Technology explained

065 VARIOCAM

Total 911 explains how Porsche’s clever variable timing chain system works, developed for the 996

For an engine to run, the intake and exhaust cams are timed to open and close at set points and for set durations, with each rotation of the crank.

Each valve opens at a set point, to a set height and for a set duration relative to the position. These valve settings were often a compromise, to balance performance at low speed, high speed and to carefully meter emissions. The results of this balance are clear in any early 911. We all know the rough, lumpysounding idle that instantly clears as we accelerate higher up the rev range. That’s because the setup favours top-end performance, with the trade-off being a less-than-smooth idle.

To combat this, Honda led the way with its VTEC system. Using three cam lobes activated across the rev range, VTEC meant performance from tickover to full scream was constantly optimised. Porsche needed its own answer to the problem, and that came in the form of Variocam on the 996. Patented by Porsche, it first appeared on the four-cylinder 968 of 1991, but the water-cooled 996 was the first 911 to use the system, having the requisite twin-cam setup.

It works using an adjusting mechanism on the timing chain that connects the intake and exhaust camshafts. By adjusting the tension between sides of the timing chain, the chain effectively lengthens or shortens. This variance is used to affect the timing of the intake cam, either advancing or retarding the timing of that camshaft as required, while controlled by the Bosch Motronic management. Note that the exhaust cam timing is fixed.

In varying the inlet camshaft timing, the time the inlet valve opens is brought forward or later, altering the ‘overlap’, or the time and point the exhaust and inlet valves operate. The result was an engine that was more flexible, offering improved torque in the mid-range, while also reducing emissions and generating a smooth, refined idle.

One benefit of Variocam is that valve clearance is kept constant, which means reduced operating noise, wear and maintenance. All useful benefits at a time when Porsche was striving to bring the 911 into the new millennium. The company later developed the system further, resulting in Variocam Plus. This is a more complicated system that also uses electronically operated hydraulics to vary the amount of exhaust valve lift.

Don’t confuse either with Varioram, which is an induction technology that’s altogether different.