OMC 400 and 800 series drive ratios - weird

[whywhyzed]

1984 OMC stringer drives overall ratios (from OMC manual and Seloc agrees):<br /><br />The 3.8 (170 or 185 HP) uses 1.77 overall<br />The 5.0 (230 HP) uses 1.5 overall<br />The 5.7 (260 HP) uses 1.41 overall<br /><br />so far the trend is looking sensible...there's a different chart for high altitude so no confusing those<br /><br />But then, the 2.5 (120HP) and 3.0 (140HP) use 1.71 overall... which is in between the 3.8 and 5.0's ratios...<br /><br />And earlier year 2.5/3.0's used a 1.64 overall ratio.<br /><br />I gotta go think about this some more.... I'm thinking something's being done to try to save the life of the splines of the w/p shaft by keeping speed up in the upper.... and then using lower to turn into torque... off to look at upper ratios vs. lower ratios....

 

[KaGee]

Re: OMC 400 and 800 series drive ratios - weird

I'm not at home, so I don't know the specifics. This I do remember... The lower gear ratio's in the 400's are different from the 800. The 800's are all identical, with the upper being the difference in all three motors.<br /><br />It's too late for all this figuring Hystat! If you can't get to sleep, drink some warm milk!

 

[whywhyzed]

Re: OMC 400 and 800 series drive ratios - weird

yes, the lowers are different- but those ratios I quoted above are overall ratios of the drive, taking into account lowerXupper.<br /><br />back to basics....<br />HP = speed X torque <br /><br />(and yes there's a constant of 5252 to make James Watt's units all jive together)<br /><br />An outdrive converts speed to torque in the HP equation by slowing the prop in relation to the engine rpm.<br /><br />The slower the prop goes, the more torque you get.<br /><br />Torque is what ultimately twists splines and breaks shafts and gears.<br /><br />So I think that's the issue....and the reason .... they couldn't go 1.98:1 like Merc because the 400 series lower unit wouldn't take the torque ... so they keep more speed to the prop shaft and then finally let a lower pitch prop convert to more torque...<br /><br />By the way, I disassembled an old 1966 V6 upper recently...the upper ratio is 25:25 and the w/p splines show zero fatigue....original parts.. by keeping the w/p shaft flying around at engine speed, it is subjected to much less torque and survives decades....