Supercharge - Chapter 5

Chapter 5 - Ignition Timing

Correct ignition timing has always been something of a problem with the internal combustion engine. With the atmospheric engine manifold pressures can vary between two pounds absolute pressure to about fourteen pounds. That is when the throttle is shut right off on the overrun to full throttle at a very low speed.

On most cars both for reasons of economy, and of performance, the manufacturer has introduced into the ignition two variables which more or less cope with the variations in pressure. He has an automatic centrifugal advance which takes care of two problems. First, poor breathing means that manifold pressures will drop as engine revolutions increase, which means lower compression pressures. Second, as piston speeds increase there is the tendency to out-run the flame front in the cylinder. The charge in the cylinder of course burns comparatively slowly. (Detonation is the instantaneous ignition of the charge and pinking is caused by part of the charge burning while the build up in pressure detonates the rest.)

This is all simple enough but the variations in compression pressures introduce other complications. As I mentioned in Chapter One, increasing the compression pressure increases the rate of combustion and of the rate of the flame front.

Now both manifold pressure and compression pressure are materially affected by the amount of throttle opening as well as the number of revolutions which the engine happens to be doing at any particular moment. To cope with this factor the manufacturer has been compelled to introduce an over-riding spark control commonly called the vacuum spark advance. Actually its prime purpose is to retard and neutralize the over advancing of the centrifugal advance mechanism. This control unit is actuated from a very small port on the atmospheric side of the carburetor butterfly. When the engine is idling there is a very low manifold pressure but since the small port to the vacuum advance mechanism lies outside the butterfly when it is in this position, this high vacuum is not communicated to the unit. The mechanism consists of a spring-loaded diaphragm which is mechanically connected to the plate on which the points are mounted in the distributor. Provision is made enabling this plate to rotate through a few degrees of arc about the distributor cam. The spring holds this in the retarded position; vacuum on the diaphragm advances it.

This all means that the engine idles fully retarded, but the moment the throttle is touched the butterfly in the carburetor opens sufficiently to include the port to the pipe connected to the diaphragm mechanism, and the latter overcomes the spring's resistance and advances the ignition timing. This gives good fuel economy on light throttle opening while the manifold and cylinder pressures are still low. If, however, the throttle is opened suddenly a good way, the manifold vacuum falls off to such an extent that the diaphragm cannot over the spring tension, and the latter retards the spark which is necessary to cope with the increase in compression pressures which follows the opening of the throttle.

If the throttle is kept in this position the engine revolutions gradually increase and so does the manifold vacuum until a stage is reached when the latter begins to overcome the force of the retarding spring and the spark is slowly advanced. At this stage both centrifugal and diaphragm advance mechanism are working in the same direction, engine revolutions are high and compression pressures have fallen away.

This system of course is far from perfect since it is designed to work at sea level. At 10,000 feet, and I have driven at this altitude, the springs tension is far too great with the result that the distributor stays retarded when compression pressures are lower than usual simply because of the high altitude. At 15,000 feet it probably would not work at all. Anyone driving at altitudes of more than 7000 ft. would be advised to simply disconnect the pipe to the carburetor and advance the spark setting. They would certainly get more power. Also the system takes no count of either atmospheric temperature or of humidity, both of which have an effect on combustion rates. However on the whole and to avoid over complication it is reasonably satisfactory and almost universally in use.

Now let us consider how this system operates if used with supercharging.

First of all when supercharged we have a much wider range of manifold pressure variations on the outlet side of the supercharger. These may fluctuate from an absolute pressure of 2 lbs.(12.7 vacuum) to as high as we may decide to supercharge, which might well be 30 lbs. absolute.

If we take a completely standard Holden 179 with a compression ratio of 8.8:1 and supercharge it with a maximum of 5 lbs., or 19.7 absolute, and if we assume that it will be operated on 95 octane pump fuel, we will find that the manufacturers specifications for ignition are not satisfactory.

We know that increasing the compression pressures increase the rate of combustion. This means that if we are to use the same type of fuel, and assuming that the manufacturer designed his ignition system to operate to the best advantage, we will have to retard the spark to avoid pinking or detonation. Logically we should use a fuel with a slightly higher octane rating, but even in this case we should retard the spark slightly.

It is generally recognized that the higher the compression ratio the less spark advance is required for a given number of engine revs. As a rough rule the spark is retarded by about three degrees crankshaft for each unit the c.r. is raised. For instance an engine on 9:1 might require 40 degrees of advance at peak r.p.m. the same engine on 12:1 would only require about 32 degrees at the same revs.

This of course assumes that in both cases the same type of fuel is being used. By type of fuel I mean that in both cases it must be a petrol base fuel, or in both cases a methanol fuel.

There is a fairly simple method for working out the theoretical compression ration for a given supercharge. We use the same formula as we used to arrive at the peak revs. When blown, only in this case we multiply the original ratio by the square root of the absolute pressure over the square root of the atmospheric pressure. This is only an approximation since a number of factors can effect the final result, but it is close enough for practical purposes.

Now if we use this formula we find that 8.8:1 with a five pound boost gives an effective 10.7:1, or an increase of two units. This means that we must retard the spark six degrees. However this assumes that we are using the correct fuel for this compression ratio. But of course 95 octane will not cope with this ratio, and the only alternative is a further retarding of the spark which inevitably means inefficiency and some power loss.

