As a general rule of thumb... from what I understand.

2.0 ratio for 4 cylinders
1.84 ratio for 6 cylinders
1.65 ratio for small block v-8 motors or motors that make less than 300 horsepower
1.50 ratio for big block motors
1.35 ratio for boats that go well over 100 mph
1.26 ratio for cats that go really really fast

Chris

P.S. this is for alpha/bravo/volvo type drives

You are correct, there are more than 2 ratio's
available for the Alpha 1 gen II lower units.

2:1 and 2.4:1 use one ratio 2:1 ( 14-28 tooth count) everything else uses the same 1.65:1 ( 17-28 tooth count)

At 4500 engine rpm, a 1.84 ratio upper will be turning
the vertical shaft at 4054 rpm ( 1.84 upper has a 1.11:1
reduction ratio) (17t pinion 19t driven)

At 4500 engine rpm, a 1.5 ratio will be turning the
vertical driveshaft at 4950 ( 1.5 upper has a .9:1 ratio,
....actually overdriven) (22t pinion 20t driven)

So, the 1.84 ratio vertical drive shaft turns slower.


It might not be enough to make a difference,
but I tend to look into every possible ramification
when changes are made.