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Lower unit Gear Ratio

M

Mich Pulse

Member
Quick question...I had a 94' Pulse harddeck; when I bought the boat ran decent #'s on gps - Put jack plate on etc. My question is: is it possible that there was a different gear ratio beside 1.86 for a 175 merc? I've since built a bass boat, and sold my mate, but both boats ran better with the old gearcase that was on the boat when purchased - and my mechanic threw out all of the old gears from the case? I switched the gearcase to a stock 200 case and on both boats the GPS #'s were down on topend (with same props etc.) Is there another possible combination/ratio (if any) beside the 1.87 that might have been in my old case?
 
Ratio

Is it possible that it's from a 150? The 2:1 will cut your top end down. There was a 1:78 gear but I can't remember what that was in or when it was used. Maybe tonight I'll find it in some of my old stuff.


Art



If it's slow it must be an OMC.
 
All the stock outboard 'alpha' size cases had either 2:1 or 1.87:1 ratios. The XR4/XR6 case was smaller diameter and had 1.78:1 gears and only used for the 150's.

Your 175 should have had 1.87:1. If your mechanic switched to 2:1 gears, you would have noticed a stronger hole-shot and your rpms would have been higher at WOT, like you had gone down in prop size.

Are you sure you had the exact SAME prop before and after? Nobody switched it on you did they? And was it possible someone switched your stock 175 case over to the smaller XR6 case before you bought the boat? The smaller case was faster by a couple mph. Of you had the XR6 case, then switched to a stock 200 case, then you'd notice a slight difference.
 
JW said:
All the stock outboard 'alpha' size cases had either 2:1 or 1.87:1 ratios. The XR4/XR6 case was smaller diameter and had 1.78:1 gears and only used for the 150's.

Your 175 should have had 1.87:1. If your mechanic switched to 2:1 gears, you would have noticed a stronger hole-shot and your rpms would have been higher at WOT, like you had gone down in prop size...........
Click to expand...

How can you tell what gear ratio you have? and how can you tell if you have a "alpha" case?
 
The 'alpha' case is basically the standard case, same as Mercruiser uses on the Alpha outdrive. It's called the Fleetmaster in the outboard line. Torquemaster is the higher performance version of the Fleetmaster, as it has lower water intake holes but is the same diameter. Sportmaster is a heavily modified Torquemaster with LWP and extended nose cone and thicker built in torque tab. The XR6/XR4 gearcase is smaller diameter and not rated for any more than 150 horse.

The only way I know to test the gear ratio without taking it all apart is to index the #1 cylinder at TDC and then spin the motor exactly one turn and try to measure how far the prop turns. Not a very exact way to do it, but 2:1 should make it all the way around ONCE with two full engine revolutions, and 1.87:1 shouldn't quite make two full turns of the motor with one turn of the propshaft.

Easiest way to determine gear case ratio is to look it up in the manual when the motor is 100% stock from the factory :)
 
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This is how it worked. I bought the cmate and it had the case that came with the boat when bought. Then I got a merc 200 parts motor that is/was the same year as my merc 175 and boat. It had a beautiful barely used case. My original case that was on when bought boat had some serious play in prop shaft, and needed some work done, so instead of fixing, I bolted on the stock 200 lower (sameprop) slower MPH. Next my cmate was re-ended behind my truck (my stock 200 lower was damaged.) So I bolted my original case on my new bass boat Running a 25 pitch high five. Boat gps many times. Next I twisted a prop shaft , and took internals out of the stock 200 case put in old case and lost MPH (same prop - Same boat). In short, I'm not sure if I had a different gear ratio, or if it is possible that I did?, and what ratio would perform better, because I have to put yet another case on this year, and was hoping to get my MPH's back. I'm assuming that I had a certain ratio in the original case that performed better than my stock 200 case and internals. Thanks I hope this help (sorry for writing this so confusing - I'm confusing myself writing it.)JK
 
The 175 and 200 should have had the same gears. No idea why it was so much slower after you installed the 200 case. What were your WOT rpms with the 175 case and then the 200 case?
 
2:1 Ratio

JW

I thought the 2:1 ratio meant 2 engine revs for every one prop rev. Am I wrong on this??

Joe
 
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WHOOPS!!! :rof: What did I say? Jeeeez.....

yep, two engine revs per one propshaft rev with 2:1. I'd better go back and fix that post! :eyecrazy:
 
I believe that I was turning near the same with both at around 6000 - 6100. I know it is a guessing game now, but thought i would see if anyone had an idea of why this occured. It is strange, the original gearcase ran better mph on both boats with same prop - jackplate settings etc. I appreciate everyones response. So should I just put the stock 1.87 that you mentioned earlier in the thread?
 
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