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Thread: T90A vs T90C, What a difference!

  1. #1
    Join Date
    Nov 2007
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    Default T90A vs T90C, What a difference!

    Recently I realized that my transmission was going south but did not really know how bad until I tore into it. My T90A was beyond any practical repair

    So I decided to Get another one from Herm, Seeing how I had to spend the bucks anyway I thought I would cough up the extra and give the T90C version with its lower 1st and 2nd gears a try...........And its great! I am not wishing for a 4 speed anymore, This is a great option for anyone with a T90A wanting lower gears but cant amortize a proper 4 speed conversion.

    The lower gearing made a new machine out of the old cj, Its noticeably easier to take off in high range from a stop and It made a HUGE difference climbing around in low range ........ And I dont have to ride the brakes hardly ever anymore going back down these steep grades The gears are just low enough the engine can now hold the rig back unassisted 90% of the time

    In high range the lower 1st and 2nd are the perfect dirt road cruising gears ( for me anyway ) with the 5.38:1 differentials, Second gear on the T90A was a little to fast on the rough roads locally unless you were nearly lugging it just above idle, Letting off the throttle for a rough spot usually meant a shift into 1st. Second gear in the T90C is low enough that you can slow down for a rough spot and gently throttle out the other side without shifting into 1st about 80% of the time and it is not over revved when you get back up to speed...........In fact my manifold vacuum gauge shows slightly higher vacuum ( 1hg - 2hg ) at the speed I like to drive in second.

    There is a slightly larger gap between 2nd and 3rd so it would mean a slightly slower pull in second through the mountains once a shift into second had to be made but its not hard to get used to, In that respect the T90A version would be a little more "street friendly " by comparison.

    I thought I would post this for anyone thinking about this option, As I am very happy with it

  2. #2
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    Default Re: T90A vs T90C, What a difference!

    Brian, I wonder if you had an OD if you could just OD quick in 2nd and then jump into 3rd? Just an idea. Sounds like a nice transmission
    If you're curious as to what ever came of my 47CJ2A, check out
    http://www.pirate4x4.com/forum/showthread.php?t=857320&highlight=

  3. #3
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    Default Re: T90A vs T90C, What a difference!

    Quote Originally Posted by sammy View Post
    Brian, I wonder if you had an OD if you could just OD quick in 2nd and then jump into 3rd? Just an idea. Sounds like a nice transmission
    After running both versions I would think the T90C and an OD would make a good combo.

  4. #4
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    Default Re: T90A vs T90C, What a difference!

    Brian,
    Thanks for the helpful post. I've been considering the transmission of choice for my new project. I want a three speed for the cost aspect of it and I just kinda like the three speeds anyway. Sounds like you and I do a lot of the same type of driving too.



    Adam Sparks

    “Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.” ~ Mark Twain

  5. #5
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    Default Re: T90A vs T90C, What a difference!

    IIRC the T-90C is only compatible with the F134. V6s need not apply. There's no long input shaft available for the T-90C.

    It would work with the Pinto 2000/2300 swap though.
    Tim Reese
    '75 CJ-6, Copper Polly 304/T15
    '77 J10 type 45 (131" WB) 258/T15/3.54
    '82 J20 type 27 360/T18/NP208/3.73 7600 GVWR
    '95 VW Golf Sport 2D 2L

  6. #6
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    Default Re: T90A vs T90C, What a difference!

    Quote Originally Posted by timgr View Post
    IIRC the T-90C is only compatible with the F134. V6s need not apply. There's no long input shaft available for the T-90C.

    It would work with the Pinto 2000/2300 swap though.
    Dangit. Guess my search continues since I only have V6s.


    Adam Sparks

    “Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.” ~ Mark Twain

  7. #7
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    Default Re: T90A vs T90C, What a difference!

    Great post Brian! <wish there was an FHead smiley that was as recognizable as the V6>
    I don't knoww if I have a "C" or not, but I think so. Unfortunately, when I do the calcs and use GPS in these hills, after normalizing rolling tire diameter, the 5 mph diff between 1st and 2nd at various PRM's, I can't say for sure.

