4.3 to 5.7 engine swap

I purchased a boat with an OMC 4.3 ltr eng and out drive. The engine is bad and I have a 5.7 ltr I would like to install. I know I will have to get different exhaust manifolds and other things to make this work. Any info on making this transplant happen will be appreciated.

One thing I did notice is the flywheel and coupler on the 4.3 have a smaller bolt pattern on them than the 5.7. Is this gonna be a problem or is there a coupler I can get to make this work? Thanks for any info!!

Re: 4.3 to 5.7 engine swap

Howdy,

You can absolutely make it work.

Just off the top of my head you'll need:

1. flywheel
2. coupler
3. manifolds/risers
4. possibly a flywheel housing if yours doesn't fit the 5.7 (It should be the same)
5. relocate the front engine mounts
6. any accessories (or brackets) that won't work or fit on the 5.7L engine
7. possibly a different drive (ratio)





Are both marine engines? There also may be an issue with what type of rear seal the 5.7 engine has....


There may be more "stuff" to consider...... Anyone else?

Re: 4.3 to 5.7 engine swap

There may be more "stuff" to consider...... Anyone else?

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Sounds like about 3k or more in parts even with a already available engine.. Be a cool project if ya got nothing but money to throw around..

Re: 4.3 to 5.7 engine swap

Hey guys, Thanks for the info. Here's my delima. The 4.3 in the boat is junk. So do I replace that one (which is about $2000 if I'm lucky) or do I Use my 5.7 (Ebay is an awsome place to purchase good used parts), I've already got a list going and so far it's not to bad and the 5.7 does mount to the bell housing.....
I guess I need to know if it is going to be worth it to do the swap? Again any input is welcome. Thanks!!!

Re: 4.3 to 5.7 engine swap

Used exhaust manifolds and risers is a waste of money. You are usually buying someone elses junk.
No one mentioned the gear ratio change in the drive.
While ebay may be awesome, but the quantity of different parts needed for the change is going to offset any savings of just getting a rebuilt long block.

Re: 4.3 to 5.7 engine swap

My two cents worth,
Do not put a high horse v8 against that drive, If you go with a 5.7 go with a stock built.
4.3 vs 5.7 IMO 4.3 will probably cost you less, will use less gas, not as heavy, and if using OEM type exhaust sounds the same (both are exceptionally quite), all of this for only a minor HP and torque gain (minor due to your drive limiting your options).
But if your like me and just like doing things out of the norm then go for it, Please post your findings, as of next season I plan on taking a perfectly good 4.3l omc out and placing a 5.7l in its place!
Good luck

Re: 4.3 to 5.7 engine swap

Thanks for the info guys. I will keep you posted on the project. As far as gear ratio goes, Whats the difference? Both engines turn the same rpm? Is it a torque issue? Just asking.

Re: 4.3 to 5.7 engine swap

By the way Joshua N. I like your profile pic. Sweet!!

Re: 4.3 to 5.7 engine swap

It would be a lot easier if you had a later model 5.7 with a one piece rear seal. I don't know if they make a readily available coupler for the older series of engines. You are correct, they turn the same RPM but the gearing in the drive allows that. I don't see an issue strength wise, the same basic drive setup was used from 4.3 -5.8 with the exception of the gear ratio, the rest of the guts are the same. I wouldn't test it with 350 HP, but 300 or so from a stock type 5.7 I would be comfortable with, just watch the high powered hole shots. With the gearing of the 4.3 outdrive it will probably allow the V8 to over rev easily, you will have to watch that. You can go with a bigger prop, but you can only make up for some of the difference that way.

Re: 4.3 to 5.7 engine swap

They do make a coupler part #38965A 2

Re: 4.3 to 5.7 engine swap

The 4.3 is a 1.68:1
The 5.7 is a 1.41:1

By the way Joshua N. I like your profile pic. Sweet!!

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Thanks.. I figure if you can have a day out with the boat where such doesn't happen to ya.. It's been a good day...

Re: 4.3 to 5.7 engine swap

Joshua Nichols said:

The 4.3 is a 1.68:1
The 5.7 is a 1.41:1

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Your top speed performance with either engine using the 1.68:1 drive will be the same (except for the additional weight) at the same RPM if you change nothing.

If you're using a 21p prop for example (which might be little "tall" for a 4.3), using a prop-slip calculator http://www.go-fast.com/Prop_Slip_Calculator.htm

you'll get somewhere around 50 MPH at 4800. (1.68:1, 4800, 15% slip)

But because you'll have more torque available, you will probably be able to install a 23 (3 blade) or 24p 4 Blade prop and still be able to turn 4800(or more) at WOT.

That'll possibly add 5 or more MPH to the top speed but give you a better hole shot.

There's a limit to all this stuff since propellers are efficient in a fairly narrow range of RPM.

You may have cavitation if you turn the prop too fast. You'll have to experiment.

It might be a better choice to just rebuild/replace the 4.3 unless you can find a complete (with acc) 5.7L engine "on the cheep"!

No one mentioned the gear ratio change in the drive.

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I thought I did in my first post....