Purely Theoretical - Merc 4.3 V6 swap out for 5.7 V8

[jasoutside]

So I know this fella. He is currently struggling to get his 1989 Mercruiser 4.3 V6 running right. He has a few more steps to take and tests to run on this engine but he's running out of possibilities (and patience).

Well, let's say for the sake of argument he had this good running 5.7 V8, of the same vintage, sitting off to the side. Let's also say he has considered yanking that V6 right out of there, sending it to the bottom of the lake, and replacing it with this V8. He understands that the engine mounts will need to move forward a bit, no problem. He gets the dog house may need to move forward a bit, no problem.

But, he might have a few other things he's wondering about.....

1. Can he use the same transom shield and bell housing that's already there? Are they interchangeable between the V6 and V8?

2. Will the Y pipe for the V8 mate up with the transom shield original to the V6?

3. He recognizes that the V6 outdrive gear ratio will be incorrect for the new V8 replacement. (1.81 for the V6 vs. 1.47 for the V8) However, he has a brand spanken new SEI drive (1.81) so it would be pretty difficult to sell that off at a major loss right away. What could/would be the consequences of running that 1.81 drive mated to the V8 replacement? He's not a speed racer or anything like that, easing into the throttle is more his style. He also recognizes he'd need to prop up to keep the RPM's reasonable.

Thoughts or advice for this guy?

 

[Bt Doctur]

1 Yes
2 Yes, part numbers for the Y pipe are the same 307130A4

 

[redneck joe]

A 'freind'......

 

[jasoutside]

1 Yes
2 Yes, part numbers for the Y pipe are the same 307130A4

Awesome, I was thinking that would be the case.

And how about #3 the outdrive question?

A 'freind'......

Yah, this guy that I know.....he's super handsome.

 

[thumpar]

You might beable to compensate with a prop but those 2 drives are 2 steps off of each other. Even if you can find a prop that gets the numbers right it may not be as efficient.

 

[Rick Stephens]

I just sold a year and a half old SEI. Sure , it cost me a few hundred in the end, however, I'm also set up correctly and not having waste hundreds on props that are mostly outside of normal operating range.

 

[Scott06]

What prop is on the boat now? If it is a heavy boat running a 17" pitch you might be able to get a 21" or 23" to work. If it's a light boat that runs a 21" now you'll run out of options and over rev unless the drive gear changes. You can get an aluminum 3 blade for about $125 try it and buy the drive if it doesn't work.

Safest is to buy the correct drive and sell the old one like Rick did.

 

[jasoutside]

I just sold a year and a half old SEI. Sure , it cost me a few hundred in the end, however, I'm also set up correctly and not having waste hundreds on props that are mostly outside of normal operating range.

I wouldn't mind losing a few hundred. I fear it would a lot more than that though.

What prop is on the boat now? If it is a heavy boat running a 17" pitch you might be able to get a 21" or 23" to work. If it's a light boat that runs a 21" now you'll run out of options and over rev unless the drive gear changes. You can get an aluminum 3 blade for about $125 try it and buy the drive if it doesn't work.

Safest is to buy the correct drive and sell the old one like Rick did.

I have a 19 on it right now which gets me to 4800 (when the engine is running right). I have a 23 on hand here so I guessing that might get me in the ball park.

 

[Scott06]

Probably a 23 will work is try it and buy the drive if you have to

 

[Bondo]

Ayuh,.... To prop it, I'm guessin' yer gonna need a 25" or larger,.....

 

[jasoutside]

Yah, that seems unreasonably big. That would prompt a sell off and replacement drive.