Supercharge!

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

by Eldred Norman

Chapter 1 - What is Supercharging?

There is nothing new about the principle of supercharging. It is almost as old as the internal combustion engine itself, although it did not become popular until the mid nineteen-twenties when it entirely took over in the racing car field.

No doubt many people wonder, if it is of practical use, why the major car manufactures have never produced a popular supercharged car. The answer is very simple. The manufacturer can bore a bigger hole and fit a bigger piston much more cheaply than he will build a supercharger.

Military aircraft of course used supercharging extensively right up to the advent of the jet engine, as there was no other way to overcome the problem of rarified air at high altitudes. Most such aircraft had superchargers fitted with clutches enabling them to be used only at the higher altitudes.

Most people these days who drive cars at least have a rough idea of how a car engine works. They know that the engine operates by breathing in a mixture of petrol and air and burning it, and that the product of such combustion pushes on the pistons and by mechanical connection turns the wheels of the car.

Now the more air/fuel you burn, the greater is the push or 'torque' as it is called by engineers, from the wheels.

Everyone knows that there are various sizes of engines in cars. Some are large with many cylinders, and some are quite small. A supercharger is a device for making the small engine burn as much fuel as the large engine, thus enabling it to supply a great deal more torque that is available normally from the small motor.

In the unsupercharged car engine, when a piston is drawn by the connecting rod to the bottom of the cylinder, a mixture of petrol and air enters the cylinder via the inlet valve and manifold, from the carburettor. This charge is compressed by the piston during the upward travel, and is finally ignited by the spark plug, and burning, exerts a pressure on the piston on its 'work' stroke.

Now this charge which was 'drawn in' by the piston can vary in size for a number of reasons. The expression 'drawn in' requires further explanation. Strictly speaking the piston moving down the cylinder, does not draw in the charge. It permits it to enter by getting out of the way.

As we all know, the earth is surrounded by an envelope of air with an effective thickness of about ten miles. Air has weight. Perhaps not very much in small quantities, but in large quantities the weight can be considerable. A cubic foot of air weighs only slightly over an ounce. A cubic mile of air at sea level would weigh 600,000 tons. Now every square inch of the earth's surface is supporting the weight of a column of air some ten miles high. This column of air exerts a pressure of 14.7 pounds on this square inch. This is know as atmospheric pressure, and it is this weight of air which 'falls' into the cylinder when the piston moves out of the way.

A number of things however can influence the actual amount of this air which occupies the cylinder. The valves, manifold and carburettor might be too small to allow the cylinder to fill at anything more than the slowest rate of piston travel, and it should be clear that the faster we can move the piston and air/fuel, the more we can burn, and the more power will be available.

Another factor, which can have a very great effect on the volume of air/fuel entering the cylinder, is the altitude above sea level at which the engine operates. The higher we go the less weight of air we have above us and the less we fill the cylinder. At 10,000 feet we loss 30% of the air by weight and this means 30% less power.

I remember driving a Ford Falcon on the Italian autobahn, la Strada del Sole. Down near sea level the car had a top speed of about 86, but a month later this was down to 78 in the high altitude of the Iranian plateau.

I will be employing the term 'absolute pressure' much in the future so perhaps I had best explain it.

If we can imagine a state of no air at all, say at the altitude of a satellite, we would say the absolute pressure was zero, or nought. At 10,000 feet, there would be an absolute pressure of about 10 pounds per square inch (p.s.i.), that is 10 lbs. more than zero. At sea level it would be 14.7 p.s.i.

I do not intend here discussing the field of engine design and methods to improve engine 'breathing', other than by supercharging. It is sufficient to say that by a suitable arrangement of manifolding, valves etc, it is actually possible to overfill a cylinder at a given number of engine revolutions. Unfortunately such an arrangement has several drawbacks.

First, such an arrangement is only effective over a very narrow portion of the 'rev' range. Second, it actually reduces power output by causing underfilling over a considerable portion of it.

