Balance Shafts — History and Purpose
Explains why the 944 uses balance shafts, how they work to cancel second-order harmonic vibrations, and the history of the system from Frederick Lanchester's invention through Mitsubishi's 'Silent Shaft' patent to Porsche's implementation.
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One of the complaints many 924 owners had about their cars was related to vibrations. Large in-line four-cylinder engines inherently produce large second-order harmonic vibrations. To solve this problem, Porsche elected to use a balance shaft system on the 944.
The balance shaft system was actually invented by Frederick Lanchester, an English inventor, in the early 1900s. However, it was Mitsubishi that eventually patented the concept under the name "Silent Shaft" system. Porsche developed their own balance shaft system for the 944, apparently using three bearings instead of the two employed by the Mitsubishi system. Porsche subsequently discovered that the Mitsubishi system worked better and paid Mitsubishi a royalty of approximately 6–8 US dollars per car to use the "Silent Shaft" system.
Each balance shaft has two counterweights which resemble small hockey pucks with the shaft running through them. The two counterweights are approximately 1-1/2 inches (approximately 4 cm) apart. The section of the shaft between the counterweights is machined for the main balance shaft bearing. The balance shaft turns the counterweights at twice the speed of the engine. With one shaft located high on one side of the engine and the other low on the opposite side, the balance shafts turn in opposite directions and cancel the vibrations of the large spinning mass inside the engine.
Note: Frederick Lanchester is also credited with inventing disc brakes in 1903.