2006 Mercruiser 3.0 Swap to 1987 Mercruiser 260

[meantobe63]

After not being able to get over 20mph with MANY attempts of fixing this, changing that, I’m swapping my 135hp 3.0 over to a 260hp 5.7. It will fit, BARELY, and will take plenty of fab work for engine mounts, adding a center toon, etc., that I’m capable of doing myself. I would never even think about doing this swap if I had to pay for it.

Here is my biggest concern.... the alpha’s are good up to 300hp is what I keep reading, that they do ok with Horsepower, not Torque like a big block has. But I’m also being told that since pontoons have so much drag, the outdrive is going to feel like it’s got more power against it. I’m not going to fully rebuild this v8 to make monsterous power, but I’d rather be safe than sorry. And who knows, once I get a taste for speed, maybe I’ll want more and find a cheap supercharger or something.

I know now it’s not going to be CHEAP, but I’m trying to make it that way. And saving money now but having to pull it all apart later to install a Bravo would cost SO MUCH later.

Should I Bravo it now for the chance I might upgrade engine later? If someone could tell me that doing a mild cam and being in a pontoon boat that the Alpha would survive, I’d prefer to go that route.
Thanks

 

[Scott06]

Anything is possible, but you'd be better off selling what you have and getting what you want.

 

[meantobe63]

What I want is $60k, I can MAKE what I want for less than half that.

 

[achris]

... doing a mild cam ...

Cars do well with a 'mild' cam, boats not so much.... Unlike cars, boats have no 'gears', so the ratio you have is ALL you have. Camming an engine has a tendency to push the power band up. Pretty much the exact opposite to what you want in a boat. Boat engines need to make power from as low down as possible, and drives tend to 'self-disassemble' at high revs (much more than about 6,000).. So the ideal boat engine has power right from idle, or just off-idle, to a peak power at around 5,000. Which just happens to be exactly how the supplier supplied the engine. Stay with the stock cam, don't overprop (prop to the top end of the recommended range, 4200-4600), and you'll be sweet.

At the end of the day, it's a pontoon, not a 60knot cruiser, so doesn't expect scintillating speed. About 25 knots is all you should expect....

Chris......

 

[Scott Danforth]

you cant use the alpha from the 3.0 behind a 5.7 without spending more money on a new gear set and the proper tools to re-gear. best bet is buy a different drive

you still may not get the pontoon going any faster, but you will burn more fuel. it is a displacement hull, and to get them to move, you need a 3rd log and lifting strakes

if you plan on running a supercharger, you will need a forged bottom end. if your going to spend the money on a forged bottom end, you may as well build a stroker as a complete forged rotating assembly is same cost if its a 3.48" stroke or a 3.75" stroke or a 3.875" stroke. you need a new drive anyway

Alphas can live behind a 500hp stroker, you just got to watch the hole shot and if you ever leave the water and become airborn, you need a rev limiter that snuffs the motor so that when you re-enter the water, the drive doesnt explode from the instant loading. Bravos are better for this. then again a B3 would be better for trying to push a displacement hull out of the water.

with wet exhaust and your 32 year old SBC, the Comp Cams XM-268H is the largest cam you can run. you will have to verify your static line height and may need risers

or you can simply use the pontoon for what its intended for...... slow cruises while drinking beer, then get a small go-fast boat such as a donzi sweet 16, or a Hydrostream with a 225 merc on the back

 

[Maclin]

You mentioned adding the 3rd pontoon as well, might do that first. It will be heavier at first, but once the 3.0 catches up it may be able to add some extra speed over what you have now.

