Supercharged Norton Roadster

SUPERCHARGED NORTON ROADSTER

GRAHAM FORSDYKE

Another 'airy 'uffer from 'agon.

START OFF with a standard machine, decide you want it to go a little faster, and you’re faced with two alternatives. You either fit larger valves, special springs, high compression pistons, lightened rocker gear, high-lift cams and dual carburetors, etc., or simply fit on a supercharger.

Alf Hagon, England’s special builderextraordinaire, grew tired of all the conventional methods of boosting engine power with the resulting loss of tractabilitÿ, and put his drag-racing knowledge to road use. He took a Norton Dominator 99 and fitted its 600cc twin engine with a 750cc-capacity Shorrock supercharger.

The experiment was such a success that now Hagon plans to market complete kits for supercharging any large-capacity twincylinder Norton. Unfortunately, the kit cannot be used on other makes of machine, as only the Norton’s Featherbed frame will allow the supercharger to be fitted without drastic modification. The American distributor will be Kosman Motorcycle Specialties.

The original plan was to drive the blower from the clutch, but it proved easier to take the drive from the engine shaft. One half-inch vee-belt carries the drive from a pulley mounted on the engine shaft, outboard of the sprocket and alternator, so that no modifications are necessary to these components. The supercharger is mounted on special plates above the gearbox. A tensioner is provided for the belt, on the inside lower run, which is covered with a sheet-metal case finished in the same cracknel black paint as the primary chaincase.

The only modification to the primary chaincase is the removal of metal from the engine-shaft dome and the provision of the belt tensioner.

Lubrication is provided for the blower by a bleed from the return pipe to the oil tank in the same way that the rocker gear is fed with oil. The output opening of the blower is connected to the Dominator’s inlet manifold by a hose-clipped rubber tube.

The supercharger feeds through a 1%inch Wal Phillips fuel injector, although an SU carburetor is to be used and should give better results at low engine speeds. Oil is carried in a compartment of the large fiberglass fuel tank.

So that the full effect of the supercharger could be better judged, no other modifications were made to the engine, although, of course, it was completely stripped and wearing parts checked.

This is the hairiest street machine I’ve ever ridden — and that includes practically every big-capacity machine on the market. The torque is fantastic. From only a little above tick-over speeds, power comes in with a Clay-like punch and continues all the way up the scale.

Although the top speed of 110 mph is only about 13 mph faster than the twincarb Sports version of the 99 Dominator, the acceleration is so different that they could be two completely different types of machines.

An electronic test of the machine’s acceleration showed it superior to such machines as the 650cc Norton and Triumph, 750 Matchless and even a l,000cc Vincent. Most of the blown Norton’s superiority was in the first 110 yards; it acted like a 650cc fueler off the line, and showed a useful advantage over the rest of the quarter mile as well.

It took only 13.8 seconds to cover the standing-start quarter mile with the machine producing exactly 93 mph on the finish line. These times and speeds are a mean of many runs done with and against a slight breeze. The bike, incidentally, was not broken in.

The relatively slow top speed — one would expect at least 115 mph from such a setup — was obviously due to the gearing. Hagon did not even increase the final drive ratio, leaving the machine geared to 110 at 7,000 rpm. With one tooth more on the engine or gearbox, more top speed should be forthcoming. So correct is the gearing for acceleration purposes that it took only 585 yards for the machine to clock 100 mph from rest.

The supercharger cannot alter the mechanical limitations of the engine and, ideally, I feel that the machine should be geared for 120 at 7,000. The Norton is not very happy much above 6,500 rpm, with valve bounce coming in shortly above this figure.

The supercharger also assures good filling and atomization of the mixture at low engine revs, allowing the twin to be ridden without any protest at speeds as low as 35 mph in top gear. Even from this speed, a tweak of the throttle would bring results that, on a normally-aspirated machine, would need a couple of downshifts.

At any stage of the power band in first gear, the front could be made to lift off. And the same goes for second cog as well — and this gear will bring the Norton to over 65 mph before valve bounce dictates a further change.

Of every machine there must be criticisms, and it would be unfair to expect what is basically an eight-year-old machine to be faultless. But wheel lifting is not a criticism as both wheels can easily be kept on the tarmac if the power is fed on gently — and the machine will still out-accelerate practically everything on the road.

What does come in for unfavorable mention is the setup of the Wal Phillips Fuel Injector, which was tuned well for top-end performance but too rich at the bottom, calling for a high tick-over speed to keep the idle reliable.

Because of this rich running at low speeds, the machine consumed a fantastic quantity of fuel. Much riding for test purposes was done in towns to gauge the practicability of the Norton as a street machine, and while it came out well on this score, 15 mpg gas mileage is almost too ridiculous to be funny.

Supercharged engines running at low rpm should return fairly good gas figures and the Norton’s poor performance on this score must be laid to the setting of the Injector and not to the blower. It is not possible to run a normal slide carburetor such as an Amal GP with the blower, as the suction through the bore is sufficient to cause the slide to stick. Experiments show that it would be necessary to use five throttle springs to get the slide to close, which would make opening the throttle a two-handed job! An SU carb seems the only real answer.

I guess that the Norton could be tuned to give over 35 mpg with no loss of performance. There was a little vibration present, but very little more than with a standard unit, and certainly not enough to make riding in any way uncomfortable.

Despite all the extra power, Hagon guessed that the standard Norton fiveplate clutch would stand up to the job — and it did not even complain after a score of fast, quarter-mile test runs.

With the engine running at 4,500 to 5,000 rpm, the clutch could be dropped with a bang, the rider having every confidence that it would provide 100 percent grip.

Starting was easy, as it should be. The speed at which the supercharger is turning over has no bearing on the starting performance. It is simply the strength of the mixture entering the combustion chamber that determines how quickly the engine fires.

But, as with other engines using the Phillips Injector, it was necessary to switch off the fuel supply before stopping the engine to allow the fuel stored in the pipe to be used. Otherwise, it would feed down the pipe and into the body of the injector via the idle jet. Provided that this routine was observed, the engine would start first time on a small throttle opening.

A very short induction system between blower and cylinder head ensures that there is no lag between the throttle being closed and the engine shutting down, so that the full benefit of engine compression could be enjoyed.

With one of these kits, the owner of a Norton can convert his motorcycle into a fire-breathing beast capable of taking on anyone and anything. This is the ultimate in bolt-on power, definitely for men — not boys.