Swapping a GM 302/350 for a Mercruiser 485

I don't know beans about inboard/outboards but, after some research I think I know enough to venture a question on engine swaps. (Searching/Googling didn't answer my questions.)

I'm looking at a 1980 Glastron-Carlson CV-27 with twin 1980 Mercruiser 485's. I think I'll want more power later on.

What would be my best option for a bolt-in repower?

Will a 1980 GM 302, 327 or 350 bolt up to the outdrives currently on the 485"s? (I understand I'd need marine components on/in the engine.)

What do you think would be the best engine swap as far as HP/Weight ratio and fuel economy?

Any help will be greatly appreciated.

Mark

Re: Swapping a GM 302/350 for a Mercruiser 485

twin 4.3's would allow reuse of the current drive ratios- the transom plates (terminology?) are different though.... you'll be best served with a donor boat- or 2 donor boats- you need the bellhousings, so basically.... complete bobtail engines.

Re: Swapping a GM 302/350 for a Mercruiser 485

kjdkjd said:

twin 4.3's would allow reuse of the current drive ratios- the transom plates (terminology?) are different though.... you'll be best served with a donor boat- or 2 donor boats- you need the bellhousings, so basically.... complete bobtail engines.

Click to expand...

Thank's! That's just the kind of info I was looking for. I never thought about swapping in a 4.3 liter V-6. I wish I had the boat now as I just found two 4.3 Mercruiser takeouts with all accessories for $500 each. Even if they didn't have the bell housings they'd be worth $1000.

Thanks for the info. Now I'm going to try and read about engine/outdrive matings and learn about bell housings.

Mark

Re: Swapping a GM 302/350 for a Mercruiser 485

The 485 is another name for the old Merc 470s Isn't it,..??....

If So,...... The 4.3l is the Best candidate for the Swap,.....

But,.... There's a Bunch of "Gotcha's" in there as well......

Mostly with mounting the Motors,.....
Most, or All of Everything behind the Rear Motor Mounts will Still be Usable,....
If you can get the Motor married to it.........

Re: Swapping a GM 302/350 for a Mercruiser 485

Bond-o: I don't know if it is the same as a 470 or not. Kjdkjd just said that the 4.3l. would be the best candidate and now you said the same. I liked the idea of the V-6 fuel economy while being able to bolt it to the existing drives.

From kjdkjd I also learned that the motor has to have a bellhousing to attach it to the outdrive and the transom plates have to fit(?). Do they sell bellhousing/adapter plates and transom plates seperatley?

I e-mailed the guy with the motors. He said he had a lot of parts and drives. I ask him some questions about the engines fitting and am waiting for him to get back to me.

I'm also trying to arrange financing for the boat purchase.

I'm REAL new to I/O's. I've never owned one. But, my boat mechanic says that the Merc 485's only run about 135 HP and that the twin setup will only get the boat I'm looking at to about 35 mph. That's not fast enough for me.

An option on this boat was the Merc 260 which, I think, is a GM 350/5.7 liter. That is why I asked if a 302-350 V-8 would bolt up to the existing outdrives. I figured that if that motor was an option, that MAYBE it would bolt in in existing motor mount locations.

Finally, I have a welder with MIG, TIG & stick capabilities. I also have a spool gun for welding aluminium. I could easily fab some motor mounts. Could I fab some transom plates?

All this really gets me thinking about how to do this. I know the Alpha or Bravo drives bolt to thes engines. I just have to learn how it's done.

Thanks a Lot, Mark

Re: Swapping a GM 302/350 for a Mercruiser 485

i dont want to rain on your plan BUT drives and motors are built as matched sets

There are many different gear rations for the drives to allow them to work best with the motor there installed with

The exaust system is also custom to each motor drive setup which is a huge PITA

Anything can be done BUT it is a hard job just swaping out things that fit toghter right when you mix and match there is no guide to tell you about the OS that may come up


Tommays

Re: Swapping a GM 302/350 for a Mercruiser 485

It's not worth dumping that kind of money into a 1980 boat. Especially if you are financing it. Find one set up the way you want it.

Re: Swapping a GM 302/350 for a Mercruiser 485

While the swap can be made, the time and effort involved is substantial. So unless the "project" is part of your goal RF's suggestion is something that should be considered. Good luck either way.

Re: Swapping a GM 302/350 for a Mercruiser 485

I'm also trying to arrange financing for the boat purchase.

Click to expand...

That Really Changes things,......

As noted above,....
What I stated above Is possible,.... But Expensive.....

It's possible to Mix & Match a bunch of Junk Parts to do what you which,.... There's No specfic Adapter Plates or Nothing, No need to Fab-up a Transom plate,... It's just Hunting,+ finding the Right pieces......

