by Richard Langworth
from The Complete History of Chrysler Corporation 1924 - 1985
Imperial Home Page -> Repair -> Transmission -> Vacamatic
Engineering efforts aimed at easier driving included a low cost alternative to Fluid Drive. It arrived this year as a semi-automatic transmission marketed as "Vacamatic" at Chrysler, "Simplimatic" at DeSoto and "Powermatic" at Plymouth.
This new transmission was similar to initial versions of GM's "Hydra-Matic", though it did not provide totally "clutchless" shifting. Basically, it consisted of a four-speed gearbox with a vacuum servo operated by lifting off the accelerator. The servo would "automatically" shift from first to second gear or from third to fourth, but moving between Low range (1-2) and High range (3-4) still required use of the clutch pedal and shift lever. The gear box designated M-3, was a heavy-duty three-speed unit cleverly converted to four forward ratios. How was it done. The key was mounting the forward (main) gear on the countershaft, with a cam-and-roller overrunning clutch providing about a 2.5:1 reduction. Power flow for first gear was normal, with gearing to the output shaft at the rear of the countershaft. For second, the countershaft was driven through a center pair of gears with a 1:1 ratio, using the same connection to the output shaft. For third, the countershaft was again driven via the forward reduction gear, but power flow was taken through the 1:1 pair to the output shaft. In other words, the power flow was reversed relative to second. Fourth gear was direct drive, with the countershaft spinning but not transmitting torque.
All normal driving with Chrysler's semi-automatic was done in High range, which was geared (approximately 1.7:1) to provide a fairly smart getaway, with slip diminishing with speed. Low range was intended only for situations like restarts with a full load on steep hills, towing a trailer in the mountains, or emergencies like getting out of soft sand or mud. The mechanical gearing for first was about 3:1, for second about 2.2:1. This transmission was a good try at greater driving convenience, but it still was not fully automatic, something Chrysler wouldn't be able to advertise until 1954.
This page last updated October 13, 2003. Send us your feedback, and come join the Imperial Mailing List - Online Car Club