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3.0 to 5.7 Swap

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I am new to the forum and have what I think maybe a noob question, but I can't seem to find any information on the subject.

I have a 1998 Alpha 1 Gen. 2 outdrive (3.0). I want to put a 350 in front of it. I know I need to change the gear ratio to a 1.47.

What I can't find is what all is needed to mate the engine to the outdrive or if anything is needed...

So, I guess my question is: What is required to mount a 5.7 in place of 3.0 to a Alpha 1 Gen. 2 outdrive. (I know the engine mounts, exhaust & etc. are different, I know it weighs more)

Thanks.
 
I have never done this but,

the inside transom is the same for all the Alpha's...........

So as long as the motor you plan to install has the correct bell housing to match up with the transom all will be ok.
Of course there will be two exhaust pipes going to the transom vs one.

The key is getting two (one on each side) stringers mounted in the boat for the motor mounts.

The motor may need to be installed while on a chain fall and get it aligned to the gimbal bearing as it would be normally and once that is done and the motor is where it "needs: to be kinda sort of...........this would locate the approximate place for the two motor mount locations on each side of the engine.

That would be the key, getting those in the right places so the only alignment needed is UP or DOWN not side to side.

The tightening of the rear motor mounts and raising and lower the motor on a chain fall should get you very close!

One issue would be that all V6 and V8 transoms have a power steering assist system in place. If your 4 banger does not have power steering then you will need to get that also.
 
Correction,

there are two exhaust pipes but they become one and mount to the same location as the 4 bangers exhaust.


Of course it is a 350 chevy..........what else is there....lol

On my local Craigslist (boat) adds there is always someone with a complete 4 cycl, V6 or V8 mercruiser for sale......so if you can get a complete mercruiser that would be Best!!

A auto engine is not the right choice.........many things have to be changed to make it marine,

core plugs, water pump, head gaskets and a few other things........so dont go there unless you are willing to make the changes and do it RIGHT!
 
You don't necessarily have to change the outdrive. You will have to reprop.
I recently had to change my outdrive on my 5.0. Went from the 1.47 to a 1.6? that's commonly behind the v6. I had to go from a19 to a 21p prop. The boat actually handles better now than it did with its original drive.
 
The OP is proposing a change from a 3.0L to a 5.7L. The difference in the overall gear reduction for either engine is considerable.
He won't be able to prop himself out of an incorrect ratio!


.
 
Earlier this summer a friend had the upper cap bearing go south........He got a perfectly good upper complete gear housing for $400 + shipping with the correct gear ratio...........made my job easy as I did not have to rebiuld his old one........lol
 
check the fuel tank size if in the belly, in case you need to refit that while engine out. you don't want a V8 with the 18 gallon gas tank a lot of 17 footers came with.
 
I just checked using a prop calculator. 4800rpm, 1.94ratio (3.0L), 15% slip, 45 speed. Came up with a 23 pitch prop. Considering that most runabouts with the alpha mercruisers have a factory sweet spot using a 19p prop that's only 4" of additional pitch which is doable. Will probably get better fuel economy too.
I'm not saying it's the best option but if money starts getting tight, and we all know how quickly that can happen with big projects, it's an option that can get him on the water for the price of a prop.

As someone said above, getting the motor mounts in, properly secured and positioned, will be your biggest chalange.
 
Dave, as much and as nice as it would be to simply use the 3.0L drive ratio, I can assure you that there is no propping ourselves out of an incorrect ratio. Hydro-propulsion dynamics and physics simply does not allow for it. If this did work, the entire stern drive industry could provide one gear ratio for all engines.

But there's a tad bit more to this:
In the marine world, these gears are refered to as a "drive" gear and a "driven" gear, as apposed to Ring and Pinion in the auto world.
As the overall reduction becomes numerically higher (more reduction), the drive gear becomes smaller, and the driven gear becomes larger.
When the drive gear is smaller, there is less gear tooth contact area at any given time.
When the drive gear is larger, there is more gear tooth contact area at any given time.
This makes a low ratio gear set less able to handle torque, and a high ratio gear set more able to handle torque.

The difference may not be all that great... but nonetheless, it does make a difference.


Here's an example using the FWD Off Road guys.
When these guys first join the FWD Off Road thing, they almost immediately install low gear sets in their differential units. Then they go out and do their FWD Off Road stuff.
Unless they have used very expensive gears, one of the first things that typically happens, is that they break the gear set.

Now... that's not to suggest that this is the only reason for using the correct stern drive ratio. But when changing from a low torque 3.0L engine to a much higher torque 5.7L engine...... there will be a risk.



One exception to ratio changes may be when a very light and/or small hull is in question... but usaully when considering using a taller ratio.
Example:
The Volvo Penta Duo Prop for a SBC requires the 1.95:1 overall ratio.
The Volvo Penta Duo Prop for a BBC requires the 1.78:1 overall ratio.
If the hull is light and/or small, on occasion the taller 1.78:1 can be used with the SBC.


Bottom line... the OEM will always suggest the correct ratio for the engine.



.
 
Last edited:
DaveG55 said:
I just checked using a prop calculator. 4800rpm, 1.94ratio (3.0L), 15% slip, 45 speed. Came up with a 23 pitch prop. Considering that most runabouts with the alpha mercruisers have a factory sweet spot using a 19p prop that's only 4" of additional pitch which is doable. Will probably get better fuel economy too.
I'm not saying it's the best option but if money starts getting tight, and we all know how quickly that can happen with big projects, it's an option that can get him on the water for the price of a prop.

As someone said above, getting the motor mounts in, properly secured and positioned, will be your biggest chalange.
Click to expand...

Ayuh,..... Change that 3.0l to 5.7l, 'n a 30-something prop won't be big enough,....

Ya Gotta regear,....
 
You might want to check and confirm that the hull is rated for the extra horse power. I suspect stringer location for mounts may be an issue also.
 
Ricardo Marine: reading the thread and your input for gear ratio. I recently removed a 4.3 and installed a 5.7 I have a 1.84 to one gear set. My 17 inch prop was 5400 WOT (4800) max. I just ordered a 21 in prop. Will this work for me or will I have to re gear also?
 
rl65028 said:
Ricardo Marine: reading the thread and your input for gear ratio. I recently removed a 4.3 and installed a 5.7 I have a 1.84 to one gear set. My 17 inch prop was 5400 WOT (4800) max. I just ordered a 21 in prop. Will this work for me or will I have to re gear also?
Click to expand...
Technically, you should re-gear for the NOW engine, and for the reasons previously mentioned.
Your call!



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another reason for re-gearing is water flow - the impeller is the same size in all Alphas and it turns at vertical shaft speed. A 1.98 ratio drive won't supply the raw water a 260HP V8 needs at full load.
 
hystat said:
another reason for re-gearing is water flow - the impeller is the same size in all Alphas and it turns at vertical shaft speed. A 1.98 ratio drive won't supply the raw water a 260HP V8 needs at full load.
Click to expand...
Great point! + 1

Merc A drive gear reduction differences occur in the upper unit, of which determines the engine to vertical shaft ratio.


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I know it’s an old post , but in case new folks still use this thread as reference , 3.0 vs 5.7 the whole mounting is different . 3.0 uses Gimbal two bolts mount plus a front bracket type mount while 5.7 uses Gimbal two bolts mount plus two side mounts
 
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