Another v6 to v8 swap question

[goodboy]

I'm so sorry if this has been covered... Honestly I think I've read every thread on v6 to v8 swaps lol

So I've got a 1991 21 foot prowler boat made by cooper yacht... It's very similar to a mirage.

It's time for a new motor and I've done a lot of research... But have a couple questions... Let me say that it now has a 4.3 with a mr drive... And I'll be installing a 5.7 vortec (97 roller motor)
I have a good grasp of requirements it get the motor marine ready... And I will be installing a new outdrive. I have the room for the engine ... But the pedestal has to be enlarged/ lengthened to accommodate the v8.

So I know that I have to add to the pedestal... Just wondering what the best way to go about it?

And I read one thread where the owner also added to the top of the existing pedestal... Saying the the mercruiser v8 mounts sit a bit higher... I believe he added a 2x4 to the top..


Any information is a huge help.

Thanks,
Corey

 

[Bondo]

So I know that I have to add to the pedestal... Just wondering what the best way to go about it?

Ayuh,..... Mount the motor's rear mounts, hang the front of the motor on the hook, slip in an alignment bar, 'n adjust to Perfect,....
That's the point you need to build out the hull's motor mounts,.....

Every hull is different, so I can't say how to go about it at this point,.....

 

[Meames1]

The above is correct, but I'll add one correction to your post.
The engine does not sit higher for a V8. The engine dimensions / mounts from a 4.3 to 5.7 are exactly the same except the 5.7 is 4" longer.
I just did this swap.

 

[goodboy]

Ok thanks....

I have the 5.7 on the engine stand... Going to pull the v6 hopefully soon...

Looking at new motor mounts online it seems like the v6 are the same as the v8s... Maybe I can reuse my v6 mounts.

What's the best way to add to the pedestal? I'm thinking lag bolts from the bottom of the boat?

Haaha... Jk.. But I really don't know how to go about it. Thinking sand paint away from existing... Make block of wood... And glass it in. Is it that easy?

 

[Meames1]

Ok thanks....

I have the 5.7 on the engine stand... Going to pull the v6 hopefully soon...

Looking at new motor mounts online it seems like the v6 are the same as the v8s... Maybe I can reuse my v6 mounts.

What's the best way to add to the pedestal? I'm thinking lag bolts from the bottom of the boat?

Haaha... Jk.. But I really don't know how to go about it. Thinking sand paint away from existing... Make block of wood... And glass it in. Is it that easy?

I used the "V6" mounts. I believe they are the same, regardless, I used the V6 mounts, all the accessory brackets, Alternator, PS pump, etc. etc.
One thing to remember, the 4.3 V6 is Literally a Chevy V8 with one set of cylinders removed. Not much engineering happened when GM made it. lol

 

[goodboy]

Thanks for the replies guys... Much appreciated.

So back to adding to the pedestal... Anyone have any tips? Not really sure what to do... Just make a block of wood that fits and fiberglass it in?

 

[Scott Danforth]

you mean the motor mounting blocks? then yes, cut out the old, hang and align the motor and then build new motor mounting blocks and glass them in to come up to the new motor mount location.

 

[goodboy]

you mean the motor mounting blocks? then yes, cut out the old, hang and align the motor and then build new motor mounting blocks and glass them in to come up to the new motor mount location.

Why would you remove the old one? Can't you just add to it?

Could I not just match the existing motor mount blocks in height?

I won't be doing the install so I was hoping to get the wood and fiberglass work done before the engine goes in

Don't doubt you just wondering... I appreciate your input.

 

[Scott Danforth]

You dont just add material, you want the motor attached to something solid. If the mount fails, usually the motor moves enough to cause issues......like sinking

 

[goodboy]

You dont just add material, you want the motor attached to something solid. If the mount fails, usually the motor moves enough to cause issues......like sinking

OK...

There seems to be some confusion as to what I'm trying to say... So I'll try to explain it better...

I have to build a new/longer spot to bolt my engine down...

From my understanding the difference between a v6 and a v8 is roughly 4ish inches. So what I need to do is extend the area where my existing motor sits another... I'm guessing 5+ inches.

I've read that some people have just added on to their existing motor mount pads by adding a block of wood and fiberglassing over it... Paint it and bolt the engine down to the new spot...

So yes I need to add to my existing... I'm not dumb... I truly understand that the engine needs to be bolted down to something substantial...

I came here wondering if the solution I read about (adding wood and fiberglassing over it was the correct way... Or if there was anything else to consider.

I'm not the first person to do this... Hopefully one of you fine fellas can steer me in the right direction

 

[goodboy]

Can anyone steer me in the right direction?

 

[Chris51280]

yes, grind away the the gelcoat. use thickened resin and glue in the extension block. then glass over it with at least 3 layers of 1708 tabbing it in the the side and the bottom hull.
Block needs to be solid seated on all 3 sides

 

[goodboy]

yes, grind away the the gelcoat. use thickened resin and glue in the extension block. then glass over it with at least 3 layers of 1708 tabbing it in the the side and the bottom hull.
Block needs to be solid seated on all 3 sides

Thank you!

 

[goodboy]

Thank you!

What do you guys think of maybe a couple of 4 inch screws countersunk... In the new block of wood into the old?

I don't know the depth of the lag bolts holding the engine... But maybe an inch or so below the depth of the lag bolts... I'd use the coated decking screws... Idk.

Thought's?

 

[Chris51280]

no screws needed if you tab in the new block correctly. grind the gelcoat up so you get good adhesion

 

[goodboy]

Ok thanks... Appreciate the help!

 

[apw30534]

I know this is an old post but I am confused by the answers given.

It was always my understanding that the distance between the motor mount bosses and the bellhousing are exactly the same for Chevy small block, big block AND 4.3 V6... which means this should be a direct drop-in swap, with the exception of the front of the engine (forward of the motor mounts) will be 4" longer.

Can someone clarify ?

 

[Lou C]

I’m thinking that the OP is saying that his motor mount pads are long enough for the V6 but not for the 4” longer V8 so he needs to extend them forward. Some boats like mine were built to take either engine so the original pads are long enough. I know the rear mount spacing is the same for the V6 & V8 small blocks but not sure about the big blocks. You might be able to figure this out by comparing part #s of the flywheel housings.

I’m thinking of the same swap when the time comes. My OMC Cobra drive ratio is 1.62:1 so I think it will work with the V8 with a prop with more pitch.