After rebuilding my engine my transmission started dying on me, I'm sure my right foot had nothing to do with it.  So I needed to either rebuild my existing numbers matching close range Muncie 4-spd or I could take it out, throw in the shed and install an overdrive transmission.

After looking into the price of a good rebuild kit for the M-21 and looking into the price of 5 and 6 speed transmissions I decided I should get the overdrive so as to get a more functional car.  My two complaints with the M-21 are not enough top end, not enough bottom end.  Basically the close range 4-spd doesn't have a big enough gear range.  My car has a 3.36 rear end and 27 inch tires.  Therefore my launch ratio is only 7.4 (2.20:1 x 3.36:1) which is hard on the clutch and isn't a very quick low end.  My cruising speed is figured at 70mph, the RPM's on the M-21 in 4th at 70 is 2928.  This is way to high at cruising speed.

With the choice of rebuilding out of the way I started looking at 5 and 6 speed transmissions, the Tremec T-56 seemed to be the good modern 6 speed choice.  The Tremec TKO-600 is the good modern 5 speed choice.  To make it more confusing there are three options for the T56, the Viper, the Camaro, and the GM Aftermarket.  Each has good and bad points.  I looked at each option with an open mind, allowing the possibility of a rear end gear swap as well.  My criteria for this swap was the following.  I wanted a good launch ratio, i.e.  between 9 and 11, below 9 your gearing is too high and its very rough on the transmission and rear end when you drop the clutch, higher than eleven your gearing is too low and you can't get any traction.  More importantly was a good highway gear, I wanted cruising RPM to be between 1800 and 2000, this lets me stay at the bottom end of my torque band while cruising.  I didn't want my top gear to turn the car into a gutless wonder.

One other thing that really limited my choices is the numbers match on the car.  Since I have the VIN/Code matching 427 and close range Muncie, the car is a candidate for future restoration.  As such all of the modifications I have made and will make are bolt on.  I must be able to restore the car to original condition, this means no permanent irreversible modifications allowed.

The following options were considered. (click on bullets to expand and contract)

• Camaro T56:

  • Requires Change over to hydraulic clutch

    • This didn't fit my hidden mod policy.  Changing from Manual to Hydraulic involves changing the pedal actuation and mounting a slave cylinder.  There isn't much room to work and this usually winds up requiring permanent mods.

  • Can't really find one that is new.  These are only available from wrecks.

  • Parts and adapters not as available as Viper or Aftermarket T56.

    • Id rather be able to order parts online, not scrounge junkyards.

    • Parts are more expensive.

  • Shifter location not at correct 19.5" 2" left offset.

    • This didn't fit my hidden mod policy.   I want the shifter to be in the correct location so at first glance the car looks original.  Also cutting the shifter hole is a permanent mod.

• Viper T56:

  • This is a larger transmission, there is some dissent but every article I found says you must cut a hole at the front of the tunnel and may have trouble getting it to fit in the tunnel at all.

  • Expensive.

  • Shifter in correct location only available from one vendor.

    • The one vendor has issues.  Ill expand on this below.

  • Gearing (2.66, 1.78, 1.3, 1.00, 0.74, 0.5).

    • With 3.36 gears

      • Launch ratio of 8.9 just a hair low.

      • 70 mph RPM of 1464 way too low.

    • With 4.11 gears

      • Launch ratio of 10.9 on the high side of what I'm looking for.

      • 70 mph RPM of 1791, still a bit low.   

    • Close ratio bottom, wide ratio top.

      • Adjusting the gear so 6th would be usable would make me drive in fifth on the street and 6th on the highway.  I could probably skip 2nd and 4th.  Kind of defeats the purpose of a 6 speed.  Im looking for a wide range of gears, but this one has a close ratio on the first 4 gears and wide ratio on the two overdrives.  Great for someone who drives 100mph plus regularly.

• Aftermarket T56:

  • Gearing (2.97, 2.07, 1.43, 1.00, 0.8, 0.62).

    • With 3.36 gears

      • Launch ratio of 10.  Perfect.

      • 70 mph RPM of 1814.  On the low side but in my desired range.

    • Wide ratio bottom, close ratio top.

      • Wide ratio lower gears, close ratio overdrives.  Most of my driving is city, this is the right combo.  This would let me use 3rd or 4th driving around town and 5th for in city highway and 6th for open road.

  • Fits in tunnel with no mods.

  • Relatively affordable.

  • No shifter exists at the correct location.

    • This didn't fit my hidden mod policy.   I want the shifter to be in the correct location so at first glance the car looks original.  Also cutting the shifter hole is a permanent mod.

• TKO-600:

  • Correct location shifter available from three different vendors.

  • Gearing (2.87, 1.89, 1.28, 1.00, 0.64).

    • With 3.36 gears

      • Launch ratio of 9.6.  Nearly perfect.

      • 70 mph RPM of 1874.  Just about right.

    • With 3.55 gears

      • Launch Ratio of 10.2. Perfect.

      • 70 mph RPM of 1980.  Nearly perfect.

  • Relatively affordable.

    • Cheapest option.

    • This lower price will allow me make a differential gear change later when I rebuild it and still come out ahead.

  • Fits in tunnel with no mods.

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