An all aluminum 20B should be about 300 pounds. P-port with turbo
  should give an easy 600HP. To get a higher HP capability gear box
   I would use the Allison HT 640/HT 650 series planetary with approximately
 3.5:1 ratio. This transmission was designed for use in 5 ton trucks
  and buses. These transmissions are readily available and so are spare
   parts. Probably a lot cheaper than the helicopter PSRU. The design
  would mimic the current Ross/Tracy box with dimensions changed
   to accommodate the Allison part dimensions. The weight should be
   close to the helicopter box and the length should be similar.

   Dale Davies

   The helicopter box is surprisingly light with magnesium castings
   at only 45 pounds..

   The drawback is; it is $30,000.

   Paul Lamar


   Dale,
     Do you know which planetary of the Allison has the 3.5:1 ratio ?
   Kelly Troyer

   Allison transmissions have a number of current and obsolete
   transmission series, all designed for medium to heavy duty use.
   AT 500 series. Kind of like an overgrown turbo hydramatic 400.
   The torque converter I am told can be used on a TH 350 giving
   a low stall RPM which can help fuel economy.
   MT 640/643/650/653 series. There are more models in this series
   but these are the ones to look at. I believe they have 6 pinion
   low planetary with the 3.58:1 ratio. This is a medium duty trans
   designed for 250HP and 640LB/FT of torque. I do not think the
   HP is a concern. The torque capability is the advantage.
   HT 740/750 series. This is a more heavy duty trans as the 445HP
   and 1435LB/FT torque capacity indicate. This has a 3.69:1 first
   gear ratio.
   CLT-3461. This trans was used in the Terex 14B scraper with
   twin engines and transmissions. The engines were Detroit Diesel
   4L71 naturally aspirated. Similar in size and power capacity to the
   AT 540 but with 6 speeds and the first gear was 3.81:1. If a wide
   cord prop was used with it I think noise could be kept down.
   A search of Allison transmissions will bring up a lot of information
   with diagrams.

   Dale Davies

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