Supercharge!

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

by Eldred Norman

Chapter 3 - Why Supercharge? Why not a general "work over?"

At first sight it might appear that supercharging is an expensive way to get more power. From the manufacturer's point of view it would be so. He can make a big cylinder almost as cheaply as a little one, certainly cheaper than he can supercharge the small one.

This book is not for him. It is for the man who already has a car who looks enviously as a Mustang swings out and surges in safety past a semi-trailer whose diesel fumes he has been inhaling for the last five minutes. Most likely he doesn't know and never will know that supercharged, his car would out-accelerate the Mustang. But there is something of Walter Mitty in all of us.

There are two ways to increase the power of the car engine without actually increasing it in bore and stroke. You can improve the breathing and cylinder filling by fitting larger valves, 'hot' camshaft, extractors, and multi carburetors, and just let the atmospheric pressure do the rest. These things will give you a considerable increase in power but will make little difference to the amount of torque. They will however move the maximum torque figure much higher up the rev. range, and since torque times revolutions per minute is the basis of horsepower, the latter will have increased materially. This increased power with the right gearing must mean higher top speed and more acceleration.

Power obtained in this way however has some disadvantages. If we have moved the maximum torque point from say 2000 to 4000 r.p.m. and if we are using one gear mainly, such as top gear, we have moved the maximum acceleration period from 40 to 80 m.p.h. This might be good on the race track where we never go as slowly as 40, but is not much use for city driving. Also, and of great importance is the fact that the usable torque range forms a smaller proportion of the total rev. range as the maximum r.p.m. figure is lifted.
For example an engine might have a reasonable torque range operating from 1500 to 3000 r.p.m. with peak b.h.p. at 4500. That is we have torque for one-third of the rev. range. Now if we raise the maximum r.p.m. to 6000 with hot bits, we find that we get the same or less torque range starting at 3000 and going up to 4500. This represents only a quarter of the available revs. This means we need a four speed box to go with our hot motor.

Racing cars with high power outputs are often out-accelerated by cars of lesser power but with a better torque range.

The supercharged motor presents a very different picture. Let us assume that we take a completely standard engine and we supercharge it sufficiently to put out the same maximum b.h.p as the 'worked motor'.

We find here that because the supercharger becomes more efficient as the revs increase, so does the engines breathing and filling improves. We find that maximum power occurs at a figure slightly higher than that with the unblown standard motor but nowhere near as high as with the 'hot' motor. We find that the torque range which originally occupied a third of maximum revs. now occupies something over half as compared with the quarter in the case of the 'hot' motor. Also the maximum torque figure will have gone up more than proportionally when compared with the b.h.p. increase. In fact so much will the torque have increased that using top gear it will out-accelerate the 'hot' motor in second. There is however one drawback, it will now be very undergeared. The hot motor will have the higher top speed unless the final drive of the 'blown' motor is changed.

A standard 1963 EJ Holden was fitted with slightly larger valves and double valve springs. With the normal manifold and carburetor it had a top speed of 84 m.p.h. It was supercharged at 10 lbs. and road tested by the Adelaide 'Advertiser'. The maximum top speed was found to be a mean 102 m.p.h. The car was then fitted with a 32% Laycock overdrive. No changes were made to the motor or boost. Top speed then increased to 124 m.p.h. in overdrive. As I mentioned before a supercharged motor does reach the point of maximum power at slightly higher revs than the same motor would reach 'un-blown'. For practical purposes when supercharged the engine will reach a peak power point in r.p.m. equal to the original peak power revs times the cube root of the absolute pressure and divided by the cube root of atmospheric pressure.

As an example:-

Peak power revs unblown:
4500
Maximum boost: 14.7 + 7 = 21.7 lbs. absolute

R.P.M. blown equals
(about)
(equals about) 5010 R.P.M

It might be thought that maximum power when supercharged would increase proportionately by the amount by which the maximum absolute pressure exceeds the atmospheric pressure. This is not so for two reasons. The adiabatic increase in temperature of the charge because of the increase in pressure; and second, an increase in pressure does not mean the same proportion of increase in volume passing through a fixed aperture. One atmosphere of pressure might pass 100 cubic feet of air a minute through a 1" hole but two atmospheres would not pass double the amount. The harder you blow the more is the turbulence and skin friction. For this reason if top performance is being sought when supercharged, large manifolds and large valves are even more important than they are with the unblown motor. An obstruction to a gas moving with a velocity of 200 feet per second is more that twice as great if the velocity is doubled. This is the main reason that torque rises faster that b.h.p when supercharging.

