re-gear or re-prop???

24'winner

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Oct 12, 2010
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13
i have a pre alpha mercruiser 120/140 out drive that went to a 3.0l i4 but would like to put it on my 302 v8 im pretty sure the v8 runs a 1.50:1 ratio and i think the 3.0l runs a 1:90ish:1 ratio, but since im not too savy on outdrives idk if i can even re-gear the 120/140 to handle a v8 or would it be more cost effective to just put a different prop on?
 

Bondo

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70,465
Re: re-gear or re-prop???

Ayuh,... It's cost prohibitive to regear it,...
It's Impossible to find a prop Big enough to make up the difference,...
So,...
Replacing the outdrive is your Best bet...
 

24'winner

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Oct 12, 2010
Messages
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Re: re-gear or re-prop???

iv herd you can mate a alpha 1 top with a pre alpha bottom if this is true could i use the top of the old outdrive (from the 302) and just have it rebuilt and just mate it up to the bottom of the 120/140?
 

Hank496

Seaman
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Jun 13, 2010
Messages
59
Re: re-gear or re-prop???

A better solution might be to sell the existing drive on eBay or Craigslist and buy the right one.
 

Bondo

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Re: re-gear or re-prop???

Ayuh,... You can mix, 'n match uppers, 'n lowers, Yes...
But,...
A 302 was Never used with an Alpha 1 drive,...
The 302 was obsolete before the Alpha 1 was introduced... Just sayin'...
 

spdracr39

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Aug 30, 2010
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1,238
Re: re-gear or re-prop???

Is your boat and transom rated for the additional weight and horsepower ?
 

24'winner

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Oct 12, 2010
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Re: re-gear or re-prop???

yes it had a 302 in it origially and the boat is rated up to 250hp with a single engine..
 

crewscontrol

Seaman
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Sep 15, 2010
Messages
52
Re: re-gear or re-prop???

Several posters have waltzed around the answer, but nobody actually said it.

It is kinda like whether to change the transmission gearing in a car or change the rear end gear (final drive) ratio. As long as whatever you change results in the same overall system change, it won't matter where you make the change, the performance will be the same.

For all practical purposes in a boat it does not matter whether you change the stern drive gearing or change the size and pitch of the prop, as long as the end result (the final drive ratio for lack of a better term) is the same, your engine and your boat performance won't know the difference.

Obviously it will be much easier and it will cost much less to change the prop, if you can find the right one.
 

nola mike

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Apr 22, 2009
Messages
5,023
Re: re-gear or re-prop???

It's Impossible to find a prop Big enough to make up the difference,...
So,...
Replacing the outdrive is your Best bet...

don't think bond-o waltzed around the answer. FWIW, i was able to reprop my drive, going from a 1.84/23p to a 2.0/25p--definitely no room to spare. i'd think it might still be possible if you were running a very small pitch before, but that would be a pretty big jump. looking at http://www.go-fast.com/Prop_Slip_Calculator.htm, seems you'd need about a 6p increase to make up for the difference in gear ratio. don't know how that would affect performance though. my boat lost about 2-3 mph top end turning at the same rpms--new prop is SS, old one was aluminum.
 

crewscontrol

Seaman
Joined
Sep 15, 2010
Messages
52
Re: re-gear or re-prop???

don't think bond-o waltzed around the answer. FWIW, i was able to reprop my drive, going from a 1.84/23p to a 2.0/25p--definitely no room to spare. i'd think it might still be possible if you were running a very small pitch before, but that would be a pretty big jump. looking at http://www.go-fast.com/Prop_Slip_Calculator.htm, seems you'd need about a 6p increase to make up for the difference in gear ratio. don't know how that would affect performance though. my boat lost about 2-3 mph top end turning at the same rpms--new prop is SS, old one was aluminum.

Sorry about the bad choice of words (waltzed around the answer). I could have (should have) worded that differently. No offense intended.
 
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