Considering your History,+ Knowledge,......
I'd recommend you find a boat that Runs,+ Learn from There......
Keep in mind,.....
Mercruisers are Good,....... Merc. 470s are NOT.....
Volvos are Generally Good,.......
OMCs are NOT..........

The Size,+ Scope of this Project is Huge,... Even for an Old Hand like Me.......
It'll Bury You.......

Re: Swapping a GM 302/350 for a Mercruiser 485

So you all are basically telling me that it is almost impossible to repower this boat and that it's not worth the effort.

I can't justify buying one set up the way I want it. I won't lay out over $10000 for a used boat. I can't touch a 20'+ boat with a cabin that will do over 35 mph for anything that I am willing to spend.

I should go back to looking at outboard boats. Their designers were smart enough to make the engine mounts for any engine option they offered unlike the I/O designers who built the hull to only accept only one engine/drive.

This topic is now closed.

Thank you, Mark

Re: Swapping a GM 302/350 for a Mercruiser 485

There is plenty of used stuff around BUT twins is what is makeing it get out of control

Right on this page HT32BSX115 just found a Big Block /Bravo 3 package used at a fire sale price and it worked out great for him

If you shop like he did the same think will happen


If you start looking at and equal boat with twin outboards you will find the cost pretty tame for the I/O


Around here boats like that get landfilled everday because of the cost of the upgrade

Re: Swapping a GM 302/350 for a Mercruiser 485

I won't lay out over $10000 for a used boat.
I should go back to looking at outboard boats. Their designers were smart enough to make the engine mounts for any engine option they offered

Click to expand...

Have you priced Replacement Outboards lately,..??..??....

Your Total Budget wouldn't even cover the Repower.......

I think I'll want more power later on.

Click to expand...

Mark,........ Don't run off just yet,...... If this quote is in Fact your intentions,.....

We can talk you thru Repowering that I/Oed boat for Less than the cost of a Single High Horsepower Outboard...........

If you Truely Love the Hull,+ Intend to Keep it for a Long Time,.......
You could be starting in a Much Worse situation.........

It might have had OMC written on it.......................................d




Btw,....... It's 305,...... Not 302......

Re: Swapping a GM 302/350 for a Mercruiser 485

Bond-o said:

You could be starting in a Much Worse situation.........

It might have had OMC written on it.......................................d

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Re: Swapping a GM 302/350 for a Mercruiser 485

btw, those 485's or big 4 cylinder Fordcruiser engines aren't always junk. A few people on this board love theirs. Have a good look at them - they make V6 kind of power with the 4 barrel and 4 barrel version of cam.... closed cooling, and you already own them... might be worth evaluating...doing the alternator conversion on one etc....

Re: Swapping a GM 302/350 for a Mercruiser 485

Howdy from Canada!

I'll second that one kjd... (their is in fact still a lot of happy people screamin' around with their old 470's)

Besides being know as a pile of sh*#!... the 470 was also called (if memory serves) the 165/170/485/488/190.
It was the only marine inboard/sterndrive engine block that Mercruiser produced and was in production from 1976 to 1989...

Apparently they built a bazzillion of em'.

My guess is Bond-o is a marine mechanic, and at the risk of being admonished by him (and with all due respect to him and his great help to so many...) I am inclined to agree with you.

Of course, all this could be a moot point if the original poster has bailed.

I owned a 470 recently and though I sold the boat last month (and am on to another project) I was excited about getting the 'big-four' into service. There's a '470-Merc' forum I learned about (Breezeworks) started by a guy down in Florida that I participated in for several months. It's chaulk-full of 470 owners who share their troubles/triumphs with these engines. I haven't participated on the forum for nearly a year but if it's still going, I highly recommend it for all 470 owners. I learned a lot about how to deal with the issues around this engine from those people... like (as kjdkjd pointed out)... changing out your cam shaft seals; the importance of running the 4-inch heat exchanger; doing the alternator conversion etc.

There was a guy (from Scotland, if memory serves me) who tricked his (4/bbl. version) out with roller-rockers, a Petronix electronic ignition conversion, then ported/polished the head (himself) and claimed he was pushin' 225+/hp...

Imagine what would happen if a guy threw an aluminum Ford Motorsport 429/460 head at one of these.

Think about that, then ask yourself - historically, what other sterndrive engine eclipsed that power-to-weight ratio?

Cheers,
Terry

Re: Swapping a GM 302/350 for a Mercruiser 485

Hey Bondo, there is nothing wrong with an old OMC-if you get an old OMC mechanic to teach you, before they all retire.