Actually the torque increases more than the increase in the ratio which the absolute pressure bears to the atmospheric pressure. This is because not only does the quantity of material for combustion increase in the cylinder under supercharging, but that extra quantity is itself responsible for an increase in compression pressure since it must be compressed into the same space as with the unblown motor.

Although the same rules relating to manifold and valve restrictions apply with regard to maximum torque figures they naturally do not have as much effect as they have on maximum b.h.p. since maximum torque occurs at much lower revs. When much less air/fuel is being passed. A valve which forms almost no obstruction to the passage of 20 cubic feet of air a minute may pass 30 'under slight protest' if however we try to pass 60 through it we may easily fail.

A 179 Holden on a wheel dynamometer gave 69 b.h.p at the wheels at 3500 r.p.m. unsupercharged. The same motor with a boost of 6 lbs. ( 20.7 absolute) gave 112 b.h.p. at the same revs. This represents a 61% increase in torque for a theoretical 41% increase in air/fuel volume.

People have often said to me "of course ports and manifolds don't matter as much when supercharged." If torque is all they want, fair enough, you can still get fairly good results through the standard manifold. But if you want 400 b.h.p from three litres you can't simply blow a standard engine at 25 lbs. You won't get it that way. You must work over the engine as if you were after the maximum power unblown and then use a supercharger big enough to give a 12 lb. supercharge on top of that.

Of course if you simply want 200b.h.p. for road use on pump fuel, you can get it with nothing more than the supercharger, and you can have a nice drivable city car that attracts no attention at the traffic lights.

Now compare the installation costs of the supercharger against those of the 'hot bits' method. The former will cost you about $400 fitted or thereabouts.

A head, ported and polished with larger valves will cost you about $130. Three one and three quarter S.U carburetors will cost you about $180. A 25/65 cam about $25. Extractor exhaust system about $70. A total cost of $405. In fact almost identical with the cost of supercharging.

Then comparing them on the road for performance all round. The supercharged car will be docile and smooth in traffic in top gear. It will accelerate from as low as 10 m.p.h. in top without snatch or jerk. It will go from 20 m.p.h. to 100 m.p.h. with out a gear change in about 20 seconds. If no change has been made in the final ratio, top speed will be about 105 m.p.h. In fact, it will be very similar to driving the 5 litre V8 except that it will have more acceleration than the later and a slightly lower top speed. Most drivers would think that it did not really require more than two gears unless a caravan was to be pulled. In top gear it would accelerate up a one in four grade and with a ton load behind, it would be much on par with an unladen standard Holden.

For city driving fuel consumption would be down some two miles per gallon and the same on a country trip. Pulling a caravan it would be about two miles per gallon better than the unblown car. The reason for this last is that even with the caravan, gears are hardly ever needed.

Now to take the unblown car. It would not be as easy to drive in traffic. Much more gear work would be required as it would not operate satisfactorily under 30 m.p.h. in top gear. Using the gears it would accelerate to 100 in much the same time as the blown car provided lots of revs. were used in the gears. It would have a top speed of about 110 m.p.h., possibly slightly more if the final ratio were lowered slightly.

Fuel consumption would fall by about four miles per gallon for city driving, that is 2 m.p.g. less than the blown car ( because of the constant gear work), and in the country it would be about one mile to the gallon better than the blown car. If pulling a caravan it would do some four miles per gallon worse than the blown car, and be most unpleasant to drive because of it's inability to pull slowly in top gear.

Wear-wise, at the end of 50,000 miles the blown car would be in far better condition. The gearbox would have done less than a quarter of the work since top gear is a straight through drive. Assuming both cars had been driven at the same average speed for the distance, the load transmitted would have been the same in both cases, in respect of the differential, but the constant gear changing with the unblown car would probably show up in increased backlash.

I am often asked how the crankshaft and bearings stand up to supercharging. Put quite simply it is easier on the motor than any other method of increasing the power to the back wheels.

Off all the forces operating in an engine the pressure of the explosion on the piston is by far the least. This pressure could be doubled and it would still not represent 10% of the stresses at work. Moreover, one of the greatest forces, centrifugal force, increases by the square of the proportion of the increase in r.p.m. This means that this force is two and a quarter times as great at 6000 r.p.m. as it is at 4000. From this it can be seen that supercharging is much easier on the motor than increasing the r.p.m. by improving the breathing.


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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