t56 magnum fitment

Discussion in 'Transmission & Driveline Topics' started by rosco72z, Sep 4, 2020.

  1. DesmoEd

    DesmoEd Veteran Member

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    That's some excellent info, it looks like that T56 is a perfect match with 4.10 out back. Seems like that .80 5th would be a nice intermediate step from the TKO's 1-1 4th gear and .64 OD

    How does its 1st gear compare with the 2.87 on the TKO?
     
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  2. badazz81z28

    badazz81z28 Veteran Member

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    the .64 is not a perfect match to 4.10s...i have 3.90s thats about as deep as I would want to go. I would go back to a 3.73 for better cruising RPMs.
     
  3. DesmoEd

    DesmoEd Veteran Member

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    Maybe you missed this part:

    "I used 6th (.63), so with a true 26.9 tire (27.3 calculated) @ 2,350 RPM got me to 80 mph/129KPH, I was just off my mains, 15 AFR's, timing almost all in, car got quiet and was back to hitting 20+mpg"

    With 3:73 out back and hitting the same 2350rpm target you would need to run over 80mph. And this is on a 27 inch tire, most guys now are running low profile 17 or 18 inch rubber thats closer to 26 inches. My experience with the .64 TKO in my Camaro was similar - the sweet spot for steady cruise was around 2400 and in order to actually use the .64 OD I needed to run 80+ mph to avoid lugging the motor. I can see a modern LS type FI motor running 1800-2000rpm in cruise but not a carbed Gen 1 400hp sbc and if so certainly not very efficiently. I had to look up the gear ratios on the T56 and it looks like some even run a .50 6th which would be useless with 3:73 out back

    Now with 4.10 2400rpm = 74 mph which is ideal for my combo which was the point of my original post.

    I dont see any point in changing from 3.90 to 3.73 as at the same 2400 rpm you only gain 3 mph in top gear
     
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  4. 76z28

    76z28 Veteran Member

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    That REALLY depends on cam though too.
    I am going to 4.10s just because of my cam... needs to stay about 1700 and will allow me to speed up or slow down when needed where 3.73 doesn't allow for that. I do have 28 inch tall tires too.
     
  5. biker

    biker Veteran Member

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    Just finished up putting in the 3.73 gears in my 10 bolt, 81 Z with 383 and tko600. Wow. Better combo than I had imagined. I had 3.08 in it, and while it still pulled hard, 1st gear was a bit tall, and 5th was under 2000 at highway speeds. The change to 3.73 is amazing for my .64 5th, and makes all the gears work better. My engine will happily putt along at 2200 in 5th with no lug feeling at all.
     
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  6. 76z28

    76z28 Veteran Member

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    How tall are your tires? Mine turned 1900 at 70 with 3.73s with 4.10s it'll be around the 2100 mark which gives me a lot more buffer room to slow down with traffic and stuff
     
  7. G72Zed

    G72Zed Veteran Member

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    Not surprised by your findings biker, and one of the main benifits of the T56 is the 5th at .80.

    It gives you that nice 20% drop from the 1:1 4th to bridge the TKO/T56's .64/.63. Nice to have, if you have the choice, but not a deal breaker or worth changing out the TKO.

    Glad to see your out there enjoying the ride.

    Tested out my customers 500hp BBC 69 Chevelle tonight, TKO600, .64 with 4.10 with 275/60/15, thing is just sweet.
     
  8. G72Zed

    G72Zed Veteran Member

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    Yep, your bang on the money DesmoEd.

    Actually I went from a 2.89 to a 2.97 1st gear, and at the same time went from a 3.25 to a 3.5 stroke, so O/A first gear/stroke combo matches up better, I was under geared by 5.3%, now I'm over-geared by 1%, and I love it.....
     
  9. biker

    biker Veteran Member

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    Currently stock size for 81Z. 225 70 15, so I think that is a tick over 27 inches. Next will be 235 60 15 which is about an inch shorter. Mynspeedo isn't quite accurate at the moment.
     
  10. biker

    biker Veteran Member

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    Yes. That 5th gear would be the ultimate in a 6 speed, especially to get into the meat of the powerband for a good top speed run.
     

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