Engine swap questions

[jallen100]

I am fairly new to boating so this may seem like an obvious or dumb question but here goes.
I have a 1991, 21.5' Thompson with a Mercruiser 4.3L and an Alpha drive.
What would be involved in swapping the 4.3 for a small block chevy outside of obviously changing the mounts and what not?
Is there anything on the motor that has to be specific for use on a boat?
Do I need to change anything over on the drive?
Is it even worth while to swap it?
Its a fairly big boat and I feel its a little under powered with the 4.3. It does have a 4bbl carb and runs about 38-40 wide open, but thats buzzing the motor pretty good. It just seems to take a while to get on plain and get up to speed. I did switch from a 17 to a 19 prop which seemed to take even more away from it but it was at around 5600 rpm's all out with the 17, although it did get on plaing much faster.

 

[Bondo]

Re: Engine swap questions

What would be involved in swapping the 4.3 for a small block chevy outside of obviously changing the mounts and what not?

Ayuh,.... At a Bare Minimum, ya need a fully dressed Marine 350, 'n regear the drive ratio...

Is Lightenin' the Load a possibility,..??
Lotta junk accumulated,..??
Hull growth,..??
Wet foam,..??

 

[jallen100]

Re: Engine swap questions

Ayuh,.... At a Bare Minimum, ya need a fully dressed Marine 350, 'n regear the drive ratio...

Is Lightenin' the Load a possibility,..??
Lotta junk accumulated,..??
Hull growth,..??
Wet foam,..??

I guess that is my main question. What makes a marine 350 a "marine 350"? What is so different from a normal automotive engine?

I dont think I have much room to lighten the boat at all. Its just a big, heavy boat. The boat alone has to weigh at least 3,000 lbs.

I am kind of wondering why that boat would have even came with a 4.3 option. I mean its not terrible but it could deffinitly use the boost.

 

[Bondo]

Re: Engine swap questions

I guess that is my main question. What makes a marine 350 a "marine 350"? What is so different from a normal automotive engine?

I dont think I have much room to lighten the boat at all. Its just a big, heavy boat. The boat alone has to weigh at least 3,000 lbs.

I am kind of wondering why that boat would have even came with a 4.3 option. I mean its not terrible but it could deffinitly use the boost.

Ayuh,.... You seem to be alluding to the pickup truck motor swap theory of boat motors...

When you lift the hood of a pickup truck,...
That motor, "Looks" little to Nothin' like the boat motor it could possibly become...
It's all in the Dressin's of a long block motor...

You need a fully dressed, Runnin', marine motor to do yer dream....

 

[jallen100]

Re: Engine swap questions

I think I found my answer. Basically every accessory attached to the motor is marine specific and the heads and cam are set up a little different for the torque curve.

 

[jallen100]

Re: Engine swap questions

Best advice I could give is to sell the boat you have and buy the boat you want. I'd bet a dollar to a doughnut it would be cheaper in the long run.

I really like my boat. I just wish it had a little more power. Just seeing if the motor swap was even worth considering.

 

[Bamaman1]

Re: Engine swap questions

It sounds as if a 19 pitch prop is a better way to go, as 5600 rpm's is probably way above redline for a 4.3. You've got a very good motor, and you probably have the higher horsepower version if it has a 4 barrel carb.

I suggest you spend $469 for a set of Bennett hydraulic trim tabs. (They're available @ Overtons.) They'll make that big ole hull plane out @ 10 mph, and your boat will pull like it's got 50 hp more. They're easy to install, and make all the difference in a full hulled boat. No more than they cost, I think every boat like yours should have them. After you get up to speed, you retract them. If you get a couple of fat women sitting on one side, you can adjust the tabs with the switch to level out the boat. I've got them on my inboard-outboard, and they're great.

 

[wca_tim]

Re: Engine swap questions

I don't mean to fly in the facce of wisdom, but advice I've been given and my experience as a result are:

of course you need appropriate spec marine long block, exhaust manifolds, intake manifold

your drive will be just fine - you'll be running something like a 22 - 24 inch prop I would bet

your "accessories" will work if you do it right - starter, alternator, power steering pump,fuel pump, brackets, morot mounts (they'll be moved about 3 inches forward), wiring harness, probably your 4 bbl carb,

that being said, it's neither trivial nor a simple swap - it is however very doable on a reasonable budget.

 

[Bondo]

Re: Engine swap questions

I don't mean to fly in the facce of wisdom, but advice I've been given and my experience as a result are:

of course you need appropriate spec marine long block, exhaust manifolds, intake manifold

your drive will be just fine - you'll be running something like a 22 - 24 inch prop I would bet

your "accessories" will work if you do it right - starter, alternator, power steering pump,fuel pump, brackets, morot mounts (they'll be moved about 3 inches forward), wiring harness, probably your 4 bbl carb,

that being said, it's neither trivial nor a simple swap - it is however very doable on a reasonable budget.

Ayuh,.... If yer doin' it Yerself....

Payin' somebodyelse to do it, ain't cost effective...

 

[wca_tim]

Re: Engine swap questions

Ayuh,.... If yer doin' it Yerself....

Payin' somebodyelse to do it, ain't cost effective...

Amen to that! As others have inicated above, it would be far more cost effective to buy another boat than to pay someone to upgrade the engine! If you don't like "tinkering", and just want to run the boat then either make some very minor upgrades, prop, tabs, etc... or buy another boat. If you've already got a 4 barrel carb on your 4.3 (and vortec heads? if so, there's not a lot you can do to increase the power output of this engine without getting silly), there next step is a bigger engine... minimum 2500+ if you do the work yourself. and don't get me wrong above - it isn't a one weekend job for the normal human being. Hope this is helpful!

ps. If I didn't like tinkering every bit as much as time on the water, I would NEVER have done some of the things I've wound up doing on my toys.

 

[jallen100]

Re: Engine swap questions

I was a Crew Chief on a Drag Race team for 10 years and worked at a race car shop, so I have a lot of experience with engines and mechanics in general. Just never on a boat. If I were to tackle it, then I would do it myself. From what I have read the water pump needs to be different and all the electronics must be sealed. Other than that I am not seeing a whole lot of difference other than a camshaft and maybe detune the heads a little bit.

 

[wca_tim]

Re: Engine swap questions

it'd be no problem for you to do if you want to spend the time...