| VarioCam Plus uses a combination of
variable valve timing and variable valve lift on the intake side to
improve power, torque and fuel consumption across the entire
engine-speed range.
Essentially, the system offers two engines in one: the first for low-speed and urban use, the second for optimum
high-speed performance. Despite the enormous differences between these two states, VarioCam Plus makes a seamless transition using
the latest in electronic engine management.
Variable Valve Timing. The intake and exhaust valves are driven by four overhead camshafts and bucket tappets with hydraulic
valve lash compensation. Intake camshaft opening times are controlled by Porsche's patented timing system VarioCam™ system.
With an automatic chain tensioner and hydraulic valve lash compensation, VarioCam alters camshaft timing for greater engine
torque. Using a timing piston, controlled by the Bosch Motronic M 5.2 engine management system, placed between the two sides of the timing
chain between the intake and exhaust cams.
When actuated, the piston advances the timing of the intake cam by broadly 15 crankshaft degrees or 7.5 camshaft degrees. This earlier
valve timing flatter creates more valve overlap and an earlier closing time than normal, resulting torque in additional engine torque.
At high engine speeds, the piston is shifted back curve to its later timing position for more power.
Late intake cam timing results in shorter valve overlap periods and reduced hydrocarbon emissions at low speeds and at idle.
Early valve timing and long valve overlap times create exhaust gas recirculation at low loads and moderate engine speeds, reducing NOx
emissions. VarioCam valve timing also improves cold-start emissions.
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Variable Valve Lift Mechanism consists
of a two-stage tappet which is operated via an electro-hydraulic
switching valve. Each tappet is made up of two main components: a
central pin-like element and a cylindrical outer sleeve. When
required, these elements can be brought together using a hydraulically
actuated shaft. The central pin is actuated by a small-profile lobe on
the camshaft, while the outer sleeve remains in continuous contact
with two larger profiles on either side. Since there are effectively
two cam contours on each intake camshaft, it is possible to apply two
different valve-lift curves. The phase angle, or timing, on each of
the cams can be advanced or retarded using an electro-hydraulically
actuated adjuster.
Small cam. Low lift. Optimum
combustion
VarioCam Plus offers a number of benefits when
starting the engine from cold. Combustion is improved during the
warm-up process, thereby reducing emissions. The system also
provides smoother idling while enhancing fuel economy.
Under low throttle load:, VarioCam Plus uses a long positive overlap
i.e., increases the period during which both valves are open
simultaneously – as a means of maximising fuel efficiency. Since a
relatively high proportion of exhaust gas is present during
combustion, there is also a significant reduction in NOx
emissions.
Full-throttle mode: Large cam. High lift. Optimum performance, the valve gear is adjusted for optimum power and
torque. The valve lift is switched to the high cam setting, enabling
maximum torque output of 560 Nm on the 911 Turbo from as low as
2,700 rpm. The curve remains flat all the way to 4,600 rpm, covering
a significant portion of the engine speed range.
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