What is the VarioCam Plus?
Porsche’s variable valve timing system, the VarioCam was presented in 1992. With the type 996 Porsche 911 Turbo, Porsche introduced the VarioCam Plus. But what is the VarioCam Plus?
Here’s how the system works in the 2001 Porsche 911 Turbo.
What is the VarioCam Plus?
The VarioCam Plus system consists of four valves per cylinder, the axial camshaft adjusters and the switchable valve tappets. The two intake and exhaust valves are arranged in a V shape with a valve angle of 27.4 degrees. In order to keep the moving masses of the valve train as low as possible, the valve stem diameter is only six millimeters.
Double valve springs are used on the intake and exhaust valves to ensure that the valves close even when the forces caused by the exhaust back pressure are higher. The intake-side camshaft adjustment in the new 911 Turbo is expanded to include the intake-side valve lift switch. This allows power and torque to be optimized on the one hand and fuel consumption and exhaust emissions to be reduced and running smoothness to be improved on the other.
The valve lift adjustment system consists of switchable bucket tappets on the intake side of the engine, which are operated by an electro-hydraulic 3/2-way switching valve. Since two different cam shapes are mounted on the intake camshaft, the corresponding valve lift curves act on the engine by switching the respective cams. In detail, the bucket tappets consist of two nested tappets that can be locked together using a bolt. The inner tappet is in contact with the small cam, the outer tappet with the large cam. A hydraulic compensation element for the valve clearance is always integrated in the power flow of the tappet.
Two Engine Concepts in One
When idling, the engine is optimized by switching the valve lift to the small cam of three millimeters and adjusting the valve timing for low valve overlap. The small valve lift results in a reduction in friction, a significantly increased charge movement due to extremely shortened opening times and little exhaust gases from previous combustion in the combustion chamber. This results in consumption and emissions reductions of up to ten percent with significantly improved idling quality.
At partial load, operation with internal exhaust gas recirculation is optimal for de-throttling the engine and thus reducing consumption. For this purpose, the camshaft adjustment is set to a large overlap area with a small valve lift, i.e. a large proportion of time for sucking back exhaust gas from the exhaust. At full load, high torque and high peak power are achieved on the one hand by a low-loss gas exchange and on the other hand by an uncompromisingly designed cam contour with a maximum valve lift of ten millimeters and correspondingly adjusted opening and closing times of the valve lifts.
VarioCam Plus also helps before a journey begins. For example, the cold start of the engine is significantly improved and during the warm-up phase of the engine, emissions can be reduced by adjusting the VarioCam Plus accordingly.
VarioCam Plus Control Unit
Both individual systems of the VarioCam Plus (camshaft adjustment and valve lift switching) are controlled by the Motronic control unit ME 7.8. The control unit has been specially designed for these requirements with high computing power. The input variables “engine speed”, “accelerator pedal position”, “engine oil and water temperature” and gear recognition are particularly required to control the VarioCam Plus.
The desired torque or power is compared with the stored maps. The decision as to whether VarioCam Plus needs to react is made in milliseconds. If so, everything runs very quietly and silently despite huge changes to the engine design. There is no sign of this in the cockpit
Image by Porsche. Text based on the Porsche press release from 2001.
