The 1941 M4 Vacamatic transmission by Chrysler was, historically, the first

semi-automatic transmission A semi-automatic transmission is a "theoretical" multiple-speed transmission where part of its operation is automated (typically the actuation of the clutch), but the driver's input would be required to launch the vehicle from a standstill and t ...

which was marketed by a major manufacturer. It was an attempt to compete against rivals' hydraulic automatic transmissions, though it still had a clutch, primarily to change range. In normal driving, the clutch is not used. The transmission itself was a fully-synchronized manual transmission, with four forward gears, one reverse; where the shifting was done 'automatically' by either

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

cylinders (early, M4), or

hydraulic cylinder A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment ( engineering vehicles ...

s (late, M6,

Presto-Matic The M6 Presto-Matic was a Chrysler Corporation semi-automatic transmission produced from 1946–1953. It was a special manual transmission with a fluid coupling. Although it had just two forward gears, an electric overdrive unit was attached and ...

). The main difference was the addition of a

fluid coupling A fluid coupling or hydraulic coupling is a hydrodynamic or 'hydrokinetic' device used to transmit rotating mechanical power.

between engine and clutch, and the shifting mechanism.

Operation

The range selector (looked like a column shift) had three positions: Low, High, and Reverse. Low consisted of what would be a first and second gear with an ordinary four-speed; High was third and fourth. The reverse was a single ratio. Driving one was/is quite different: Upon starting, one would disengage the clutch, then choose a ratio (High for normal driving), and, whilst stopped with a foot on the brake, engage the clutch. Pressing the accelerator would move you forward, in third, with the fluid coupling connecting the engine to the transmission. Upon reaching , the accelerator is lifted, a 'clunk' would ensue (the automatic shift), and you would continue in fourth gear. The low range was quite similar, but much slower speeds. It is possible to start in Low, allow the shift, then move the range selector to High, using the clutch, and continue in High. This allowed somewhat faster acceleration but was quite clumsy. Kickdown for passing was possible through electrical connections made by flooring the accelerator, though only in one range (i.e.: fourth to third).

Commentary

Never a performance transmission, it was nevertheless fine behind the six- or eight-cylinder

flathead engine A flathead engine, also known as a sidevalve engine''American Rodder'', 6/94, pp.45 & 93. or valve-in-block engine is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, as ...

but once the V8

Hemi engine A hemispherical combustion chamber is a type of combustion chamber in a reciprocating internal combustion engine with a domed cylinder head notionally in the approximate shape of a hemisphere (in reality usually a spheric section thereof). An en ...

was introduced, the shortcomings of such a slow-shifting transmission were magnified greatly. Over the years, though, this transmission has acquired a reputation for absolute reliability.

References

{{unreferenced, date=March 2008 Chrysler transmissions Automatic transmission tradenames

Operation

Commentary

See also

References