Re: [Amc-list] Looking for a torque tube T-96 trans w/OD

[Frank Swygert <farna@xxxxxxx>]

Almost forgot! The OD unit spreads the gears out better, just like a 
four speed would. It reduces rpm at cruising speed, but everything 
depends on the rear axle ratio. Non-OD cars were geared for best 
take-off and cruising speed with a standard three speed transmission. 
That means the gearing will be too high (low numbered) when the trans is 
in OD. The Borg-Warner OD units have a gear ratio of 0.70:1. A standard 
three speed car with a 3.08 axle ratio would be like having the same car 
with a 2.38 rear axle. Too high for first gear with a three speed, and 
too high for the engine to pull effectively in OD. Engine rpm would be 
so low in OD that the engine would struggle to hold speed even on a 
level. So you have to reduce the axle ratio to compensate. A typical OD 
car would have a 3.31 rear axle or even lower (like a 3.78). How would a 
lower gear help economy? A 3.78 car with OD would be just like the same 
car with a 3.08 at cruising speed, right? Yes, it would, but it would 
also have a much lower first gear and a better gear ratio spread between 
first and fourth (third + OD). The engine would work less getting the 
car moving, and that increases economy in city traffic. The 3.31 would 
be a better choice for a highway cruiser of course.

So why did the auto OD unit become popular in the first place -- a four 
speed trans makes more sense! The OD unit was developed in the late 
teens/early 20s, way before the automatic transmission, which was 
developed in the very late 30s and still expensive until the late 40s. 
The old engines were big and torquey at low rpm and didn't put out much 
horsepower for their size (a 1935 Nash Ambassador had a 260.8 cubic inch 
straight six that developed 100 hp @ 3400 rpm, 110 lb/ft torque @ 350 
rpm, compression ratio was 5.25:1). First was only used to get a car 
rolling from a dead stop, after that only second and third were used to 
reduce shifting. With the automatic OD unit once you got up to third 
gear there was no shifting needed at all, just let off the gas to shift 
into OD, floor it to shift out. Sort of a poor man's two speed auto 
trans. With careful driving one hardly ever had to shift the trans down 
at all, unless the car was at a dead stop. Any other time just lug it a 
bit in third until it got back up to speed -- the old long stroke motors 
could handle the lugging! With changing times came changes in 
engineering thinking and transmissions. You can't drive an old car like 
a new one, or vice versa! Try to lug a modern stick shift car and you'll 
get "the jerks" or choke it down!

-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)

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