Balance shafts: principle and typical problems
Why the 944 has two balance shafts, where the technology came from and where they typically leak.
In short: Two counter-rotating balance shafts cancel the second-order forces of the large four-cylinder. They run on their own belt, and their seals are typical leak points.
Principle
The two shafts turn in opposite directions at twice engine speed. Their eccentric weights create forces that cancel the free inertial forces of the inline four, so the engine feels as smooth as a six.
Origins
The principle goes back to Frederick Lanchester (1904) and was developed further and patented by Mitsubishi. After its own attempts Porsche chose to license it.
Typical problems
- Leaking front shaft seals: oil around the timing cover.
- Leaking balance shaft housing covers: not difficult but labour-intensive.
- Incorrectly fitted belt: can cause idle vibration – the same symptoms as tired engine mounts.
The belt
Tension and replacement of the balance shaft belt are covered in the article on the timing belt and balance shaft belt. An over-tight balance shaft belt is said to often whine – more on that there.
Sources
- Secondary source Porsche 944 (Abschnitt «Production»)
Secondary source. Production tables per model year, no book citation; totals are partly inconsistent.
- Secondary source Buyer's Guide: Porsche 944 / 944 Turbo
Enthusiast buyer's guide (1998), checks on the lift and on the test drive.
- Secondary source Engine Mounts – Checking
Symptoms and check dimension for engine mounts.