Gear Ratio Question

[beerman57]

I have this ’84’ Johnson 40hp, it came with a 1970’s L/U. 4" diameter gear tube, blunt nose. I got a good deal on what is probably the right L/U for it, 3.5" dia. tube, more streamlined nose. BUT, the old L/U has a little better gear ratio – 24 revolutions of the driveshaft = 10 rev. of the propshaft, the newer L/U only gets 9 rev. of the P/S for 24 rev. of the D/S. It’s going to be a little hot rod motor, high pitch prop. Is the difference in gear ratio enough to care about? The newer, lower gear ratio L/U does have the smaller, more sleek gear tube.

[dan-in-tn]

The 4" gearcase was basically the 3 cylinder gearcase with a different D/S to work with your 2 cylinder engine. That bigger gearcase is stronger with regard to the pinion gear design. (Diameter) The smaller gearcase is faster and what has been used on 2 cylinder motors since 1975 I believe? (Definitely 1976)! Of course the larger gearcase takes the larger prop (V-4) type. Depends on what you are doing with the engine as to wheather it is better? Pontoon boat bigger prop is better. Small lite boat, small gearcase better. These are generalities. Always exceptions to any rule. All props have to be tested to find out how they run in an application.

Dan in TN

[beerman57]

Dan, guess what I’ll be doing if I use the smaller gearcase, that’s right – disassembling the big case again to get the D/S (haven’t checked part numbers yet, but if I remember right, the D/S will work in both). But my original question, will the 2.4:1 ratio (big case) be that much better than 2.66:1 (small)? When you multiple that by 5000 or so RPM’s, it might make a big difference – like about 320 prop revolutions in one minute (5000rpm divided by 2.4 versus 5000 divided by 2.66). That’s over 5 prop revolutions per second at only 5000rpm, starting to sound like a big performance difference to me, if my thinking is right.

[garry-in-michigan]

The blade area and pitch have as much (or more) to do with how the horse power is translated to water speed. You are worrying about gear ratio, but that works only if you use the same propeller. Generally speaking, the smaller gear case will need a higher pitch propeller when used on the same boat. (If it was running at the proper RPM with the 10/24 gearcase.) You can only find out by trying it. If the motor runs too fast, you can gain a half inch of pitch by cupping the blades (If brass or stainless steel) … Good Luck.

[beerman57]

Unfortunately, I can’t test just by changing lower units, the new (small) L/U is long shaft. I guess I’ll run the older (big) L/U with my friends 4 cylinder 21" pitch prop. One prop shop in north Georgia says it won’t go, will just "dig a hole", I think he’s wrong. If my QD11 could turn a heavy, high pitch 2 blade brass prop, why wouldn’t a 40hp (soon to be 50hp) turn a 3 blade SS prop? Of course what I wanted to run was a 2 blade, 25" pitch SS prop, but I guess it’s a 6 cyl. prop and can’t be rehubbed to 13 spline.

[bh]

I would go with the small unit regardless of ratios, because if that is what I think it is ,
ie; the Nitro unit used under the 3cyl 497"motors in racing, then many good, used and proven hi-speed props
are available for it . Not that there are not props for the 4" unit, just not as many fast ones .
1:1 ratios are not all they are cracked up to be. Mercury gained performance with their 20H
back in the ’50s by going from 1:1s to 16:21s.
Go figure ?

[dan-in-tn]

First off do your research. I don’t think that shaft will work in both cases. The different ratio is obtained from a larger pinion gear which won’t work with the smaller drive shaft splines! The smaller pinion won’t work with the gears from the gearcase either, but I figure you know that. You may be Screwed !**!#! As the small gearcase didn’t come along until ’75/’76. Didn’t we figure out this big case might be a ’72/’73 because of the shift mechanism in the front? Hydro mechanical? I may be confused as to where you are headed at this point, but I dont think the gear ration is enough to be your biggest problem. These motors don’t mind turning a full 6000 RPMs stock. Hoping for the best. You got to try it to see. Plenty of prop sizes in both styles.

Dan in TN

[kerry]

What boat will the motor be used on? For what purpose? If a small light weight hull, little towing or skiing, only a few riders, the smaller case will be ideal. Heavier hull, many people, towing a lot, larger case would be ideal.

If you have too many, AND not enough, you're a collector.

[beerman57]

Dan, yes on the early 70’s L/U and shift mechanism. I’m starting to like the higher gear ratio in that big, old L/U the more I think about it. The motor will be on a light boat and I’m looking for speed, that’s why I like the more than 5 extra prop revolutions PER SECOND as compared to the newer L/U, and that’s figuring at only 5000RPM. Whatever, I’ll be trying the old L/U and if it’s fast enough, I’ll probably just stay with it. First I have to get the parts to fix the steering arm/tiller from the motor’s lame attempt at a front flip.

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