#4 Speedmaster Gear Ratio
 

As most of you know gears for the 4 & 5 uppers are hard to come by: Question, someone rebuilt a upper using a 16/21 gear count = 1.31 ratio, the other upper is 15/20 gear count =1.33, lower on both 1.125.
Being the power is evenly match, has anyone used a ratio this close and was there any RPM spread, the props are matched and ran even RPMs prior to this. Maybe I am splitting hairs, matched gear sets are like props and matched engine power.

My guess is it would be a minor variance, very minor. maybe 50rpm? But, just a guess.

i agree,most marine tach,s it would be hard to see a 50 rpm difference from one to the other,and engine load will be almost identical.

That's the question, how much? By the time you figure slip.
This is on an older 33" Executioner, came with TRS back then, changed to #4 back 8-10 yrs ago. X # was in the TRS location. It has been moved up about 2 ", so now we changed everything. Same power 780 @ 6200, now the RPMS will increase and 1 drive is slightly different. I know the props will need some tweaking, but now the props will have to adjusted for gear ratio.

If you have a chance; check all the different ratios that were available. The gear counts are close. Why would MERC offer so many ratios?

gears
 

if you want answers to you questions call craig corabella of corabella performance he was aperformance drive guy at lake x really knows thoose drives

Is it standard or counter rotation? We need the details to solve the mystery!

I spoke with Craig, back at the beginning of this. I called; him to find out if he had some 1.33. The gear that is a 1.31, is from the guy; that purchased the molds and tooling from MERC. I was just trying to get a heads up on the difference in RPM, the drive guy's answer is bend the prop. My customer would have had no problem buying a matching gear set for the other. It's just screwed up to me. Guess we will what and see.

You have been out of town to long! Get some rest over the weekend, and ask me that question Monday

Wasn't it your boat, the one that someone installed the drives on up-side-down, that you had to drive it with scuba gear on??

1.31
 

I hate to disagree guys but the gear ratios need to be exact.:party-smiley-004: Years back a friends boat had 1.58 on one side and 1.50 on the other. It never handled right and would sometimes roll side to side(the diff was only a couple hundred rpm)....guess what fixed it? The next winter he had the uppers matched up and she tracks straight well past 100mph now. The difference in acceleration from side to side I believe can mess with a boats ability to "set" correctly. just my.02. I also have seen super fast skaters dial in some handling issues by adjusting throtle cables to pull exactly even. Your 1.33 upprer has helical cut gears for the #3 and #4. Your 1.31 drive has the stronger #5 straight cut design that GK makes to put in the older SSM's. Guess it all depends what you are doing. Good luck.

Hey Andy! Good point about the straight cut/helical gears.

When I replied earlier, based on his gears he posted, I came up with one drive being a 1.49 Ratio, and the other being a 1.47 ratio?? Is that correct?

Plugging those numbers into a slip calculator with 10% slip, 6000 engine rpm, 28p prop, 97mph, going from 1.47 to 1.49 changed the engine RPM by 50.

I never ran anything fast like you cat guys. A couple seasons ago I ran a set of ssm props, that were both stamped 17x27. MY engine RPM was off by about 200. I brought them in and had them scanned on a prop scan machine. one prop was 27.2'' of pitch, the other 28.0. Other than the rpm difference on the tachs, I noticed nothing ill handling wise. But, that's a 85mph vee, not a fast cat, so its probably moot.

You are correct on that info. I know the 1.33 upper and the 1.125 lower work out to be 1.5:1, what does that work out to be with a 1.31? The math formulas for gear ratios I found need to know the teeth count of the lower gears. I was unable to find the teeth counts in the manual only for upper. Anyone know the final ratio?

I do.

Multiply the 1.31 upper gear set ratio by the 1.125 lower gear set ratio and you get 1.47325 overall ratio for the drive. In terms of overall strength the 1.31 net forged, straight cut upper gear set is far superior to the spiral cut 1.33 original gear that came standard with the #4 drive. Mercury never produced (as far as I know) a 1.31 straight cut, net forged gear for the #4 or #3 drive. When I had the upper gear sets, forged by Mercury's OEM supplier, I had them make a "hybrid" gear with the same forged straight cut gears as were used on the #3A'a and #5 drives and the "shank", as I call it, to fit #3 & #4 drives. I had 10 of these sets polygon'd to use in the #4's. I have, in stock, new forged straight cut, 1.31:1 ratio upper gears for all the SSM drives, #3, #3A, #4 and #5's as well as new LH and RH 1.125:1 lower gear sets and new "big" propshafts to fit all the SSM drives (including the #3 and #4 drives). I hope this adds clarity to the overall ratio and gives a little more background as to what is still available for the SSM drives. My goal in doing all these gears, shafts and parts for the SSM drives is so the owners of these drives do not have to scrap them if they experience any drive issues.

Also, if you are really concerned about the ratio of the drives, you could convert the 1.33 drive, whose upper gear set is NLA from Mercury, to the still available and much stronger 1.31 upper set. Granted I have a vested interest in selling gears, but this seems to be the only way to make them the same ratio.

Gary
612-247-7081

Gary, Hey thank for the help and information. I called you about these gears last year, with concerns on this issue. It was out of my hands on the drive rebuild and I am VERY DISAPPOINTED, in the out come, almost a year for the guy, to resolve getting gears and actually doing it. Why, he wouldn't just have let me get the gears from you and we handed them over for set-up, is way beyond me! I will say; it's non of the top guys we know out here. They would have never taken as long, nor would they follow this procedure of mixing ratios.

#4 gears
 

Footnote. I just spoke with a friend that has been doing drives for 30+ years for one of the premier offshore shops in Fla. and he informs me that when Mercury switched from the spiral cut upper gears in the SSM drives (#3,#3A, #4 and #5) to the straight cut forged gears, the strength was increased by 50% if not 100%, (his words)

Yes, Thank you everyone for you input on this. Spoke with Gary, today; he informed me 50-75 rpms difference. The bad thing is the person that did the work, charged my client for 2 sets of gears , 2 different ratios. One drive with 1.31 and the other has 1.33 that the parts came from the better of the 2 drives. The other new gear set is on the shelf in his shop, which I informed my client to get. All said and done; he could have had 2 new sets of gears in both drives, so now this is what we are doing.

I have the 1.33 that are used and a new set from Merc. that I will have in my hands by Friday, at that time I will verify the ratio of the new set. If you or anyone is interested in buying the used and new set, let me know. Needless to say; my client is not happy about the drive issue. The guy; told him flat out, they matched in ratios. Pretty sad deal!!!

That sucks.....So is he going with all new straight cut gears? Is he running the 26 spline or 19 spline propshafts? How much power is he putting thru these iv's? Only reason I ask is I too have iv's. I have no idea the history on my drives, as in hours, etc. Been running them for 3 seasons with no issues, but Im starting to worry about them.

26 splines. Right at 780 hp, naturally aspirated.

upper gear set
 

if the price is right i may be interested in the used set if it is splined for #3 .... not the 4 poly gon shaft .. i have a good drive mabe interested for spares mike 708 308 9584