Can I replace a Mercruiser 5.0L with a 4.5L engine?

[AnthonyT]

I have a 26ft SeaRay with a seized Mercruiser 5.0L 350HP Bravo III sterndrive engine. Since they don't build these anymore I need a different replacement engine. I'd prefer to go down in size to a 4.5L 250HP Bravo III sterndrive engine. Will it fit? Will it be compatible with the current electronic starting setup? If I am buying the engine do I need other parts to make it work?
I'm not an engineer and I welcome any advice.

 

[dubs283]

In your case it'd be easier and cheaper to get a 5.7 long block

 

[Earl Cordova]

That drop in horsepower will really slow down your boat.
Plus as already mentioned you are better off just getting a rebuilt motor with the same specs.
No use reinventing the wheel, especially if you don't know exactly what you are doing.

 

[stresspoint]

first off let me say , i like your thinking , from what i read the 4.5 is a great little motor and very economical i don't know in a 26 foot searay how good economy you will get but a dare say better than the 5.0 .
BTW i would like to see that 350 hp 5.0 ,

the 4.5 is the same footprint as a 4.3 , you will most probably need to do lots of cobbling to get the older electrics to talk to the new stuff.
there is also engine mounting points that will need to be changed , not a simple task.

? can it be done A ; yes but why would you want to when there are better options , like as posted a reman 5.7

 

[Scott Danforth]

I have a 26ft SeaRay with a seized Mercruiser 5.0L 350HP Bravo III sterndrive engine. Since they don't build these anymore I need a different replacement engine. I'd prefer to go down in size to a 4.5L 250HP Bravo III sterndrive engine. Will it fit? Will it be compatible with the current electronic starting setup? If I am buying the engine do I need other parts to make it work?
I'm not an engineer and I welcome any advice.

First, Welcome aboard

Second, the most power from a 5.0 was 260hp out of the 5.0 MPI. the most power from the 5.7 liter was 300hp, the most hp out of the 6.2 liter from mercury is 320hp.

350 hp is non mercury small block or big block territory. the 4.5 is a V6 producing 250 hp. however if you look at the prop curve for your boat and the torque curve of the motor, you will see you need two 4.5's or a 6.2 liter to get you the power to get out of the hole

most likely your boat has a 350 (5.7) or a 377 (6.2). if it was me, I would build a 383 stroker (0.030" over 377) and go that route.

using proper pistons, cam, intake and you can get 350-400hp out if it, however more importantly, about twice the torque of the 4.5 liter V6

 

[Chris1956]

A 26 footer with a single I/O needs to be a big block, or it will be a dog, in my experience. A Bravo should be able to handle the 330HP from a BBC.

 

[Lou C]

Also check the parts prices for that Merc 4.5, when Mercruiser took the old cast iron GM 4.3 design and slightly re-engineered it, they made it more complex and jacked up the price of many parts 2-3x what they were in the past. I wouldn't reward Merc's greed and BTW, you can still buy brand new GM long blocks, 5.0s and 5.7s. It would be a foolish waste of money to go with the 4.5. If you're buying a newer boat and that's what it has, fine, but would I choose one over the older tech engines, no way....I don't see the need or desirability of complicating things that much.
For what these engines cost, and the parts price, including the fact that they have to have cat converter exhaust, IMHO they have priced themselves out of the common sense market. They make outboards, look like a good deal financially. Because while an outboard price and parts price is very high, the maintenance is lower by a lot, especially for boats kept in salt water and outboards have safety advantages over I/Os in a few important ways.

 

[Bt Doctur]

6.2 I think would be the best, a drop in motor

 

[Scott06]

I have a 26ft SeaRay with a seized Mercruiser 5.0L 350HP Bravo III sterndrive engine. Since they don't build these anymore I need a different replacement engine. I'd prefer to go down in size to a 4.5L 250HP Bravo III sterndrive engine. Will it fit? Will it be compatible with the current electronic starting setup? If I am buying the engine do I need other parts to make it work?
I'm not an engineer and I welcome any advice.

Go up in engine size not down. a 4.5 would struggle in a 26 ft boat…

as mentioned a 6.2 will be a good fit And take all the accessories off your 5.0 As both are small block chevys.

Is your 5.0 carb or mpi ?

 

[QBhoy]

As said. I think you have the hp wrong there.
Anyway. It will cost you a fortune to do that idea…and end up in disappointment too. You can still get a direct replacement 5.0 or 5.7 gm engine, if you wanted. Look at the repower site.