Ratio question
 

My son and I changed out the motor in his boat, took out a 4.3 and put in a 5.0, the original 4.3 drive ratio should be 1.84:1 and the 5.0 calls for a 1.50 or 1.65:1. can we make corrections by changing the prop pitch?

The sea water pump on a alpha drive is
mounted on the vertical drive shaft.

The 1.84 ratio drives vertical shaft turns
slower than a 1.65 or 1.5 does, so your
coolant volume will be less.....and you now
have a larger engine to cool.

Then there's the issue of the added torque of
the larger engine with that ratio....the splines on the vertical
driveshaft probably wont like that.

It's not all that larger of a n engine, the 4.3 is rated at 175 hp and the 5.0 is 228 wasn't sure if the different ratio would turn the prop faster or slower..thanks for the heads up on the sea pump we will watch the water temp

It's more about the additional cooling capacity
of the v-8 over the v-6.

The 1.84 ratio is going to turn slower than the
1.65 or 1.5, so it will require a larger pitch propeller.

gears
 

i did a 4.3 to a 350 mag in a rinker 206 the drive keep the the eng. cooled but after about 50 hrs. with the bigger prop the upper gears went south. i put a set of 1.50 hd gears .

the different ratio of the out drive will not make any difference in the cooling of the engine and the 1:84 ratio will turn the drive shaft faster not slower.

Is that why 1.5 geared applications run larger
props than 1.36 geared application ?:rolleyes:

An engine running 4500 rpm is going to have a 3000 rpm
propshaft speed with a 1.5 gearing.

An engine turning 4500 rpm is going to have
a 2445 rpm propshaft speed with 1.84 gearing

we use the 184:1 gears all the time for high altitude on the 5.0 engines, just prop it for the proper rpm and it should be fine. and we have had no cooling issues.

what is the speed of the DRIVE SHAFT ???? you know the one that drives the water pump impeller?? and the over all gear ratio is not made in the upper uint any more there is more than one lower for alpha gen 2 drives with two gear ratios.

it's a Alpha one drive, isn't the ratio made in the upper?

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.