AJS Porcupine - The Mechanicals - The Motor

The Motor

The E90S Porcupine motor was a unit construction, aluminium alloy, 500 cc, DOHC twin, with horizontal cylinders and heads, to give the bike a low centre of gravity. A later version of this motor was named the E95, re-engineered to have its cylinders inclined at 45 degrees for better cooling and easier carburettor installation, and is claimed to have produced 55 bhp @ 7600 rpm

The gear drive for the camshafts was on the right of the motor, while the gear primary drive was on the left. The geared primary drive meant that the motor ran “backwards”. The cam gear drive also drove a jackshaft at the rear of the cylinders, which drove an oil pump, a fuel pump, and (via a chain drive) a Lucas magneto. The four-speed gearbox output was on the right.

As originally designed, the E90S was to be supercharged, with the blower mounted above the gearbox, and driven by the clutch. The loss of the supercharger meant that the design was compromised, lacking sufficient fly-wheel effect, which caused problems with magneto failure. The main initial change was to reduce the valve angle to 90 degrees for a more compact combustion chamber.

It had rubber mounted twin GP carburettors, inclined at 49 degrees, with an unusual float tank system used rather than float bowls. Plain bearings were used for the big end bearings and the centre main bearings. The outer main bearings were rollers. One problem cited for the engine is the use of non parallel valves with conventional rockers.

The 1947 model engine was mostly made of alloy, but, as an experiment, the head was cast in silver for increased thermal efficiency. Due to the softness of silver, it had to be alloyed to make it hard enough for racing use. By the time this was achieved the thermal efficiency gains were lost, and the experiment was abandoned. There is dispute as to whether the silver cylinder head went beyond feasibility-study stage.