Supercharge!

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

by Eldred Norman

Chapter 4 - Supercharging Of “Hot” Motors

Like an engine a supercharger has a swept volume. That is the amount of air it displaces or compresses in one revolution. Since it is a four-cycle motor, a Holden 179 displaces 179 cubic inches each TWO revolutions or near enough to 90 c.i. per rev. This of course is purely theoretical. Owing to breathing restrictions the actual displacement falls off enormously as the revolutions rise. The same rule of course applies to the supercharger but to a far less extent. The supercharger’s port area is usually enormous when compared with its swept volume. A typical unit would have an inlet area of 10 square inches with a volume of 90 c.i. And this area is open the whole time.

If we compare this with the Holden motor we find that we have six valves which if open altogether would have an area of about 6 sq. ins. But these in fact only open for about a quarter of the time; that is for the inlet of the strokes of the cycle. In effect we really only have about 11/2 sq.ins. of inlet area, and to make the position worse, the flow of air/fuel is an interrupted one. It is quite clear from this that the supercharger is at least six times as efficient as the motor in this respect.

If we were to fit a 90 c.i. s/c to a standard 179 and drive it at engine speed, and assuming that it had a sufficiently large carburetor, we would find that at 500 r.p.m. we had no manifold pressure. At 1000 we would have perhaps 2lbs. at 4000 we would probably have about 12lbs, and this pressure would continue to rise until the motor disintegrated or the valves remained open causing misfiring and relieving the pressure.

The standard Holden 179 or 186 has virtually no valve overlap. That is the period at the top of the stroke when the exhaust valve is about to close, and the inlet valve is about to open. I mentioned earlier in this chapter that the engine’s breathing is interrupted. That is, that the column of air/fuel in the port comes to a complete stop when the valve is closed and has to be started moving again when the valve opens to admit it to the cylinder.

A great improvement in breathing can be obtained by opening the respective valves earlier and closing them later in their operational cycle. This of course causes overlapping of the exhaust and inlet valves at the top of the piston stroke. In a ‘hot’ motor this overlap can be quite considerable, occupying at least 100 degrees of the engine’s cycle. Now in supercharging a motor like this we come up against some difficulties. Firstly the supercharger itself must be larger than we would consider adequate for an engine of this size. The reason is that a great deal of the boost pressure from the supercharger will be lost into the exhaust system owing to the overlap just mentioned. This will particularly noticeable at low r.p.m. and secondly, there will be a considerable wastage of fuel for the same reason.

These effects can be somewhat mitigated by closing the exhaust valve earlier than would be the case with a ‘hot’ atmospheric motor, the extractor effect of the exhaust charge not being quite so vital to the introduction of the new charge when blown. On the other hand since the product of combustion will be greater with the blown motor the exhaust valve should still have the very early opening common to a ‘hot’ motor.

The effect of extreme valve timing can be seen from the following example. A 65 c.i. supercharger driven at engine speed on a standard 1500 Cortina Ford motor with a 2” SU carburetor will give 18 lbs. of supercharge at 6000 r.p.m. With the Cosworth conversion and a 6A camshaft, it will only give 12 lbs. at the same revs.

With a supercharged motor boost pressure will fall by about 5% of absolute pressure for each 10% by which valve overlap is increased. To take an example.

If an engine had a valve overlap of 30 degrees and a boost at maximum revs. of 9 lbs., that is 23.7 lbs. absolute, if the valve overlap was increased to 70 degrees, that is 40 degrees increase in overlap, then the absolute pressure would fall by 4 x 5%. This would reduce the absolute pressure by almost 5 lbs. leaving only 4 lbs. of supercharge.

It might be thought from this to leave the motor standard and use the higher boost. Unfortunately high boosts mean high temperature in the air/fuel charge, with a corresponding loss of efficiency. The Cosworth motor on 12 lbs. would put out at least 60 b.h.p. more than the standard motor on 18. Of course straight methanol fuel would be essential in both cases.

All of this leads up to one thing, high r.p.m. are essential even when supercharged if you want maximum b.h.p. This means the lot; cams, valves, manifolding, all are important and even more so when supercharged. Pressure alone cannot make up for poor design. If you had a 3 litre motor of 200 b.h.p and wished to raise it by supercharging to 400, logically you should increase valve and port areas by at least 50 % and more if possible.

In the mid nineteen fifties a small 1500 cc. engine the B.R.M., using a fantastic supercharge of 72 lbs. of positive pressure, achieved 600 b.h.p. at 12,000 r.p.m. ("B.R.M." by Raymond Mays and Peter Roberts, PAN edition). This small 16 cylinder engine had a total inlet valve area of 20 sq. ins., or just three and a half times that of a standard Holden motor of double the capacity. It was only this enormous valve area which permitted it to use such a high boost pressure.

The compression ratio of this motor was 7.5:1 and with this much supercharge it would have had compression pressures equivalent to those achieved by a ratio of almost 20:1 in an unblown motor. No wonder it was somewhat unreliable.

The valve timing may be of interest to some of my readers.

Inlet opens at 55 degrees BTDC, closes 70 degrees ABDC. Exhaust opens at 70 degrees BBDC, closes 70 degrees ATDC

In mounting the supercharger it should be provided with some sort of reservoir or ‘plenum’ chamber to stabilize port pressures. This should have a total capacity in volume equal to about half the swept volume of the motor. This should be kept as near to the ports as possible. If a pressure ‘blow off’ valve is incorporated it should be as near to the supercharger itself as possible. In this case if a flash occurs the flame front, which always travels towards the s/c, will push out some of the unburnt charge as the pressure builds up, and will reduce the intensity of the explosion. A relief valve at both ends of the manifold is a good idea. Relief valves should be of considerable area. With a 3 litre motor they should have an area of at least 4 sq. ins. They should be set to at about 50% more than the manifold pressure maximum. The best type consist of an aluminium disc on an edge seal and located by three guide studs arranged at equal distances around the circumference of the disk The same three studs can be at their outer extremity to hold a triangular plate. A spring goes between this plate and the aluminium disc. A valve spring is often used for this task.

A relief valve is essential with the extreme valve timing as backfires are very common when running low revs, such as when warming up an engine. With a standard car engine and using boost up to only 5 lbs. a relief valve is hardly necessary provided the manifold volume is not too great. Also a V belt drive is an advantage since it can always slip when there is a check to the supercharger.

I mentioned that a ‘plenum’ chamber should be as near to the ports as possible. The closer and direct is the passage of the air/fuel the less heat it will generate by skin friction, and the less it will acquire by conduction from its surroundings. With reference to the latter, under the bonnet of a car it is more likely to gain than to lose heat via the manifold.

With a standard car every effort should be made to introduce a cooling draught of air both to the s/c casing and to the air induction to the carburetor. ALL superchargers heat the charge, certainly some worse than others. If a gas is compressed it must rise in temperature, and superchargers can get mighty hot when operating on petrol. The vane type compressors which I manufacture are all water cooled unless I am specifically informed that they are to be operated on Methanol. Of which more later.

Inter-coolers are not really feasible. If they are large enough to be effective they form a too large reservoir for the mixture, and when there is a backfire it is almost of nuclear proportions.

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