Volkswagen never accepted that it was safe, or indeed necessary, to increase the power output of the basic flat-four engine. Indeed, in Germany, Britain and America, VW dealers went to great lengths to point out that any unauthorized alterations to the engine would invalidate the manufacturer's guarantee. The Beetle, they said, was built to transport four people from A to B with the minimum of fuss. Outright speed was not a prime consideration.
However, as is always the case, human nature dictated that some people could not resist tinkering with their VW engines - even Ferdinand Porsche was guilty of that. Companies like Okrasa and Denzel favoured the normally-aspirated approach with long-stroke crankshafts, special cylinder heads and twin carburettors, while others such as MAG (Switzerland) and Judson (USA) preferred to supercharge their engines.
Back to School
To understand why supercharging was a popular tuning tool, we need to take a look at some basic engine theory. When the pistons are travelling down the cylinders with the inlet valves open, they draw air and fuel into the combustion chamber via the inlet manifold and carburettor. In a perfect world, if the swept volume of a cylinder is, for the sake of argument, 300cc, then precisely 300cc of air/fuel would be drawn into the cylinder. However, as we all know, this isn't a perfect world and so what happens is that only maybe 85% of the possible maximum volume of air/fuel mixture is sucked into the engine. This is because there are restrictions to the flow in the form of restrictive inlet manifolding, poorly designed inlet ports and valves that may be too small. This percentage is referred to as the Volumetric Efficiency (VE).
At both low and high engine rpm, this cylinder filling ability can fall off dramatically. The Volkswagen engine was deliberately designed to have poor cylinder filling ability, restricting its high-rpm potential and thereby ensuring a long life.
The ultimate power output of an engine is the product of the amount of air/fuel mixture that can be burnt per cylinder filling. If the engine can only draw in a restricted amount of mixture, then it can only be expected to produce a restricted amount of power. However, if there were some way of introducing more air/fuel into the engine, then the power output will be increased - assuming, that is, that the ignition system is up to the job of burning the extra fuel. A supercharger - which is effectively an engine-driven pump - will force air/fuel into the engine under pressure, thus greatly improving the cylinder filling and thereby increasing the Volumetric Efficiency. In fact, it is possible to make raise the VE to well over 100%, thus making the engine act as if it is of larger capacity. As the engine is then able to burn more fuel, it will produce more horsepower - with a significant increase in torque at lower rpm.
OK, so this is a very simplistic approach to what is indeed a very complex subject, but it outlines the basic theory behind supercharging. However, there are three different designs of supercharger: the centrifugal, the traditional lobe type and the sliding vane.
The first of these, the centrifugal, needs to run at extremely high rpm to be effective, operating, as it does, on the principle of the fan. It does not pressurise the incoming mixture per se but does drive it at a higher velocity into the inlet port. As not all the air/fuel can pass through the port, pressure does build up, as a result of which inlet temperatures will rise, increasing the likelihood of detonation.
The lobe - or Rootes-type - supercharger consists of two interlocking rotors that scoop the mixture in, compress it and force it under pressure into the engine. Again, inlet temperatures rise and detonation can occur but, it has to be said, the Rootes-type supercharger remains the most common of all designs.
The design favoured by Judson, however, was the sliding vane type. Here, the air/fuel is scooped into the supercharger by a serie of sliding vanes mounted on an eccentric hub. As the hub rotates, the vanes are thrown out under centrifugal force and seal against the inside of the supercharger body, pressurising the mixture and finally blowing it out into the inlet port. The inlet temperatures is kept down to reasonable levels as the gases are allowed to expand once they are free of the blower unit.
Temperatures are further kept down by virtue of the fact that vane type superchargers, being more efficient, usually operate at engine speed, as opposed to being overdriven as with other designs.
The 30-horse Judson supercharger is driven by a pair of drive belts run off a special crankshaft pulley that still allows use of the original generator drive. The blower itself is mounted on top of the standard inlet manifold, with the original Solex carburettor then fitted to the inlet side of the supercharger.
As the vanes need to be kept well lubricated, oil is fed to the inlet manifold of the Judson from a glass container filled with engine oil. This oil is drawn into the supercharger by inlet vacuum and, after lubricating the vanes, passes into the engine to be burnt along with the fuel. Judson claimed this helped engine longevity by acting as an upper cylinder lubricant.
Fitting the Judson kit was a straightforward enough task, taking just a few hours using regular workshop tools. The engine did not need to be disassembled, nor did it even need to be removed from the car. Apart from the obvious fitting of the supercharger, oil bottle and drive pulley, all that needed to be done was to change the carburettor jetting (new jets were supplied) and fit a slightly stiffer spring to the fuel pump so as to deliver a little extra fuel. As a consequence of the greater power output and increased fuel delivery, fuel consumption rose by around 5% - a figure that Judson claimed to be a small price to pay for the increased performance on offer.
In practice, fitting a Judson supercharger made quite a difference to the performance of an early Beetle, boosting the power output to around 48bhp and increasing the top speed to 85mph. Acceleration from 0-60mph fell to 15.5secs, approximately half the time taken by a standard Oval. If the Judson had any drawbacks, they were only those shared by all aftermarket supercharger installations: increased mechanical noise at low rpm (the vanes in the blower tended to be rather noisy until engine speed increased), higher underbonnet temperatures and increased fuel consumption. There was also the matter of having to remember to top up the oil container every 500-1000 miles. However, offsetting these are the benefits of greatly improved performance, ease of installation and low initial cost - less than $150.000 US back in the late 1950s. You just had to make sure that your engine was in good condition to start with.
Today, the Judson is a much sought-after conversion and while not in the rarest of rare category, still commands top dollar, especially in NOS (new old stock) form. Practical and fun, the Judson supercharger from Conshohocken, PA, will certainly liven up even the most lethargic of Beetles. Buy one and drive with a smile.