Supercharge!

0 to100 m.p.h. in 14 seconds in a HR Holden

by Eldred Norman

Chapter 2 - Types of Superchargers

Superchargers can be classified into three main types. The centrifugal, with which I will also include the axial, since it works on the same basic principle. The Roots type, which operates by transference of air through itself to the manifold. And the vane type, which also works through transference, but compresses the mixture within itself before releasing it into the manifold.

The Centrifugal Supercharger

This operates on the principle that since air has weight it will have considerable force or pressure of you throw it hard enough. It is exactly the same as the household vacuum cleaner with an impellor with fixed radial blades rotating in a casing with a tangential outlet. The inlet is to the centre of the impellor introducing air to the axis of rotation, and the air is flung by centrifugal force to the outside of the casing and into the manifold.

To build up much pressure such a unit must operate at very high r.p.m., 50,000 r.p.m. is quite common for this type. Unfortunately because of a physical law the pressure from this supercharger increases by the square of the increase in r.p.m. That’s is if it gave 1 lb. pressure at 20,000 r.p.m. it would need 60,000 r.p.m. for 9lbs. boost.

To stand r.p.m. of this order of course requires a very high standard of mechanical precision and high quality materials. The centrifugal supercharger is mechanically very efficient in itself in that it requires less power to drive than either the Roots or the vane type to achieve a particular boost. Its usefullness however is rather limited because of the very great difference which the revs make the pressure. These superchargers have however had a considerable amount of success lately in the tractor and earth-moving fields. This type of machinery operates at fairly constant r.p.m. and in any case such superchargers are usually driven by gas turbine from the exhaust of the engine on which they are employed. A by-pass system on the exhaust permits the unit to operate at almost constant speed. This is the type of unit which was on the Offenhauser which won the 1968 Indianapolis 500.

Centrifugal superchargers are often two or more staged. That is one supercharger is fed into another so that the pressure is increased without increasing the r.p.m. A two-stage compressor of this type will have more than double the pressure of the single stage operating at the same revs. This is because the second stage receives its charge at a greater density than did the first stage, and since this means a greater mass travelling at the same velocity we must get a more than proportionate increase with the second stage.

Owing to the extremely high centrifugal forces involved in this type of supercharging it is impossible to suck the air/fuel mixture through the supercharger. Petrol is more than 500 times heavier than air, thus immediate and total separation would take place, and although a certain amount of remixing might occur in the passage through the manifold and valves, a great deal of raw fuel would enter the cylinders and remain unburnt during the firing stroke.

Where a carburetor is used it is necessary to enclose it in a complete box with glands for the controls. It is also necessary to seal and enclose the fuel pump and pressurize it also, otherwise it would not be able to pump fuel into the pressure surrounding the carburettor.

Of course when fuel injection is used there is no such problem provided the injection system operates between the supercharger and the valve.

The exhaust operated centrifugal supercharger is called a turbo-charger because it is driven by a turbine. It is expensive to produce, and this alone is likely to severely limit its employment.

The belt driven centrifugal supercharger is very ineffective at low speeds. I had a Paxton of this type on a Falcon Pursuit 170. It made little or no difference to performance between 40 and 50 m.p.h. in top gear, but had a very considerable effect at about 90 m.p.h.

The centrifugal supercharger was invariably the type chosen for aircraft. It has an enormous volume capacity to weight ratio, which puts it far ahead of either of the other two types. Of course aircraft engines, like tractors, operate at almost constant r.p.m. under flying conditions. The variable pitch propeller makes this possible.

There is another type of supercharger which also makes use of the fact that air has weight to obtain pressure. That is the axial flow compressor. Undoubtedly the best known of these is the Latham unit. This uses the same principle as the turbine. Dozens of small blades like tiny fan blades are arranged in rows on a revolving drum or cylinder and set at an angle to the axis of rotation. The outer casing has a similar arrangement of blades except that in this case the blades are arranged on the inside of the cylinder. With both rotor and outer casing the blades are arranged in regular ordered rows in such a way that blades of the one alternate with the blades of the other. The blades on the interior of the outer casing are all arranged to deflect air in the same direction and that direction is opposite to the arrangement of blades on the rotor. In effect, air entering at one end of the space between the drum rotor and the outer casing is deflected many times in its passage of the whole length, and each such deflection adds to the density and velocity of the charge and builds up to a considerable pressure at the outlet. Like the true centrifugal, this supercharger must operate at five to six times engine speed to achieve a useful boost. It enjoys a considerable advantage over the centrifugal in that it does not tend to cause fuel separation to any greater degree than the Roots or vane type, in spite of the very high r.p.m. employed. This is because there is no real centrifugal action. A particular molecule of air would simply pursue a slightly zig-zag course from one end of the casing to the other, the mean direction remaining parallel with the axis of rotation.

This unit is equally as efficient as the centrifugal. It also suffers the same disability in that it requires fairly constant r.p.m. if it is to maintain fairly steady boost pressures. Also, it is an expensive unit to manufacture because it is almost a precision instrument. The Latham unit is very compact and beautifully made in stainless steel. It is very light for its capacity, and like the centrifugal requires no lubrication other than that already built into the bearings.

[ continued... ]


This is a special Technical Info article, reprinted from the original (and rare!) book that was supplied with superchargers purchased from Eldred Norman, Aussie racing legend and manufacturer of Norman Superchargers.

Although not a common method of modifying an FE or FC, the theory and information about fuel induction, carburettion and so on is fascinating. Many thanks to Tony (IhadaV8) for obtaining the book and providing it to us. Tony in turn thanks Mike Norman, for supplying a copy of his father's book.

Important Note: This document is intended as a guide for those persons interested in repairing or modifying their vehicle. The FE-FC Holden Car Clubs of Australia take no responsibility and accept no liability for the information contained herein. You must ensure that all work carried out and/or modifications made to your vehicle are legal in your state, and we recommend you contact an engineer or your local Traffic Authority for further information.


If you have a technical question about repairs or maintenance on your FE or FC, please post a question on our Discussion Forum.

Home || NSW || Queensland || SA || Victoria || Sign the Book || Chat about FE/FCs || Nationals
|| Tech Data