engine swap

I have a 18.5 ft pleasure boat with a 3l GM 4cyl & cobra stern drive. I
would like to swap-in a 5.0 Ford V8. Aside from losing some space at
the back of the boat it seems my biggest concern is chevy vs ford bolt
patterns. what is available for the potential conversion or do I have a
fabrication project on my hands changing from chevy to ford products?
Where do I find max torque/power handling capability of my cobra
stern drive?

 

bolt patterns may be the least of your problems.
Before anything else, check out the weight difference. A 5 L V8 is usually heavier than a 3L 4 cyl. and your 18' might not be able to handle it.

 

Welcome to the forum.

I'd be more concerned about that Cobra. I don't have a very good opinion of them, having worked on too many with the same set of issues. Gear ratios will be different for a V8, then the 181 L4 GM engine.

If the drive is in exceptional condition maybe (I've never seen one), otherwise consider leaving it as is, using it and having fun or find a better unit, possibly a new boat, with something isn't a 1/4 of a century old and long out of production, which was fairly short anyway.

 

Yes, even if it could all be happily mated up mechanically, you have a lot more iron back there, too much for an 18 footer, most likely.

 

Figure out how to get more hp out of present engine.

 

There are plenty of 18'ers with small block outdrive power.

 

Yes but chance are if they were designed for 4 cylinder then an 8 we tough. Outdrive won't handle it. An outboard might way to go

 

The 400 gear case has smaller bits than the 800 (V6/V8) case, so . . . This drive is designed to fit the Mercruiser transom cutout.

 

good info, thanks. In my area, I'm ok with a big 4 cyl. I haven't had the
boat long. Its in great shape but for the engine. The block is cracked. The
previous owner tried to "patch it" and made the matter worse, ha. I have
a spare V8 at the moment and am anxious to get this thing on the water.
I'll stay with a replacement 4 and have the drive gone over.

 

Would a 4.3 V6 Mercruiser bolt up to it? There are a lot of them and the Crate version with most of the marine stuff on it is around 7K

 

PAR has good info! Yes, the Sunbird is a Bayliner, Chapparel, etc. type of boat. It's probably made to be from concept. I live in/am from SC, so the brand caught my attention
right away. The boat itself is in grand shape; not much more than age has degraded it cosmetically and NO hull damage inside OR out (my main buying decision). I'm the 3rd owner. I've never owned a boat in my life...several cars, no boat (wife, home, kids, first).
I'm sorry to hear the drivetrain has a low reputation...I tend to believe THE OWNER has
much input on that as with a car!!! haha...whew!!! See my info about the boat's
cracked block?...what a dork!
There are some good big lakes/reseavoirs not too far away. In the upstate SC county I live in, 35-40 hp is the MAX power you can put on the water! In several area's, a
trolling motor, IF THAT is IT!
Some boats of that design came with V8's. Give or take adjustments in its max seating
capacity, one could easily compensate for the difference between a 4 and an 8.
The boat is capable of 85knots plus? folks, 100mph is fast enuff 4 me! I've seen guy's
stand those things on their stern on take-off, too...plenty of power there as I see it.
El Gringo has a good idea with a V6 swap but that returns us full circle to my idea of
saving money with an engine to begin with...I have a V8 around with nothing to do
and it didn't come out of my grand-pa's chevy caprice last year just because he died.
I can get more power out of the 4 if I want it...

 

Ford got out of the marine market years ago, so locating parts to marinize that motor will be pricy and possibly difficult. Then the outdrive won't be up to the task and would need to be upgraded to a different model, you’re probably looking at a few grand minimum to convert it.
It’s far cheaper to sell what you have and buy what you want.

 

Again, the 400 series lower gear case has smaller, lighter parts, than the 800 series (V6's and V8's). If you put a V8 on a 400 series OMC, you just blow up the gear case, so good luck with that. The Sunbird was never capable of 85 knots, not even close. Mid 30's MPH with a V6 and low 40's MPH with a 302 V8. Some prop and performance upgrades can get the small block versions into the high 40's, but that's about it. The 4 cylinder versions hit the low 30's at best.

 

There is an upper v8 OMC gear case and I ended up finding one years ago when I went from the early OMC V6 of the sixties to a chevy 350 V8.

The upper v6 gear case did strip its gears when run with the v8 350.

I also had the prop pitch increased by 2 points as otherwise the v8 over revved to 6000 plus rpm.

That boat sits here unused now for about 15 years when I got the bigger sedan cruiser.

When I got the v8 gear case, it was complete with lower case.
I completely tore it down and repaired whatever it needed including a new rubber impeller and shaft seals.
And a new piece of teflon sheet for the lower bearing.

After repairing it all, I decided I really do not like the OMC stringer design, too many failure points.

My lower gear case is the long leg version suitable for deep v style hull.
And i used marine grease on the SS bolts so they would not corrode the aluminum housing and get stuck.
I found heat from a torch the only way to get the SS bolts loose.

 

Don't even start me up on the Stringer and Cobra OMC design issues . . .