 

[meantobe63]

you cant use the alpha from the 3.0 behind a 5.7 without spending more money on a new gear set and the proper tools to re-gear. best bet is buy a different drive

you still may not get the pontoon going any faster, but you will burn more fuel. it is a displacement hull, and to get them to move, you need a 3rd log and lifting strakes

if you plan on running a supercharger, you will need a forged bottom end. if your going to spend the money on a forged bottom end, you may as well build a stroker as a complete forged rotating assembly is same cost if its a 3.48" stroke or a 3.75" stroke or a 3.875" stroke. you need a new drive anyway

Alphas can live behind a 500hp stroker, you just got to watch the hole shot and if you ever leave the water and become airborn, you need a rev limiter that snuffs the motor so that when you re-enter the water, the drive doesnt explode from the instant loading. Bravos are better for this. then again a B3 would be better for trying to push a displacement hull out of the water.

with wet exhaust and your 32 year old SBC, the Comp Cams XM-268H is the largest cam you can run. you will have to verify your static line height and may need risers

or you can simply use the pontoon for what its intended for...... slow cruises while drinking beer, then get a small go-fast boat such as a donzi sweet 16, or a Hydrostream with a 225 merc on the back

I bought a complete 1987 Sea Arrow. Would use the drive out of it. Has 400 hours on it. It’s an Alpha One 1.5:1.

 

[meantobe63]

you cant use the alpha from the 3.0 behind a 5.7 without spending more money on a new gear set and the proper tools to re-gear. best bet is buy a different drive

you still may not get the pontoon going any faster, but you will burn more fuel. it is a displacement hull, and to get them to move, you need a 3rd log and lifting strakes

if you plan on running a supercharger, you will need a forged bottom end. if your going to spend the money on a forged bottom end, you may as well build a stroker as a complete forged rotating assembly is same cost if its a 3.48" stroke or a 3.75" stroke or a 3.875" stroke. you need a new drive anyway

Alphas can live behind a 500hp stroker, you just got to watch the hole shot and if you ever leave the water and become airborn, you need a rev limiter that snuffs the motor so that when you re-enter the water, the drive doesnt explode from the instant loading. Bravos are better for this. then again a B3 would be better for trying to push a displacement hull out of the water.

with wet exhaust and your 32 year old SBC, the Comp Cams XM-268H is the largest cam you can run. you will have to verify your static line height and may need risers

or you can simply use the pontoon for what its intended for...... slow cruises while drinking beer, then get a small go-fast boat such as a donzi sweet 16, or a Hydrostream with a 225 merc on the back

My go fast boat will be my 1963 Aristocraft Funliner with the I/O out of the pontoon attached to it!

 

[Scott Danforth]

I bought a complete 1987 Sea Arrow. Would use the drive out of it. Has 400 hours on it. It’s an Alpha One 1.5:1.

Thats a step back in drive technology going from the A1G2 to an A1

 

[meantobe63]

Thats a step back in drive technology going from the A1G2 to an A1

i know. Bummed about that. I like the built in speedo pickup (which I heard is pretty inaccurate, and I don’t have a Speedo yet anyways, and I liked the reservoir feature. I’ve been told other than that they are the same, gears and all. I spoke with Sterndrive Engineering about swapping out the top half to correct ratio for the v8 and they said that I have the only Gen 2 drive that won’t work on. Apparently to get the 2.00 ratio the lower half is regeared as well, called a “high altitude” or something. They said they have a LOWER that I could swap out and get close to v8 ratios. I think somewhere like 1.65 or something.

 

[achris]

As you're looking at a fairly low speed anyway, a 1.65 drive would actually work better than a 1.5. It would get you into a slightly high pitch prop... And that's a good thing.

 

[meantobe63]

As you're looking at a fairly low speed anyway, a 1.65 drive would actually work better than a 1.5. It would get you into a slightly high pitch prop... And that's a good thing.

Then maybe I’ll go that route. Do you think the 135hp 3.0 in that little all fiberglass Aristocraft I have can spin that gen 1 1.50 ratio decent? All it had in it before was a 110hp but no idea what the ratio was. But I know the boat is light as a feather.

Thanks everyone one for the help.
I’m not looking to haul ass, just keep up with my buddies same size pontoon with a 4.3 running a 19p prop in the low 30’s.
And my other buddy has a tritoon with a 305 MPI that ran close to 40.
So with a 3rd toon and some kind of strakes eventually I don’t think 40 is impossible. But 30mph I’d be happy.

 

[Scott Danforth]

3.0 wont spin the 1.5 drive under load. the 3.0 is normally paired to a 1.98:1. you wouldnt be able to find props with pitch low enough

your current 'toon with the 3.0 with a 3rd log and lifting strakes would hit 30. as Maclin stated, I would go that rout first.