With reference to the three degrees retarding necessitated by the one unit increase in compression ratio it is best to reduce the centrifugal advance maximum by this amount, rather than simply retard the distributor three crankshaft degrees. A spot of weld at the end of the slot which limits the amount of advance in the distributor, will do this.

It is probable that about four degrees of retard will have to be made to the distributor itself to cope with the higher compression ratio in its relationship to the octane rating of the fuel.

Of course engines vary in their ability to cope with a particular compression ratio with a given fuel. Much depends on the design of the cylinder head. Removal of projections and sharp edges in the combustion chamber will usually permit a slight increase in compression ratio.

Instead of retarding the spark the compression ratio can be reduced by fitting an extra gasket, or a copper shim and a gasket.

In my own 186 Holden a solid copper sheet .040 thick with a standard gasket reduces the ratio from about 9.2:1 to about 7.8:1. This permits an eight pound supercharge when operated without water injection, of which more in a separate chapter.

A factor which has a great deal to do with ignition timing is the rate of boost pressure increase in relation to the engine's r.p.m. increase. For instance if we had a large supercharger, and fitted it with a small carburetor we would get a high boost at low engine r.p.m. on full throttle, but this boost would gradually fall off as the revs. rose since the supercharger can only put into the engine what it can get through the carburetor. This of course would mean that the s/c was of not much use, since it would increase compression pressure low down in the rev scale where we can least stand an increase. No adjustments to the ignition would be of much avail under these circumstances.

I will be discussing carburation in a later chapter but for the moment we must assume that the carburetor is of sufficient size for the work.

It is important to note at this stage that when supercharging it is always safer to be slightly too far retarded than too far advanced with the spark. Over advancing is hard on a motor always; when supercharged it can blow gaskets and crack pistons.

If it is desired to run a car on pump fuel there is not much alternative to reducing the compression ratio if boosts of more than five pounds are intended. It is possible to make another modification to the distributor which enables somewhat higher boosts to be used. This also gives better fuel economy with manifold pressures below atmospheric pressure. Unfortunately the advantage of the higher boost pressures suffer to a considerable extent under this modification.

I mentioned earlier that the vacuum advance mechanism has a spring which holds the plate mounting the points assembly, in a retarded position. If the spring can be put on the other side of the diaphragm it will advance instead of retard. The diaphragm can then be coupled to the pressure manifold of the supercharger so that a positive pressure in the manifold retards the spark, which of course is what is required with high manifold pressures. It is difficult to get the correct spring tension and amount of travel correctly balanced with the boost pressure. However for those interested in trying this a total movement of the plate mounting the points, in the region of six degrees of the distributor will cope with an eight pound supercharge. The distributor itself will have to be advanced about eight degrees over the normal setting to compensate for the total loss of what was formerly the vacuum advance mechanism.

In supercharging for 'dragging' or track work, ignition timing is seldom much of a problem since most of these vehicles operate on methanol fuel. The drivers of these cars all regard the highest octane petrol as being only something suitable for warming up tractors. For them there is only one fuel, methanol. This will operate satisfactorily on compression ratios as high as 20:1,although it is generally recognized that no increase in power takes place by exceeding 16:1

A 'hot' motor operating on 10 to 1 with a fifteen pound supercharge has an effective ratio of about 13 to 1, which methanol easily copes with. This fuel has another very great advantage it has a latent heat value of about three times that of petrol, of which more later. This means that it is a far better coolant of superchargers, valves etc. As a fuel however it suffers from the disadvantage that it has only about 40% the energy content of petrol, pound for pound, and must therefore be used in much greater quantity. This problem never seems to trouble its advocates even at $1 per gallon.

Methanol is a slower burning fuel than petrol and in general requires about 5 degrees more spark advance than the latter. For this reason it is appreciably less critical of ignition timing. Most 'alky' addicts on the track don't bother with spark advance and retard mechanisms. They use fixed advance of somewhere between 30 and 35 degrees. They undoubtedly loss some performance at low revs, but since the revs are high most of the time, they don't worry. Spark plugs play a very important part in the process of ignition. It is essential that the correct heat range of plugs be used for the particular case.

For average city running with an occasional boost of up to 6 lbs. the original plugs will be suitable, but with hard driving and sustained positive manifold pressure it will be essential to use colder plugs. Compression pressures are not the only things which govern the heat requirements of the plugs. The number of revs which the engine is doing has a big effect on them. A plug which will give no trouble while towing a caravan at 40 m.p.h. with a five pound boost, will fail completely at 80 m.p.h. with the same boost.

A car operating on methanol fuel can use a hotter plug than one using the same compression ratio and boost but using 115 octane petrol.

With a supercharged car under racing conditions and using a very 'worked over' motor, plug failure is usually symtomised by a series of violent backfires into the manifold. These explosions are caused by the hot electrode of the plug igniting the incoming charge from the supercharger. As mentioned previously if adequate relief valves are not provided these backfires can damage the supercharger drive or even the supercharger itself.

With the higher compression pressures achieved in the blown motor and using coil ignition, it is usual to reduce the plug gaps to about .020 otherwise misfire can occur at high speed without the driver being aware of it. Of course in a racing car with open exhaust it is easier to notice this.

If water inhalation is used when supercharging, it has much the same effect as the use of methanol on the heat requirements of the plugs. That is a hotter plug can be used in this case.

This brings me to another method of overcoming the problem of the use of low octane fuels with high compression pressures. The introduction of atomized water with the air/fuel mixture.

Supercharge!

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

by Eldred Norman

Chapter 5 - Ignition Timing