Mercruiser Alpha gear ratio question

[1kwik72]

I have an 84 Sea Ray 23' cuddy. The engine is a 5.7 260hp Mercruiser with a 1st gen Alpha 1. The gear ratio on the current drive is 1.47. I'm replacing the entire drive and I found a 1st gen Alpha 1 that is a new drive and a good price, but the gear ratio is 1.62:1. The seller says there will be no change in cruising rpm/efficiency, hole shot or top end if I run a smaller pitch prop. I currently have an aluminum 19". He's suggesting 16"...

Anyway, if prop pitch/diameter can make all things equal then I'll buy it. If not I'm looking at an SE106 in the 1.47 ratio.

Thanks in advance for any input!

 

[Don S]

Re: Mercruiser Alpha gear ratio question

The seller says there will be no change in cruising rpm/efficiency, hole shot or top end if I run a smaller pitch prop. I currently have an aluminum 19". He's suggesting 16"...

Obviously the seller knows little of what he is talking about.
Props change will get you the correct WOT rpm range, but it will not be as efficient as the 1.47.
That 1.62 is for a V6 which has less torque than a 5.7, that is the reason for different gear ratios.
Besides, to get your rpm down some (which you will need with a 1.62 drive, you will need more pitch., not less.

Moving this over to the prop forum.

 

[Lyle29464]

Re: Mercruiser Alpha gear ratio question

He is backwards


oops don was just ahead

 

[45Auto]

Re: Mercruiser Alpha gear ratio question

You would be putting on an outdrive with 10% more gear reduction (going from 1.47 to 1.62, prop will be turning 10% slower), therefore you would need a prop 10% bigger (go from a 19" to 21") to theoretically get the same performance.

It's easy to calculate exactly what the equivelent prop would be:

old gear ratio / new gear ratio = old prop pitch / new prop pitch

1.47 / 1.62 = 19 / x

x = 19 x 1.62 / 1.47 = 20.94"

Put a 21" pitch prop on the 1.62 outdrive and you'll never notice any difference.

 

[dvandsm64]

Re: Mercruiser Alpha gear ratio question

Obviously the seller knows little of what he is talking about.
Props change will get you the correct WOT rpm range, but it will not be as efficient as the 1.47.
That 1.62 is for a V6 which has less torque than a 5.7, that is the reason for different gear ratios.
Besides, to get your rpm down some (which you will need with a 1.62 drive, you will need more pitch., not less.

Moving this over to the prop forum.

our 1997 19' b.r. has a 5.7liter that has a 1.62 listed on its mercruiser outdrive?

 

[dockwrecker]

Re: Mercruiser Alpha gear ratio question

I changed to 1:62's with a 19 stainless in my Carver which is the same size/weight/motor/drive as your Ray. (I had previously thought they were 1:7? until I counted the teeth) Much better than the 1:47. WOT RPM went up about 400, hole shot improved massively and it seems happier at a cruising speed. I use my boat at an altitude range of 4200-6600 ft. It works well for me.

 

[Bondo]

Re: Mercruiser Alpha gear ratio question

I changed to 1:62's with a 19 stainless in my Carver which is the same size/weight/motor/drive as your Ray. (I had previously thought they were 1:7? until I counted the teeth) Much better than the 1:47. WOT RPM went up about 400, hole shot improved massively and it seems happier at a cruising speed. I use my boat at an altitude range of 4200-6600 ft. It works well for me.

Ayuh,.. That's a very important component to this conversaion that most of Us don't have to deal with....

 

[1kwik72]

Re: Mercruiser Alpha gear ratio question

So no matter what prop I run, the 1.47 is the best match to my engine for strength and efficiency...right?

Thanks for moving this for me!

 

[dockwrecker]

Re: Mercruiser Alpha gear ratio question

So no matter what prop I run, the 1.47 is the best match to my engine for strength and efficiency...right?

Thanks for moving this for me!

Unless this shoe fits or something close altitude wise...


" I use my boat at an altitude range of 4200-6600 ft. It works well for me."

Or if you're just needing a bit extra snap performance-wise with that heavy boat..

 

[Tail_Gunner]

Re: Mercruiser Alpha gear ratio question

Get the SEI drive...Its new... underwarranty....and you have no ideal how the other drive was driven or maintained..

If the other drive was 600...your still investing 600 into blue sky ...plus installation time ...

 

[dockwrecker]

Re: Mercruiser Alpha gear ratio question

The OP stated the 1:62 is a new drive also but no warranty But given the seller's total ignorance about everything else, I'm beginning to question the "new" part too. SEI is probably the smartest choice. I bought a lower unit from them about three years ago that was a bit noisy out of the box until the gears wore in and has been fine since. The ratio question remains, but we need more info from the OP. What altitude does he boat, is his wife like mine who packs for a day trip like she's going on a 3 month assault on Everest, was he happy in the past with his performance etc. The SEI's cost the same in either ratio I believe.

 

[1kwik72]

Re: Mercruiser Alpha gear ratio question

I boat on Lake Lanier 45 minutes North of Atlanta. 1071 MSL.

I just purchased the boat, no idea how it ran but WAY back in 84 my buddy had one just like this with the 5.7/alpha setup. Ran great, had plenty of top end, got out of the hole well, etc. I'm sure the 1.47 would be fine, just wondering about the 1.62 and the prop question. The 1.62 is a brand new in the crate Mercruiser OEM outdrive for about $1000 shipped...

BUT, after our discussion here, I'd rather have the same basic drive that it came with. I've heard good things about SEI and their prices and warranty are hard to beat.

I'll be doing the install of the engine and outdrive in my shop. Maybe I should start a restoration thread.

Anyway, thanks for all the great input!