454 Mag uses 502 heads and is a MPI
I don't know of a Merc 7.4 Mag that uses a carb

454 Mag MPI 385HP
502 Mag MPI 415HP

 

[QBhoy]

I can give you a naturally aspirated boat motor at 7.4hp running a carb close to 500hp or 250 hp.

I can take that same 7.4 liter displacement in a non-marine application and get well over 800hp with a carb or EFI

has nothing to do with how big the cake is, it depends on the ingredients and how it is baked.

once you understand engine architecture, its not a mystery.

Totally understand all that Scott, to be fair.
What I was asking around, was in particular, the standard, numerous and factory straight into boat, engine.
Like I say, totally get gains from a built engine with all sorts of variance. Just not too convinced in particular around some of the quoted power figures or gains, between some of these commonly fitted engines over the years.

 

[QBhoy]

Talking Merc only, the 454 Mag uses 502 heads and is a MPI
I don't know of a Merc 7.4 Mag that uses a carb

454 Mag MPI 385HP
502 Mag MPI 415HP

There is a 454 magnum 4 barrel carb AD. 100%. Was working on a boat with one just a few months ago and know of a few others too. Supposed to be something like 350hp. Opposed to the usual 7.4 carb of the same era, producing something like 310 or similar

 

[QBhoy]

Talking Merc only, the 454 Mag uses 502 heads and is a MPI
I don't know of a Merc 7.4 Mag that uses a carb

454 Mag MPI 385HP
502 Mag MPI 415HP

Here is the actual one I was messing with not long ago. Fitted to a formula. Some of her components may have been altered over the years, I might suspect though. But yeah…they definitely exist!

 

[alldodge]

My point exactly
As per Merc manuals, the Mag is listed as a 454, the standard is listed as a 7.4

 

[QBhoy]

My point exactly
As per Merc manuals, the Mag is listed as a 454, the standard is listed as a 7.4

Maybe picking you up wrong. I thought you were perhaps suggesting there wasn’t a carb 454 magnum? There was a magnum in both carb and mpi/efi forms. At the same time as the magnum 454 carb…there was also the usual and more common 7.4 carb with less power.
Later as you’ll know, there was the 7.4 mpi with the 310/330hp or whatever they are…but also the more potent mpi magnum too. With different intake and cam (as Scott mentioned). As you’ll know too, obviously

 

[alldodge]

I'm talking Merc terminology, the way they are "listed" in manuals, parts and on cowling. Going back to the original "carb" comment; All "Mags" are 454, all 7.4 are standard

Now so far as 7.4 MPI it is 310HP, the as 7.4 carb is 300HP

 

[QBhoy]

I'm talking Merc terminology, the way they are "listed" in manuals, parts and on cowling. Going back to the original "carb" comment; All "Mags" are 454, all 7.4 are standard

Now so far as 7.4 MPI it is 310HP, the as 7.4 carb is 300HP

Ah. Think I’ve got your thinking now. I see now that you were maybe being very specific about the wording and literal name on the decal. I was thinking much more simply with things.
So now that’s out the way…what’s the difference between a 4 barrel 7.4, a 4 barrel 7.4 lx and a 4 barrel 454 magnum? Just the intake manifold maybe on the 454 ? I think I have in my head that there are three different HP ratings amongst the above mentioned carb engines ? Could be wrong.
I might even confuse myself further and dare to ask if the older carb versions were prop rated figures and the later mpi were flywheel rated ? Maybe for another time…haha.
Jeez…I could go further and ask why the much later 300hp 350 mpi (5.7) is named in Ci terms and nothing else. So no such thing as a 5.7 mpi…but the 5.0 mpi is just called the 5.0 mpi, rather than a 305 mpi !!
Or why the older 350 magnum carb has 260hp, but so too does the alternatively named 5.7 lx 4 barrel carb ? Minefield

 

[alldodge]

Might need to apologize to @Westerndiesel25 for getting off topic on his thread

So now that’s out the way…what’s the difference between a 4 barrel 7.4, a 4 barrel 7.4 lx and a 4 barrel 454 magnum? Just the intake manifold maybe on the 454 ? I think I have in my head that there are three different HP ratings amongst the above mentioned carb engines ? Could be wrong.
I might even confuse myself further and dare to ask if the older carb versions were prop rated figures and the later mpi were flywheel rated ? Maybe for another time…haha.

It's a mix bag. The MIE ratings are different then the MCM models.
This may help a bit and just shows MCM's from manual 16 top and manual 23 bottom



Use this link to review parts
https://www.mercruiserparts.com/v-8-sterndrive-engines

 

[QBhoy]

Might need to apologize to @Westerndiesel25 for getting off topic on his thread



It's a mix bag. The MIE ratings are different then the MCM models.
This may help a bit and just shows MCM's from manual 16 top and manual 23 bottom

View attachment 393341

Use this link to review parts
https://www.mercruiserparts.com/v-8-sterndrive-engines

Very helpful AD. Appreciated! See there is a difference in compression ratios they’re right enough. Answers a few questions for sure. Cheers.

 

[04fxdwgi25]

My boat has a 454 Mag / 4 bbl installed.

 

[tpenfield]

The OP only hung around for an hour after registering and posting (so presumably they saw Scott's initial reply). So, for now there has been talk amongst ourselves.

I agree a Gen6 BB would be the way to go. I've owned the Mark IV and the Gen6 . . . really liked the Gen6. Not sure how much of the fuel system from the cracked block could be swapped over though. Staying with a carburetor would be easiest.

IIRC, the exhaust is the same.

 

[Scott Danforth]

If the old engine and new heads are both oval or both rectangular. The whole induction can be reused. The exhaust can also be reused.

With a small spacer, the whole accessory drive can be used