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View Full Version : 1:84 Drive...Is it possible?



Sagbay32
05-31-2002, 11:44 PM
On my '95 classic (18) I swear the Alpha is stamped 1:84 on the port side sticker. Is this possible and how to tell the drive ratio (for a beginner).

Magic Float
05-31-2002, 11:55 PM
1.84 was normal for most V-6's in the mid 90's

GeneD
06-04-2002, 11:06 PM
I was always under the impression that it went like this...
1.8 4 cylinder
1.6 6 cylinder
1.5 8 cylinder
1.3 Big Block or high output SB
Basically, the only way you can tell without ripping the drive apart to read gear numbers is the old differential method.
Rotate the input shaft, and watch the output shaft. Count rotation.

AVickers
06-05-2002, 10:06 AM
You can also pull the top off the drive and count teeth on the pinion...and compare with the count on new components. The ratios are changed in the upper unit input pinions.

RickSE
06-05-2002, 12:13 PM
Sounds like you may have high-altitude gearing, assuming the stamp is correct which it may not be. Didn't Bryan T. go through this on one of his replacement drives, upper was mismarked? On all the boats we order out here we request lower gearing (higher numerical ratio) to help make up for the altitude. We usually order 1.68 though.

The case on my 96 Alpha says 1.47. I'd rather have 1.68 gearing but I just run a smaller prop.

The prop size and boat performance should help you figure out what gearing you have. What pitch are you running and what's your top speed & RPM's?

I'd think with a 1.84 ratio you'd have to have a pretty big pitch to run correctly at sea level, 25-27 pitch.

Sagbay32
06-05-2002, 05:09 PM
The numbers that I know of so far:
1995 C-18
350 mag 4bbl HP?
14 5/8-23p Michigan Wheel Stainless
1:84 drive ratio ?
4000 rpm
Speedo says 58mph (660 ft elevation) when trimmed up, and four in boat w/ full tank
No GPS readings for speed yet.

I was out last weekend and noticed that the tach would not go over 4000 rpms. I was wondering if I should try to experiment with different props. Thge boat has very low hours and I don't want to put too much into her due to the fact that I am going to be selling her if the right deal on a bigger boat comes along. I just thought that this quest for speed would keep me busy for the time being.
How exactly do you go about counting teeth on the top half?

Rootsy
06-05-2002, 05:45 PM
to count teeth on the upper drive gears look straight down on the top of the sterndrive, you'll see (4) 3/8 12 point bolt heads. Remove them, don't damage the o-ring. you may have to drain the gearlube down some to see the gears also. This method is best performed with the sterndrive OFF of the boat so you can rotate the input shaft after marking the gears so that you can count the teeth. Once you have the teeth numbers divide the one with the most teeth by the one with the least and taht gives you the ratio.

4000 rpm for your boat is way low.. that motor i believe was rated 4600 - 5000 rpm WOT if my memory serves me. if anything with the 23 pitch prop you should almost overrev the motor with that gear ratio. I believe ranman may have something similar except his motor is the newer 300 hp version and he spins a labbed 25, correct randy?

Sagbay32
06-05-2002, 06:11 PM
The drive may be mis-marked by the factory (ran out of decals on a friday and slapped on what was left).
Is there a way to figure out a ballpark on the drive with the performance numbers that I have? I will not be taking the drive out of the boat :D

If I am only revvin' 4000 with the 23p prop and have a stock motor should I be looking at a different prop? the sticker on the shroud says WOT is 4400 to 4800. Which way should I go with prop size and pitch? Maybe my tach and speedo are both off ? I will be GPS'ing soon.

PaulO
06-06-2002, 09:02 AM
At that rpm and pitch, sounds like you have a 1.5:1 ratio. If you had a higher ratio, you would be spinning that prop faster assuming the engine is running correctly.
PaulO

Ranman
06-06-2002, 10:16 AM
Jamie, You're close. Here's my specs

97 C-18
350 MAG MPI 300hp
1.65:1 Bravo One
Labbed 25P Mirage +
WOT on tach is 4900rpm
She'll run 66-67 with 40 gallons of fuel and 2-3 passengers.

Mike, I would check your tach to make sure it's accurate. The speedo is less important but you could check it too. My guess is that at some point your stern drive unit was changed or modified. Like PaulO said, a standard C-18 Mag 350 1.5:1 Alpha would be on the money with a 23P Mirage +. This should net you 4600-4800rpm and in the upper 60's for mph.

The only reason I turn a 25p Mirage + because of the 1.65:1 gears. I think this is a high altitude option from MerCruiser. I think this was ordered because my boat was originally from Arizona.

With 300hp and 1.84 gears turning a 23P Mirage +, you would way over rev the boat unless you put on a bigger prop. Again, you most likely have 1.5:1 gears as they are most common, maybe your case has been changed or the sticker is simply incorrect.

Sagbay32
06-07-2002, 06:25 AM
Maybe I should look at the tach? I will verify speedo this weekend.