Mercruiser 3.7L/Alhpa 1 to 4.3L chevy swap...

[chevy-aholic]

Hey guys, I'm new on the site and I need help on a swap that I have in mind. Now, I'm not really a boat guy all that much, but rather I build performance cars and street vehicles. Now the deal of a lifetime fell into my lap not to long ago....an 18' celebrity with the 'imfamous' 3.7L mersruiser for only $500! We have tried for a while not to get this finnicky motor to behave, and most of the time it doesn't do too bad, but after paddling this whale into the dock for the second time today I just want to be done with it!

This is the part where I need you help. I have in my shop an array of engines left over from past projects. Among those are a couple of Chevy 4.3L 90 degree V6's. What I am wanting to know is if the alpha 1 drive will mate up to one of my 4.3's OK. I know that I am going to have to make a custom mount to attach to the front of the motor, but that's well within my bag of skills. the plan is to re-wire this boat similar to a car (3-wire hookup) and if possible do away with the emissions altogether. I was also wondering how I would route my exhaust. I know that to make the outdrive happy, I need to incorporate the water cooling into the exhaust; and to make my neighbors happy I need to refrain from running tuned headers out the back!

thanks guys, great site!

 

[Don S]

Re: Mercruiser 3.7L/Alhpa 1 to 4.3L chevy swap...

None of them were ever designed to interchange with each other. There will always be glitches you have to work arouind.
Unless someone has used EXACTLY the same engine and drive switch, then all bets are off as to how it all fits together. To many minor and major changes over te years.

 

[chevy-aholic]

Re: Mercruiser 3.7L/Alhpa 1 to 4.3L chevy swap...

Well, lets take this step by step......

1) Is the bolt pattern from a GM V6 the same as a Mercruiser V6? (are they the same motor?)

2) Does a Mercruiser V6 fit the same (Aplha Gen 1, 1986) outdrive as the Mercruiser 3.7L?

3) How does Mercruiser couple the engine flywheel to the Gimbal? Is it a simple "coupler" to convert from the flywheels' bolt pattern to the driveshaft's splines?

4) What is the maximun HP rating a Aplha Gen 1 outdrive can handle?

I am able to have quite a few parts custom machined or even one-off fabricated through a couple connections of mine. I guess the main thing is if the GM motor will even bolt up to the outdrive. If it won't then this idea is dead in the water.

 

[Don S]

Re: Mercruiser 3.7L/Alhpa 1 to 4.3L chevy swap...

1) Is the bolt pattern from a GM V6 the same as a Mercruiser V6? (are they the same motor?)

Bolt pattern for what? Flywheel cover? Yes the Merc and Chevy bolt pattern are the same.
But the 3.7 is not Chevy and the flywheel cover does not have the same bolt pattern.

2) Does a Mercruiser V6 fit the same (Aplha Gen 1, 1986) outdrive as the Mercruiser 3.7L?

Yes

3) How does Mercruiser couple the engine flywheel to the Gimbal? Is it a simple "coupler" to convert from the flywheels' bolt pattern to the driveshaft's splines?

Yes, but the coupler from a 3.7 will not fit a 4.3. And yes they get expensive.

4) What is the maximun HP rating a Aplha Gen 1 outdrive can handle?

300 hp seems to be the magic number, but don't push it. It's still a small rubber coupler and an Alpha drive. Merc learned that a 330 HP big block didn';t work. So they invented the Bravo drive.

You forgot to ask about how you could adapt the exhast Y or the stringers as motor mounts instead of a front mount, or the different exhaust manifold seti. or all the other good stuff that will come along.

 

[chevy-aholic]

Re: Mercruiser 3.7L/Alhpa 1 to 4.3L chevy swap...

I'n not as worried about the exhaust as the physical mating of the motor to the outdrive. If I have to, I can run shorty car headers and some small mufflers and run them out the back....my neighbors will hate me, but oh well. I am mainly concerned that a GM V6 will bolt up to a merc. outdrive. I have a cool idea in mind to mount the engine into the boat. My boat is only setup for the front, cast-ironmount for the 3.7L..so I was just gonna make a plate to mount to the front of the block (the 4.3) and then two soft rubber mounts and a solid plate going to the hull. It's not a huge geal if the flywheels' bolt pattern isn't the same (I kinda expected it not to match up there) as I can have a friend of mine machine whatever adapter I need and then broach the inner bore for the driveshaft spline. Does this coupler have to be made out of rubber, or could I fabricate it to rubber mount to the flywheel and have hardened steel splines? I was planning to keep the engine relatively mild, only around 210-240 HP with a mild cam. I didn't want to go crazy cause I've heard thet marine applicationa eat up high peformance engines. I guess it's worth asking.....what kind of emmissions are there with boats that I should be concerned about? I at least would like to know what laws I'm potentially breaking before an unhappy DNR officer has to explain them to me...LOL!

I know it sounds like alot of custom work, and a rather huge P.I.T.A, but I am very use to these sort of hurdles while building cars. The only thing I can't fix is the motor not bolting up to the outdrive. Thanks again for the quick replies tho.