Toyota Supra exhibits persistent turbine hub failure
Recently a few transmission shops have questioned whether or not a stronger torque converter exists for the Turbo Toyota Supra, used in the A341 transmission. The problem that persists is constant failure of the 20 spline input shaft turbine hub. Under high performance conditions the turbine hub strips out after two to three thousand miles. Replacement hubs are not yet available through usual suppliers.
One suggestion to curing this problem is to use 4340 material, machine and broach new hubs and finish up by using a unique heat-treating, annealing (softening), and cryogenic tempering process. This procedure, known as Cryo-Tek, is a hardening process differing from common heat-treating procedures by converting the retained austinite (by-product of heat-treating) into martinzite. This allows the turbine hub to sustain higher torque levels up to 4 times stronger than its conventional hub.
In addition, a second problem plaguing these units is converter clutch failure. It is suggested to use the well-know “red” clutch liners as a replacement material, available from your regular converter supplier, such as Sonnax, Tri-Component and Raybestos. This offers the highest coefficient of friction available for this performance converter. A surface finish of 10-RMS or better is required to ensure proper clutch performance. Clutch clearance should be reduced from the average .075” to .035” which can also improve clutch performance. Replacement of both stator caps from the factory-cast to solid billet aluminum replacement caps also improves durability. By substituting factory-cast aluminum for a stronger replacement part, the chances of stator cap cracks and breakage is greatly reduced.
Another area of concern is the using the proper transmission fluid. Only Toyota TO4-type fluid should be used in this model transmission. Do not substitute fancy synthetics or universal-type ATF. The consequences are poor gripping ability of not only the converter clutch, but also the transmission clutches present in the unit.
Thanks to Dennis Sneath, member of TCRA board of directors, for supplying this information and helping the TCRA provide useful technical data to rebuilders through this the monthly newsletter. Midwest Converters, located in Rockford, Illinois, and specializes in torque converters, heat treating processes and lubricants . He may e-mailed at firstname.lastname@example.org.