
44 LIGHT AND HEAVY VEHICLE TECHNOLOGY
Cam
+
I Base circle
I
i* diameter *1
Figure 1.82 Extent of
cam
lift
of 1.5:1. A further point is that valve lift will always be
reduced in the presence of tappet clearance, so in the
absence of valve rockers it would be more correct to
the state that valve lift is nearly equal to cam lift.
The maximum lift of a valve generally approximates
to one-quarter of its head diameter, since any greater
opening than this gives diminishing returns in gas flow
capacity. That is, the restriction to flow in the port and
through the valve opening becomes equal when opti-
mum lift occurs.
The period of valve opening or