Variable Valve Timing in your Twin Cam Jaguar Future ?


Jaguar Clubs of North AmericaJCNA Home

Variable Valve Timing in your Twin Cam Jaguar Future ?
By Harry Parkinson, Jaguar Association of New England


One of the latest innovations in automobile engine technology is the introduction of variable cam timing as a method of increasing output (horsepower and/or torque), improved emissions and improved economy. JANE members and JCNA president, Gary Hagopian, has designed, manufactured and installed a variable cam timed cylinder head for a XK engine. Gary retired as an executive in the machine tool and die business and has had many years of experience in design, fabrication, and assembly of tools and dies. It was a natural to design, fabricate, and assemble a Variable Valve Timing head as performance improvement for XK twin cam engines.

This is one of a series of articles covering the project. Part One will cover the design/development, fabrication, and assembly. Part Two will cover the initial run up on an engine. Part Three will cover dynamometer testing and timing profile development while Part Four will encompass the results, final design and conclusions of the project

Most all manufacturers have either introduced this technology or are developing it. Japanese, European, and United States companies including Ford and Jaguar have engines with variable cam timing technologies that are typically coupled with the computed controlled engine monitor/management systems.

What is variable cam timing? One must get back to the basics of the automotive engine to understand the principles involved. In conventional engines, the cam is coupled to the crankshaft (and pistons) by either a gear system, a chain (Jaguar XK engines) or by timing belts. A four cycle engine has a suction, compression, ignition and exhaust cycle which is two crankshaft rotations while the cam makes one rotation. The cam rotates at 1/2 the speed of the crankshaft and controls the valve opening and closing in relation with the piston moving up and down in the cylinder. The cycle begins with both the intake and exhaust valves closed and the spark plug firing. The ignited fuel charge expands an pushes the piston down as the exhaust valve opens and the exhaust goes out the exhaust pipe. After the piston purges the burnt fuel, it starts to pull in fresh fuel and air with the intake valve open. It continues on to compress the charge with both intake and exhaust valves closed to start the ignition process. The cycle repeats over and over again for all the pistons.

A conventional camshaft has fixed lobes that open and close the valves in relationship to the crankshaft and pistons. The timing of valve opening is measured in degrees while the amount of opening is measured in thousandths of inches(mm) and is called lift. The size of the cam lobes determines the lift while the shape of the lobe determines the timing. Since the cam is a ground shape in steel, it can't change from the fixed dimensions of the shape of the lobe. The cam is a compromise between low speed and high speed. At low speeds, some of the charge is allowed to escape during the compression stroke reducing the chance for engine knock. At higher speeds, the fuel mixture is more completely compressed allowing a more complete charge ignited.

This relationship between the cam, valves and the pistons can be altered in a variable cam timing engine. This allows the engine to be "tuned" differently at low speed and high speed. The implementation of variable cam timing takes many forms and often is computer controlled, but the simplest is to change the relationship of the cam to the crankshaft by rotating the camshaft dynamically by rotating the cam. Gary's design does this by pushing the camshaft in and out of a spiral cut into the camshaft and into the camshaft gear. The other end of the camshaft, opposite from the timiing chain, has a hub /bearing combination that has a mechanism to pushing the cam in and out to change the timing.

Because the the camshaft lobes move back and forth, the tappet buckets in the must be notched for to clear the cam lobes. That is the only modification to an otherwise stock engine. The modified camshafts are bolt in's on the head! The complete cylinder head onto an engine without any other changes! Gary's design means that a cylinder can be assembled to any XK engine!

Over the Christmas holidays Gary and I completed the first cylinder head at Gary and Sue's Sunapee house. The well thought out design went together as planned and is ready for installation on a rebuilt XK140 motor. After inital runs, head will be removed and shipped to be tested on a dynamometer. Data will be taken on torque and horsepower on various cam settings at a variety of engine speeds. Once this is done, the "timing curves" for the cams can be established.

The next report cover the initial running of the motor and set up for the dynamometer runs.

The gallery below contains 9 detailed images of the head.