What Gear Ratio do I have? (Alpha 1 Gen 2)

[JoLin]

Can anyone tell me? They're the original drives, circa 1992, and the original engines were 4.3 LX's. Both engines have been replaced with Vortecs along the way. I have the ratio documented as 1.84, but pretty sure that's a typo... and I don't remember where I found that number.

Drive Serial Numbers: 0D656659 0D697552

Original Engine S/N's: 0D803315 0D803389

Appreciate any help you give me!

 

[achris]

Both of those drive numbers come up as 1.84:1. Which is correct for that series of 4.3 (I had one, and it had 1.84:1)... Now, if someone has changed the gearing in the top box, then all bets are off...

The only way to be completely certain it to remove the gears and start counting teeth, or pull the plugs on the engine, mark the propeller and start counting engine turns verses propshaft turns (in forward gear of course)... Or... give me your WOT engine speed, GPS speed and prop size (pitch and diameter) and I'll plug them into my prop calculator and see what ratio lines up the most accurate...

HTH,

Chris......

 

[JoLin]

Sorry, guys- I found it myself, tho I don't know why I couldn't find it before. They ARE 1.84, which (according to SEI) Merc changed to 1.81 in 1996. That's what was confusing me and why I thought I'd made a typo when I wrote it down. Was only finding references to 1.81 and 1.94 ratio.

 

[achris]

Do you know the actual difference between the 1.84 and 1.81 (also goes for the other ratios, 1.50 vs 1.47, 1.65 vs 1.62 and 1.98 vs 1.94)? The differences should be a hint ...

 

[JoLin]

How's this?

In 1996 Mercury changed the gear ratio in the lower unit to achieve a stronger gear set. Prior to 1996 the lower unit utilized 17 tooth and 28 tooth gears. In 1996 this was changed to a 13 tooth and 21 tooth gear set. SEI uses the newer style gears in our lower units. This changed the overall ratio of the drives when added to the ratio of the upper unit. The new ratios can be substituted for the old ratios with no discernible performance changes. The following chart shows the old and new overall ratios.

1995 and older (22/20 tooth upper, 17/28 tooth lower) 1.5 ratio
1996 and newer (22/20 tooth upper, 13/21 tooth lower) 1.47 ratio
1995 and older (24/24 tooth upper, 17/28 tooth lower) 1.65 ratio
1996 and newer (24/24 tooth upper, 13/21 tooth lower) 1.62 ratio
1995 and older (17/19 tooth upper, 17/28 tooth lower) 1.84 ratio
1996 and newer (17/19 tooth upper, 13/21 tooth lower) 1.81 ratio
1995 and older (20/24 tooth upper, 17/28 tooth lower) 1.98 ratio
1996 and newer (20/24 tooth upper, 13/21 tooth lower) 1.94 ratio

Chris, the drives are on the boat and I ain't pullin' 'em to count gear teeth. I have the maintenance records on this boat and no reason to think anything's been changed. Thanks for the input!

 

[achris]

How's this?

In 1996 Mercury changed the gear ratio in the lower unit to achieve a stronger gear set. Prior to 1996 the lower unit utilized 17 tooth and 28 tooth gears. In 1996 this was changed to a 13 tooth and 21 tooth gear set. SEI uses the newer style gears in our lower units. This changed the overall ratio of the drives when added to the ratio of the upper unit. The new ratios can be substituted for the old ratios with no discernible performance changes. The following chart shows the old and new overall ratios.

1995 and older (22/20 tooth upper, 17/28 tooth lower) 1.5 ratio
1996 and newer (22/20 tooth upper, 13/21 tooth lower) 1.47 ratio
1995 and older (24/24 tooth upper, 17/28 tooth lower) 1.65 ratio
1996 and newer (24/24 tooth upper, 13/21 tooth lower) 1.62 ratio
1995 and older (17/19 tooth upper, 17/28 tooth lower) 1.84 ratio
1996 and newer (17/19 tooth upper, 13/21 tooth lower) 1.81 ratio
1995 and older (20/24 tooth upper, 17/28 tooth lower) 1.98 ratio
1996 and newer (20/24 tooth upper, 13/21 tooth lower) 1.94 ratio

Chris, the drives are on the boat and I ain't pullin' 'em to count gear teeth. I have the maintenance records on this boat and no reason to think anything's been changed. Thanks for the input!

Yeah, that's pretty close.... :thumb: