General Motors Turbo-Hydramatic 200C
A little history on the THM200 and THM200C. The THM200 was introduced in 1976 as a more efficient 3 speed automatic as compared to earlier design 3 speed automatics such as the THM350 and THM400. The need for this new automatic was brought about by the gas crunch that came about in the 1970s. The THM200 was produced from 1976 through 1978. This unit used an open style torque converter (non-lockup). The THM200C unit was introduced in 1979 and used a lockup torque converter clutch to further increase vehicle gas mileage. The early designs of lockup torque converters gave the 200C a very bad rap since most of the failures were brought about by the torque converter. Having built many 200s and 200Cs during the 1980s, I had grown a respect for the unit. It was simple, quick to rebuild and when done correctly was very reliable. Of course, we always crossed our fingers when it came to the torque converters, but GM eventually straightened out the design problems of the early lockup torque converters and from that point on lockup torque converters took over the industry. The 200 was the first to introduce a radical design change in the hydraulic control systems that has carried on to all future designs for GM. This was probably a very big factor when it came to the acceptance of this new unit by transmission technicians that struggled with trying to understand why they couldn't get these units to operate correctly.
After performing some research and development work on the 200C for use as a high performance transmission, I have become very optimistic about this unit. It is light, simple, efficient, periodic maintenance or checkups can be performed very quickly and cost is low, although some very specialized recalibration has to be done to the unit to insure that line pressures are sufficient for high horsepower levels. Lets take a look at the THM350 and THM400 for a moment. The main weak points of the THM400 and THM350 transmissions are the intermediate (2nd gear) roller clutches or sprag as in the case of the 400. The case lugs that support the intermediate clutch pack retaining snap ring is another weak point for the THM400. Not to say that the 400 and 350 are weak by any means, but every transmission has its Achilles heel. During the course of a racing campaign the cost for periodic repairs is something that should always be looked at. The 200C uses a band instead of clutch packs for second gear as is the case for the 350 and 400 and this feature gets around the problem of catastrophic 2nd gear failure which eventually catches up to the 350 and 400 due to simple metal fatigue brought about by the stresses of racing. The bad part about the 400 is when those aluminum snap ring case lugs break, there goes the case. Not a cheap prospect. Those people that drag race their car and use the 200-4R as is the case with the Buick Grand National or Turbo TA vehicles usually do not use overdrive during the 1/4 mile run and since the major weak points for the 200-4R are in the overdrive section even after upgrading to a billet intermediate shaft, the 200C becomes a nice alternative for racing purposes. The main advantages of the 200C over other transmission model options are less rotating mass, lighter weight (96 lbs. 1 pound less than an aluminum Powerglide) and most importantly less parasitic horsepower loss. Weight comparisons of common transmissions. With the two units on a bench, just the difference in turning effort by hand of the input and output shafts is amazing and very profound. This can't help but translate to less horsepower loss through the transmission and more getting to the rear wheels. I put the horsepower level for these units at 600 HP. The limiting factor being direct clutch (3rd or high gear) capacity.
DRW Transmission Specialties, Inc. 10728 Kenney St. Ste. B, Santee, CA 92071
Office phone: (619) 258-4240 Fax: (619) 258-9105
Copyright 1998-2008 DRW Transmission Specialties, Inc. All rights reserved.
