Why a 1:32 ratio for a 260 merc?

[chris650]

Hello everyone, just want to become more knowledgable on the uses of certain gear ratios...advantages, disadvantages etc.. For instance...i have heard that for most merc sbc's, 1:5 is the way to go. But I happen to have a 1:32 upper unit in my garage that was used on a 260. Was hoping someone could tell me why? Was there a special edition or something? thx

 

[thumpar]

The 1.32 was meant for the 454. Those stickers may be newer than the drive. I don't know why there would be that ratio behind a 350. Have you verified the ratio stamped on it is accurate? If it truly is a 1.32 it is worth some money since they stopped making them years ago and only 1.5 is available.

 

[chris650]

yep, i verified it. "H" is stamped on the yoke, and i even pulled the cover off and counted the teeth on the gears... 20 and 16. Maybe that 260 sticker was placed there at a later date?

 

[chris650]

so help me understand...why wouldnt you want to use a 1.32 on a 260?

 

[thumpar]

Do you know the history of the boat? Normally if the drive ration was not "normal" it would be higher (in number) not lower like if you had a bigger boat or higher elevation. The 1.32 is coming to be a rare item since they were put behind 454 and unless careful the power was too much. They stopped doing that combo and the Bravo came out.

 

[chris650]

so let me ask...in good shape, how much would you say this upper unit is worth?

 

[HT32BSX115]

so help me understand...why wouldnt you want to use a 1.32 on a 260?

Propellers are efficient in a rather narrow range of RPM. Turn them too slow and you increase the slip. turn them too fast and you increase the drag and cavitation................So if you do not have the torque available to turn it through a 1.32 drive, you have to run a lower pitch prop.

But turning a low pitch prop at too high an RPM will result in additional (RPM) drag losses The ratio "Gods" determined that the optimum ratio (to produce the most efficient RPM running most SB engines was 1.4-1.6:1 or so. (the 454 produces it's optimum torque 200-400 RPM lower than a SB (my 454 is rev limited at 4700 and max recommended RPM is 4600.) A LOT of 5.0/5.7L engines are rated at 4800-5000 RPM MAX.

It's not that a 1.32 wouldn't "work" .............it would probably work "ok".............. It just wouldn't be as efficient as a 1.5 or a 1.68 ratio turning a 2-4" higher pitch.

5000 @ 1.32 would result in a prop RPM of 3787 RPM

5000 @ 1.50 .................................................3 333 RPM

There is likely an upper limit on how fast you should turn a prop before the losses start to become more of a factor. (probably somewhere around 3000 prop RPM)

I know what you're thinking. Racing engines turn props at 6000 and higher RPM! But look at those props. They're designed quite a bit differently

If you look at the MAX and cruising RPM for most of the 4, 6 and 8cyl (SBC/BBC) engines, the recommended drive ratios deliver a very narrow (similar) prop RPM range.

so let me ask...in good shape, how much would you say this upper unit is worth?

Whatever you could get someone to pay you for it! (really)


regards,


Rick