    I'm responding because what you describe as the "C" performance is exactly what I have ... I have to UPshift sometimes because the chosen gear is too low ... in 1st, low range steep descents on old trails is almost too slow and, in just slightly wet conditions, a slide occurs (tires inflated for paved mountain driving).

    As has been eluded to, my main interest in OD is splitting 2nd gear on 5%-10% grades
    Last edited by sparky; 02-02-2009 at 11:21 PM.
    FHead, T90, D18, 31x10.5x15's, 1" springs, Weber 32/36, MSD 6a, 2" exhaust, M6000 D1 winch, daily driver
    "It is easy to love the V6, but it takes real connoisseur to love the L & F heads."
    "Jeeps" were manufactured by brilliant men long ago. Street rods, buggies and crawlers are "built".

  8. #8
    Join Date
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    Default Re: T90A vs T90C, What a difference!

    Wasn't the "c" installed in 64 and later CJs with the 4.27 gears?
    • In the land of the blind....the one eyed man is king!
    '53 CJ3B F head, lockrights, Koenig king winch.11" brakes at all corners. Nice rusty PA frame and body. Just more work in the long run

  9. #9
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    May 2005
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    Default Re: T90A vs T90C, What a difference!

    I second the improvement of the t-90c gearing. I had Herm install the c's gears in my t-90a when he rebuilt it. Not a big change, but you notice it in underdrive.

  10. #10
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    Default Re: T90A vs T90C, What a difference!

    The following is an excert from
    "Everything You Ever Wanted to Know About CJ-3B Speed and Then Some".
    This also applies to models CJ-5,6.

    For our model of jeep there were two differing transmission input ratios in use.
    See bulletin # 600:

    http://www.film.queensu.ca/CJ3B/Phot...B600Tranny.JPG

    Since slightly more hp was realized by the 134 engines progression,a slightly faster vehicle speed became feasable.
    So in model year 1963 the transmission low gear ratio was increased.
    This transmission change was done to offset the simultaneous axle change from the 5.38 ratio to the 4.27 ratio.
    With this combination of 3 changes including the ,engine, transmission and axle;
    the maximum vehicle speed would be increased without adversely raising the minimum (crawl) speed.

    In the transmission this change was manifest by changing the main drive tooth count from 18 teeth to 16 teeth
    plus also changing the driven cluster gear tooth count from 33 teeth to 35 teeth.
    Remember these particular driving and driven gears can only be interchanged as a set.

    Here then we have two different T-90 variants that were used in the 3B.
    Below are the gear ratios for both the pre 1963 (early) Borg Warner T-90 A and the post 1962 (late) T-90 C.

    T-90 A T-90 C
    (INPUT) (INPUT) (OUTPUT)
    1st gear 2.798 3.339 1.00
    2nd gear 1.551 1.851 1.00
    3rd gear 1.000 1.000 1.00
    reverse 3.798 4.531 1.00

    As you can see from the numbers the velocity of the transmission output drive will be affected by gear selection.
    The transmission velocity is to be understood as a numerical ratio, input to output.

    Low gear ratios reduce a vehicles resistance to the force by decreasing traveled distance.

    The minimum output for the early T-90 A in 1st gear is ( 600 divided by 2.798 =) 214 rpm.
    The minimum output for the latter T-90 C in 1st gear is ( 600 divided by 3.339 =) 180 rpm.

    High gear is always a straight through drive without gear reduction.
    So the maximum output for both T-90's in 3rd gear is 4000 rpm.
    As you can now see the T-90 C has a greater gear ratio spread.
    Ken Bushdiecker
    1953 TRANS-VINTAGE CJ-3B Second full rebuild using only the best from all 3B vintages.
    1962 OPTIONAL-STOCK CJ-3B / Warn O.D. / Tigertop / Transport yellow (orange)
    Currently serving as my one and only DAILY DRIVER

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