February 11, 2009

What Lies Beneath: Porsche Sportomatic

Porsche’s Tiptronic system has become quite familiar to the automotive community by now. However, the Tiptronic system was not the first time Porsche offered a clutch-less manual transmission. In 1968, the Sportomatic semi-automatic 4-speed was introduced as an optional transmission to the standard 5-speed manual on the 911T and E. (though there were approximately 5 Sportomatic 914s made) It was quite an interesting design indeed. Porsche took their type 905 transmission and modified it to contain a torque converter along with a vacuum operated clutch.

When a driver changes gear, he simply grabs the shift knob and selects a different ratio. Beneath the shift knob, there is a micro switch that is triggered when the knob is depressed slightly. The switch then activates a vacuum valve that in turn operates a vacuum diaphragm. This diaphragm then operates the clutch and thus allowing the changing of gears and therefore eliminating the need for a clutch pedal. Before power from the engine reaches the clutch, it flows through a torque converter. This allows the driver to stop without putting the transmission in neutral. A feature that is particularly helpful in stop-and-go driving. Though the Sportomatic is a ‘clutch-less manual’, the ratios in the transmission are not shown as 1,2,3,4 and so on. Instead, Sportomatic ratios include L, D, D3, and D4. Furthermore there is a Park feature, as on an automatic, and naturally reverse.

Porsche’s period brochures indicate how to chose the correct gear:

L (Low): For ascending and descending steep grades or for slush, mud and snow.

D (Drive): Normal driving from 0-60 miles per hour. For rapid acceleration, the transmission can be shifted through all ratios like a typical manual transmission.

D3 & D4: For highway cruising. D4 is essentially overdrive, while D3 can be used for passing and downshifting under braking.

P (Park): This is necessary since due to the torque converter there is no mechanical link between the engine and transmission.

R (Reverse): Acts as it would in an automatic. It can only be selected if the car is at a complete stop. Slight increase in engine speed may be necessary to actually move the car.

Later, as 911 engines increased in power, a stronger Sportomatic was necessary. In 1972 the type 925 Sportomatic was introduced using the same casing as the type 915 manual. By 1975, engine capacity and power in such cars as the 911S had increased to the point where once again the Sportomatic transmission was too weak. Since the system incorporates a torque converter, only three speeds were necessary. This allowed for larger and stronger gears without increasing the size of the transmission case.

By 1979, Sportomatic had ceased development and was dropped from the line-up. Porsche released the 928 in 1977, which was available with a fully automatic transmission sourced from Mercedes-Benz, and later the 944 also had an optional 3-speed automatic. Porsche didn’t come out with another ‘clutch-less manual’ until 1992 with the introduction of Tiptronic.

2 comments:

Slow Joe Crow said...

The VW Beetle's "Automatic Stickshift" is the same basic system, and I even saw one on a Formula Vee car in the mid 70s.
The PDK/DSG is a much better way to go but it does work.

Anonymous said...

It's a great design ideal for modern busy traffic - often criticised by people who have never used one and although slightly slower than a manual not as much as you may think.