merc outdrive

[boaterboy71]

3 yrs ago I repowered my 1983 21' criscraft. Went from a 260 hp to 310 hp small block v8. This past summer my drive let go, it had 1.50 gears and was original to the boat. I was planning to replace with a new 1.47 gear ratio drive. Then I found a used drive (in excellent condition) with 1.32 gear ratio. was wondering if this would be a good alternative option, and would it work with my boat set up. From what I have found out I believe I would have to drop 1" in prop pitch to maintain wot rpm. Any advise wold be great. Thanks

 

[thumpar]

You may have to change the prop by more than 1" but it should work with that amount of power. Those were originally put behind the 454.

 

[achris]

Theory...

Required prop change for any given ratio change. New ratio/old ratio * old prop size = new prop size.

So, what was your current prop size? Multiple that by the ratio of the ratio changes, 1.32/1.5=0.88. So if you were running a 17", you'd need a 15", in theory.
Where that comes apart (only slightly) is the efficiencies of different pitch props is not the same, so there will be a little 'fiddling' to get it spot on, but that formula is where you start...

Also, be aware that finding a decent prop of less than about 15" is not easy. I'm talking about efficiency and availability, not quality...

Chris......

 

[Scott Danforth]

welcome aboard.

the 1.32:1 drive was used on a few big blocks for a short period of time. not sure what you are looking to pay, however an SEI drive is a brand new drive for about $1500

 

[Bondo]

3 yrs ago I repowered my 1983 21' criscraft. Went from a 260 hp to 310 hp small block v8. This past summer my drive let go, it had 1.50 gears and was original to the boat. I was planning to replace with a new 1.47 gear ratio drive. Then I found a used drive (in excellent condition) with 1.32 gear ratio. was wondering if this would be a good alternative option, and would it work with my boat set up. From what I have found out I believe I would have to drop 1" in prop pitch to maintain wot rpm. Any advise wold be great. Thanks

Ayuh,.... Welcome Aboard,.... You could use that drive, but repropin' will be necessary,....

What prop, 'n at what Wot rpms are ya seein' with the original dive,..??

 

[boaterboy71]

Didn't mean to post question more than once. I was getting an error message when trying to post it so I didn't think it was working. I'm running a 21' mirage prop and wot is approx 4500

 

[Bondo]

Didn't mean to post question more than once. I was getting an error message when trying to post it so I didn't think it was working. I'm running a 21' mirage prop and wot is approx 4500

Ayuh,.... A 17" prop might yer yer rpms at Wot up to 4800,....

Might haveta go to a 15",...

You'll be hard pressed to find anything smaller, that'll fit the drive,....

 

[achris]

Using the formula, a 21" at 4500 on the current drive, changed to a 1.32:1 would require an 18.5" prop to maintain the current rpm. As you can't get an 18.5, you have 2 choices, a 17" or a 19" (I would stay with the 'Mirage' style prop as changing to a different style throws all the numbers right out the window). A 19" will push the revs down, and you are already in the lower half of the recommended range. So what will a 17" do to the revs? Using the same formula for revs will have the engine at 4800, which I would say is exactly where you need to be....

Good luck.

Chris........
(I deleted your duplicate post )

 

[redneck joe]

welcome aboard.

the 1.32:1 drive was used on a few big blocks for a short period of time. not sure what you are looking to pay, however an SEI drive is a brand new drive for about $1500

SEI will not warranty anything behind a 454 - that I know - ask if you are looking at this option. That said; the math the others supplied should work.

 

[boaterboy71]

Thanks for all the info. I was considering the 1.32 drive just for the strength of it as I know that an alpha drive is only rated for 300 hp and the motor I put in is rated at 310 hp. However I think I might stick will the alpha 1.47 drive then I don't have to spend more $ on a new prop also

 

[thumpar]

The 1.32 is still an Alpha and the power rating is the same. They only used the Alpha behind the 454 for a while until they figured out it was too much power for the drive.

 

[boaterboy71]

I figured that the 1.32 alpha would be rated for more hp since it was used behind the 454. Since it's not, that make's my decision easier because buying a new 1.47 drive from sei is cheaper than what the guy is asking for the used 1.32 alpha drive. Plus I save money on not having to buy a new prop

 

[thumpar]

Definitely go with the SEI if he wants more for a used drive. You will get a 3 year warranty with it.

 

[Howard Sterndrive]

this small block is rated 310HP at 4600 rpm? or 310HP screaming away in a steet/strip car?