Camshaft Engineering Company

The Rolls-Royce Phantom III V12 engine was, like it's precursor, the Phantom II L6, designed with a camshaft and curved profile followers 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. If these forces are sufficiently high, and notwithstanding the cam and lifters being mostly submerged in lubricant, metal to metal contact with the sliding type followers can result in scuffing or partial seizure at the contact line, leading, over time, to wear and subsequent failure of the lifter/cam combination.

In addition, for the Phantom III, a zero valve lash system was incorporated, introducing additional spring and hydraulic forces for it's operation. These add to the cam to follower contact forces, further aggravating the operating conditions for these components.

Experience has shown that these severe conditions prevail in both the Phantom II and the Phantom III engines. Many attempts to understand the problem and determine and apply solutions have been made. A solution found successful for the Phantom II engine was the redesign the cam and follower system by our Company, replacing the rubbing style follower with a roller mechanism.

Following the success of this conversion, our Company was approached by a client seeking to overcome similar camshaft durability problems in the Phantom III engine. The problem was studied through measuring and reproducing the dimensions and motion of the complete valve train as drawings within computer programs, the valve motion was measured and analysed by computer based camshaft lobe profile measurement equipment, and possible alternate roller based camshaft follower geometries established within the constraints of the engine structure. A proprietry standard roller follower was located which, with a revised design follower sleeve, and some re-machining of the tappet block castings would allow the conversion from sliding to roller follower.

It was also found possible with the small roller radius of the selected follower, to design the the necessary new camshaft lobe profile to maintain the original valve lift versus cam angle profile, yet not introduce unmachinable negative flanks to the lobe profile. In this way, the conversion to roller followers is expected to maintain if not improve the original power output of the engine as well as overcoming the durability problem.

The Phantom III angine was originally designed with a hydraulically operated tappet adjustment mechanism to automatically maintain the tappet clearance at zero, presumably as a noise reduction measure. Unfortunately it also served to mask the onset of cam and tappet wear, allowing the wear to continue, leading to major wear of these components until the limit of clearance compensation allowed noise to be generated, thereby indicating the problem.

In addition, the relatively poor oil quality of the day resulted in gum and varnish blocking the small oil passages and clearances within the system causing malfunction of the hydraulic clearance control system. Temporary measures to disable the system were introduced, before a revision to replace the system with a normal mechanical adjustment to a set clearance. A revised profile camshaft was introduced with ramps designed for 'solid' followers with standard tappet clearance.

The client's engine was of the original hydraulic zero tappet design but had been modified to disable it's operation. He was keen to accept the challenge to restore the original design to working order however, as a backup, a second shaft was designed for use with a solid lifter system. These profile designs were applied to camshafts machined from alloy steel bar, and case hardened on bearing and lobe surfaces.

The special cam follower guide was designed and manufactured to replace that used with the standard follower design, and special arrangements were made to ensure the roller and camshaft axes remained parallel. Special follower extension pieces were also designed to allow use of the standard push rod to complete the valve train assembly. An engine set of components has been manufactured together with the two camshafts. Installation of the system is expected to be straightforward, and will essentially follow that for the standard valve gear assembly.

Numerous articles and letters have been written about the problems with the Phantom III cams and followers, and the automatic zero clearance system, in an endeavour to determine the cause and a solution. Some are contradictory, and show differing views on the operation of the system and it's contribution to cam and lifter failure. Questions arise regarding the 'balancing 'of spring forces within the system.

As the roller follower conversion design resulted in some increase in the valve train mass, a basis for establishing valve and helper spring requirements for the conversion to operate effectively was necessary. The author therefor made a force and motion study of the original systems, both hydraulic and solid lifter, to determine the forces developed within them. This was then used to define the spring system for the roller conversions. At present this study has not been prepared for publication.

Assembly of the conversion components to the engine has been interupted by the untimely death of the owner of the car concerned. Arrangements were in process (September 2001) for the restoration of the car and engine to be completed by another Rolls-Royce expert. At this time (March 2007) no action has occured. The restoration, once implemented, is expected to be documented such that interested owners can be informed of the process, and the perfomance of the conversion once completed.