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How does variable valve timing work on the 996 engine?


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Hi. So there are four camshafts right? two on each side of the engine? 4 valves per cylinder? I'm familiar with how BMW's VANOS system works: pretty simple: uses the engine oil as a hydraulic fluid (controlled by a solenoid) to move both the intake and exhaust cams around. Does porsche have something similar (is that what variocam is)? If so, are there two systems? One on each side of the engine?

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Yes, 4 camshafts. Yes, 4 valves per cylinder.

From the MY99 Porsche Carrera Tech Book...

"VarioCam, the adjustment of the intake camshafts at engine speeds of approx. 1,300 rpm* and 5,920 rpm, produces a major part of the engine output, the engine torque as well as the good exhaust values at idling speed. Engine speeds below 1,300 rpm produce a slight overlap in the valve stroke curves for the intake valves in relation to the exhaust valves.

This particularly affects low proportions of hydrocarbons (HC proportions) in the exhaust gas before the catalytic converters. If the engine exceeds the 1,300 rpm mark *, the intake camshafts are adjusted by 12.5° (by 25° if measured a the crankshaft).

Adjustment of the camshafts to produce a greater overlapping of the valve stroke curves for the intake and exhaust valves results in improved cylinder charging and thus an increase in engine torque. In the case of engine speeds above 5,920 rpm, the intake camshafts are returned to the basic control times (as for engine idling speed). If the intake valve closes later, additional amounts of intake air can be drawn in. This produces a recharging effect since the flow of air is not interrupted. This also results in improved charging of the cylinders and an increase in performance.

* The value 1,300 rpm increases to 1,480 rpm with engine oil temperatures of approx. 133°C."

post-1-1244047216_thumb.png

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VarioCam

To increase the torque and to improve cylinder charging, the engine has two VarioCam actuators which are installed in the chain tensioners of the camshafts. The two actuators are operated

by electromagnetic valves activated by the DME control unit.

Functioning

When the ignition is switched on, the electromagnetic valves of the camshaft adjusters are supplied with positive potential. If the engine is started, the control unit applies ground to terminal 25 (camshaft adjuster for cylinder line 4 - 6) and to terminal 52 (camshaft adjuster for cylinder line 1 - 3) if the following conditions are fulfilled (VarioCam activated):

1 - Engine oil temperature between -3°C and 133°C

2 - Engine speed > 1,300 rpm

3 - Throttle opening > 5%

or

1 - Engine oil temperature > 133°C

2 - Engine speed > 1,480 rpm

3 - Throttle opening > 3.9%

The ground potential is deactivated by the DME control unit (VarioCam deactivated) if:

Engine speed > 5,120 rpm

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Thanks. Are those electromagnetic valves, item 1 in the ValveTrain picture you posted? If not (I don't think they are), where are they located? I think this variocam system seems like a pretty good, simple design.

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In my '03 C2 there is a noticeable surge at 3,000 rpm. If it's not caused by the vario cam, what is happening at that rpm?

Thanks.

VarioCam Plus started in MY02 with the 3.6 liter engines. A slight hesitation at about 3000-3400 RPM.

VarioCam is a bit more complicated and takes 4 page of the tech book to explain it. As a Contributing Member you can read it here. Look for the MY02 Carrera Technical Information Book.

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Thanks Loren. So only the intake cams are adjusted. ok. But how are they adjusted? I don't think I see any solenoids in that picture.

Here is a picture of the variable camshaft timing actuator. The cylinder is the solenoid that controls the oil pressure on the chain ramps:

post-18633-1244222343_thumb.jpg

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