From 1953 to 1978, British automaker Rolls-Royce manufactured its own versions of the General Motors Hydra-Matic transmission for use in Rolls-Royce and Bentley luxury cars. One version was very similar to the Hydra-Matic used in American cars and trucks, but Rolls-Royce later created an improved aluminum-case version with modifications to provide smoother shifting. This version was offered in some versions of the Rolls-Royce Silver Shadow I and Bentley T Series from 1966 to 1969. The older version remained in use in the big Phantom V and Phantom VI limousines until 1978.
THE SILVER SHADOW AND THE GM 400
In the autumn of 1965, Rolls-Royce unveiled two all-new models to succeed the Silver Cloud III and Bentley S Series: the Rolls-Royce Silver Shadow and closely related Bentley T Series. The new cars retained the 6,230 cc (380 cu. in.) aluminum V-8, but were otherwise thoroughly updated, now using monocoque construction (with front and rear subframes), fully independent suspension with hydraulic self-leveling (using technology licensed from Citroën), and four-wheel disc brakes.
Left-hand-drive cars also had a new transmission: the latest GM Turbo Hydra-Matic (a.k.a. TH400), shared with some contemporary full-size General Motors cars. Although Rolls-Royce and Bentley units had serial numbers beginning with “RR” (and, according to some sources, tighter production tolerances than the GM norm), these three-speed torque converter transmissions were manufactured by Hydra-Matic Division rather than in Crewe. Rolls-Royce literature generally describes this transmission as “GM 400,” but the factory workshop manuals also use the Turbo Hydra-Matic name.
Early right-hand-drive Silver Shadows and T Series Bentleys did not have the GM 400 transmission, instead continuing with the fluid coupling four-speed automatic. It would be natural to assume that this was a carryover of the transmission from the last Silver Ghost III and Bentley S Series, but in fact the four-speed transmission had undergone some substantial changes, creating the final new variation of the original single-coupling Hydra-Matic.
The first major change was a new case. Where earlier Hydra-Matic transmissions were primarily cast iron, the new unit had a cast aluminum case, including the bell housing, tail extension, and some internal castings. (Crewe-built transmissions already had an aluminum side cover and sump.) We haven’t found any figures suggesting what weight savings this change achieved, but we would guess something on the order of 60 to 70 lb (27 to 32 kg), offset somewhat by some of the other internal revisions.
A second important revision, shared with Rolls-Royce and Bentley GM 400 transmissions, was an electric shift actuator system, providing fingertip control of the various driving ranges. The actuator was an external add-on, mounted on the side of the tail extension. It used a double-wound electric motor to move the transmission selector shaft lever, with a solenoid-operated brake to hold the actuator securely in place after each selection. Apparently having noted the troublesome results of earlier American attempts at electric shift control, Rolls-Royce added an access port in the transmission tunnel through which a small tommy bar (described in the workshop manual as the “Get-You-Home Bar”) could be inserted to manually operate the selector level if the electrical system failed. The electric shift actuator also had a new R-N-4-3-2 shift pattern; on earlier cars with the four-speed automatic, reverse was below “2” at the bottom of the pattern.
The third and most significant change was in the transmission’s internal layout, which was revised to incorporate one of the changes GM had made for the Controlled Coupling Hydra-Matic: a one-way sprag clutch that took over the role of the rear brake band in “4” and “3” ranges. The effects of these changes are explained in more detail in the sidebar below, but the object was to smooth out the transmission’s occasionally harsh shifts.