Such an engine may be very good in a racing car when used in conjunction with a five-speed box, but would be most unpleasant to drive on the road in city traffic conditions.
The manufacturer of the ordinary road car is forced to compromise. He aims for good cylinder filling at speeds between 30 and 60 mph, the normal driving range, and sacrifices performances at higher speeds.

No doubt there will be those who will say ' why not use bigger valves and manifolding at least, even if the valve timing is left very 'soft', this would help the high speed performance without spoiling the low speed'. Unfortunately it is not quite as simple as this. The engine designer is not a complete idiot although you might feel so at times. To get good fuel economy over the driving range it is necessary to restrict the valve and manifold sizes so that the air/fuel velocity is kept fairly high. This keeps the mixture in a highly atomized state and prevents the petrol from separating out into large droplets, which cannot burn quickly in the combustion chamber where they would be largely wasted. As I said previously, the manufacturer must compromise, and the result is that we have proportionally less cylinder filling as the engine revolutions rise.

There is another factor besides the quantity of air/fuel in the cylinder, and which has a very great effect on the final torque delivered by the engine. That is the amount we compress the charge before we ignite it.

The smallest particle of any compound, which can exist on its own, is known as a molecule. Now, the power of the explosion of a mixture of petrol and air in the cylinder is very much increased if the millions of molecules making up the mixture are squeezed closer together before igniting them. This causes them to burn more rapidly, generate more heat, and give a bigger push to the piston.

The compression ratio (c.r.) of an engine is a numerical value of the amount of this squeezing together of the molecules in the cylinder.

If we take the volume of the cylinder as displaced by the piston, and add it to the volume of the combustion chamber in the cylinder head, and then divide this total by the volume of the combustion chamber, we get a ratio of the two numbers.

As an example, if the volume of the cylinder was 40 cubic inches, and that of the combustion chamber was 5 cubic inches, we would get 40 plus 5 divided by 5, that is 9:1

For a very rough guide, increasing the compression ratio by one unit, say from eight to one to nine to one, will increase the torque by about 10%. Unfortunately such a simple solution to a power increase is not always possible. There is always a limit imposed by the type of fuel being employed. Petrol fuels are rated by the oil companies by the octane number system, which I will not elucidate. It is sufficient to say that the higher the octane rating of a fuel, the higher the compression ratio that can be used with that fuel.

The process of compressing generates heat. Too much heat and the charge will explode or detonate at the wrong time without it being ignited by the spark plug. This can cause considerable damage to the motor, as well as causing loss of power.

If you think about it you must understand the expression 'compression ratio ', is only a theoretical concept. Because of the fall-off in breathing and cylinder filling the actual compression ratio is not a constant. This is know as compression pressure, and varies all the time with changes of throttle opening and increases in engine r.p.m. Using a compression ratio of 16:1 an engine would idle quite well on ordinary pump petrol because there is almost no compression pressure under these conditions, but the moment the throttle was opened allowing a vast increase in pressures, violent detonation would set in and in a few seconds could do considerable damage.

There are fuels which would permit your car to operate on this ratio but they are very expensive.

Compression pressure is influenced by another factor besides the amount of actual cylinder filling.

Anyone who has used an ordinary cycle pump knows that it gets quite hot while in use. As I stated before compression of a gas or mixture generates heat. Now if a gas is heated in an unconfined space it expands. If it is confined the pressure will increase. This means that the temperature of a gas entering the cylinder and the subsequent increase in temperature due to compression will influence the final compression pressure. This temperature increase due to compression is know as adiabatic, that is the temperature change comes from within the mixture itself, not by contact with an outside source such as a hot cylinder wall.

In this chapter we have seen that cylinder filling falls off with an increase in revs at the same time reducing compression pressures which in turn mean less torque from the motor. It is here that the supercharger comes into the picture. All superchargers have one thing in common; they give increased pressure as the revolutions rise. A supercharger is simply a pump, which forces fuel/air into the engines induction system so that compression pressures can remain more constant over the entire revolution range.


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.

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