causing it to extend. At the same time, the rising lower piston allows the lower chain
run to be shortened. The overall effect is that the intake camshaft is rotated
anticlockwise in relation to the exhaust camshaft, thereby retarding the intake timing
back to its nominal setting. Above 1500 rpm, the DME de-energises the Variocam
solenoid, causing the shuttle valve to move to a different position. Engine oil pressure
is then redirected to lower the pistons, extending the lower chain run and shortening
the upper chain run. This causes the intake camshaft to rotate clockwise in relation to
the exhaust camshaft, advancing the intake timing by 15 degrees. Above 5500 rpm,
the solenoid is again energized, and the intake cam is retarded back to its nominal
position.
Possible Causes of Variocam Failure
Although it is impossible to reach a general consensus on this, from the public
discussion in the “968.net” community and actual examination of failed Variocams,
the following appear relevant reasons for their failure:
1)
Aging of the plastic pads (rails). With time and miles, the pads became much
more brittle than when new, leading to an increased chance of breakage, and
in any case to an increased rate of wear. In failed 968 Variocams, the pads
appear very worn with deep tracks, or partially broken, or brittle with some
“craters” on them, meaning the material has lost its original properties.
2)
Stretching of the camshaft chain. Some stretching is inevitable with use, but
the stretching increases the effective pitch of the chain links, accelerating wear
of the camshaft sprockets.
3)
Variocam on/off action. As described above, the Variocam is “on” (intake
timing advanced) in the 1,500 to 5,500 rpm range, and “off” outside this
range. The engagement of the chain with the intake/exhaust camshaft
sprockets differs between these two positions, and this may affect the wear
patterns on the sprocket teeth. The wearing pattern is different with respect to
a non-Variocam car (eg 944 S2), and apparently reduces sprocket life. The
driving style (average RPM), can also produce an asymmetric wear of
upper/lower pads, and sometimes the lower pad wears more than the upper
one: this is a problem, because the lower pad is more difficult to inspect.
4)
Camshaft slope. To support the timing advance, the slope is “aggressive”, that
is has an elongated form, to open the valves very quickly. This means
additional stress on chain, and when the chain gets stretched, a “hammering”
effect on sprocket teeth, particularly at high RPM.
5)
Given all the stress on cam sprockets, there has also been speculation that the
thermal treatment during manufacture of the cams was inadequate or
inconsistent.
It appears that any 968 car with more than 60,000 miles (100,000 km) can potentially
develop a Variocam failure, even if luckily only a small percentage will effectively
experience it. Whether problems develop or not, depends on many factors: driving
style (high RPM means asymmetric and accelerated wear of Variocam parts), age of
the car and the type of oil used (influencing pad/chain wear), car abuse or poor
maintenance (high engine temperature and insufficient lubrication mean accelerated
wear), previous Variocam maintenance, and total mileage.