Timing Drive Mechanism; Camshafts With Cylinder Specific Cam Contours - Porsche Cayenne V6 2008 Service Information

Engine – Cayenne V8
The chain is guided by two specially coated
guide rails. The lower guide rail on cylinder row 1
to 4 is also designed as a tensioning rail. The
hydraulic chain tensioner is connected to the
engine oil circuit and is totally maintenance free.
The chain drive consists of a 3/8" 8 mm wide
duplex bush chain, which drives the two intake
and exhaust camshafts.
Page 1.28

Timing Drive Mechanism

Camshafts With Cylinder Specific Cam Contours

The intake and exhaust camshafts for both engines have a basic outer
diameter of 38 mm. The intake valve lift is 3.6 mm and 11 mm. The
exhaust valve lift on cylinders 1, 2, 6 and 8 is 8 mm, while the exhaust
valve lift on cylinders 3, 4, 5 and 7 is 9.2 mm.
The engine design, with a V8 crankshaft and 90° throw, guarantees
superb mass and torque balancing. In this engine design and a design
with normal cam contours (same cam strokes), individual cylinders
would be hindered during exhaust outflow into the exhaust manifold. The
reason for this is that the surge of exhaust gas that emerges during the
early (sooner than normal) exhaust valve opening for the respective
cylinder (e.g. cylinder 2) goes into the overlap period of the next
cylinder (cylinder 3). This would have a detrimental effect on the
charging of the cylinders. Too many residual exhaust gases would also
have a negative effect on the knock limit.
The firing order of the Cayenne (1-3-7-2-6-5-4-8) would put cylinders 3,
4, 5 and 7 at a disadvantage in terms of volumetric efficiency. These
cylinders therefore have a larger cam stroke. This means that the
cylinders are charged evenly, which results in an optimized torque curve
in the entire rpm range.