1993 Big Block Ford Mustang - Hybrid Drive

The TH400's claim to fame is its integrated center support. Its large center bushing cradles both the gear train and clutch packs, which provides excellent internal stability. The center support is critical to overall durability and longevity, since heavy-duty transmissions like the TH400 pack lots of rotating mass. Neither the Ford C6 or Chrysler 727 transmissions have a center support which, according to Phoenix, gives the TH400 a definite edge in strength.

Through the years, TH400s have been equipped with three different types of intermediate sprags, which are responsible for bringing the heavy direct drum to a halt on the 1-2 shift. The super strong 34-element unit (top) is far more durable under high torque loads than the early- (center) and late-model (bottom) sprags.

Opting for a 34-element sprag mandates the use of an early model drum (left). The difference in the sprag race design is quite evident between it and the '72-and-up unit.

The stock steel clutch apply piston (bottom) is the weakest link in the TH400 design, and has just a narrow band of surface area to engage the clutches. The aftermarket aluminum piston (top) is a step up, but will only allow loading five clutches in the drum. The aluminum piston (right) used in our build is the best alternative, as it provides additional strength and accepts six frictions.

The front band is only applied when Second gear is selected in manual mode. The heavy-duty aftermarket piece (bottom) features a heat-resistant Kevlar composite lining, and a weld-reinforced end.

When loading the high clutch drum into the case, careful attention must be paid to prevent binding. Rotating the assembly back and forth ensures that the drum engages both the intermediate clutches and the sun gear.

To accommodate the larger-diameter 300M billet input shaft (right), the spline area of the forward drum has been enlarged. This dramatically reduces the likelihood of the shaft breaking or twisting the splines in the cast drum.

Compared to the stock input shaft, the 300M piece is larger in the drum area and features longer splines. In metallurgical terms, 300M is basically 4340 steel with higher silicon, carbon, molybdenum, and vanadium content. The stock shaft is very stout, but Phoenix suggests upgrading to a 300M unit at the 650hp mark for added insurance.

The factory cast iron forward clutch hub (right) is a rugged piece in its own right, but high-load drag applications call for a billet steel hub to avert spline failure. Phoenix uses billet hubs in all of its ST- and SX-model TH400 transmissions.