Camshaft Engineering Company

The Rolls-Royce Phantom II engine was designed with a camshaft and curved profile follower which produced a valve motion profile of quite high lift, very high initial lift rate, and small overlap (ie. period during which both inlet and exhaust valves are open at about the piston top dead centre position).

We presume that this was done to achieve a smooth, relatively low speed idle, good low speed torque, maintained to sufficiently high engine speed to provide the 'adequate' engine horsepower advertised.

In achieving the fast initial lift, very high values of acceleration of the valve train components apply, resulting in correspondingly high contact forces, particularly between the cam lobe, and cam follower. These conditions resultis in scuffing or partial seizure at the contact line, leading, over time, to wear and subsequent failure of the lifter/cam combination.

Attempts have been made in different parts of the world to overcome this problem with varying degrees of success. In some cases, the curved face follower has been replaced by a conventional flat (actually slightly domed) cylindrical follower with minor modifications also to improve the flow of lubricant to the contact area. Unfortunately, the maximum diameter of follower which can be introduced is limited due to space restrictions. This limits the camshaft profile to one of lesser lift velocity, to ensure that the contact between cam lobe and follower is contained within the follower face diameter. As a consequence the modified profile results in a significant reduction in power available from the engine.

When approached by a client seeking to overcome the camshaft durability problem, Camshaft Engineering Company studied the problem, analysed the camshaft/follower geometry characteristics using computer techniques, and examined the alternative options available within the constraints of the engine structure. It was found possible to closely maintain the original valve lift profile by use of a roller follower, and a corresponding cam profile computed to reproduce the lift curve of the original engine. Such a modification was expected to maintain the normal output of the engine as well as overcome the durability problem.

Computer measurement and analysis of the original cam profile showed initial and final acceleration rates (mm per crank degree squared) at the cam, approximately six times that found on typical cam profiles, both roller and flat follower, and including those used for race car engines. It was decided to modify the profile slightly in the initial lift and closure stages to reduce this acceleration to a more acceptable level, without significantly affecting the effective working area of the profile, that is, the lift profile above approximately 1mm cam lift. Taken into account in the cam profile design was the offset of the follower centreline relative to the camshaft centreline (approximately 3mm).

This profile design was applied to camshafts machined from alloy steel bar, and case hardened on bearing and lobe surfaces.

A proprietary standard roller follower was selected and a special cam follower guide was designed and manufactured to replace that used with the standard follower design, and special retainer plates were designed in conjunction with the follower guide to maintain the roller and camshaft axes parallel. Special push rod extension pieces were also designed and manufactured to couple to the standard push rod to complete the valve train assembly.

Installation of the system is relatively straightforward, and essentially follows that for the standard valve gear assembly.

Due to the modification of the initial opening profile to reduce the acceleration values, the timing procedure differs from standard. This is only in respect to the tappet clearance used for setting. Instead of the 0.020" (0.5mm) published setting clearance, a setting of 0. 040 " (1.00mm) is used. The operating clearance of 0.003" (0.08mm) is retained for the new shaft.

At the present time six engines have been successfully converted to the CEC roller cam system, by an associate of the Company, however, the Company has decided not to offer the conversion without his involvement and experience..