posted September 04, 2001 10:40 AM
RUN IT RICH!
We are now in a position to consider the use of Nitromethane blends in practice.
Like methanol, nitromethane has a strong tendency to pre-ignition, but unlike methanol it has a much lower knock rating, that is to say it will detonate.
Both these conditions will be fully explained at a later stage, but in the meantime by making sure the mixture ratios are well on the rich side, these two conditions should be reduced to manageable proportions.
In addition to rich mixtures it is highly desirable to have a very clean combustion chamber, giving both freedom from Carbon deposits and a smooth flowing surface with no sharp edges that can get too hot.
While polished combustion chambers are the subject of much debate in conventional high performance engines, they have a real use when using nitromethane fuels.
Since, as has been stated, the figures quoted tend to provide a rich mixture so as to be on the safe side, it will be as well to know the signs of an over rich mixture.
Difficulty in starting coupled with mix-firing during the early part of the run, cleaning out at a later stage, or large quantities of liquid fuel coming out of the exhaust system are the two major signs.
Plug readings are another method and can be taken without the usual method of cutting the engine at full power.
An examination when the engine has completed a run will prove quite satisfactory, provided new plugs are used.
Signs to look for are as follows whenever the amount of nitromethane is 25 per cent or over, starting with a weak mixture, as this is the most dangerous condition and to be avoided at all cost.
WEAK. The center insulator rather white looking in color and may well have the surface rough or blistered, even in some cases with the insulation chipped, a fairly sure sign of pre-ignition.
One or both electrodes on careful examination may show very small beads of metal attached to them evident to the naked eye, and almost always considerable blueing of one or both electrodes.
CORRECT. The porcelain center electrode insulator light grey brown in color, often with the earthed electrode just showing signs of heat.
RICH. Sometimes difficult to distinguish from a weak mixture as in both cases the center insulator will be rather white looking, but in this case the surface will be smooth and both electrodes will be almost as the original metal.
Another check as a rough indication is that with the engine being turned over with the ignition off, signs of vapor should be seen at the exhaust, and if not, a weak mixture could well be suspected.
Fuel pump pressure is of importance since with Nitromethane (as already mentioned) we are dealing with a fuel that is liable to become unstable when confined and subjected to shock.
If you now consider a high pressure pump forcing the fuel under pressure along the line, it is quite possible for an air bubble to form, which can then be regarded as a slug of air, which by the pressure behind it, will be forced along the line in a series of pulses hitting the fuel in front of it, now compressed in a narrow space, thus providing the ideal conditions for an explosion, hence the limit of 100 Ibs. per square inch mentioned.
In view of this you must consider placing the fuel cut-off valve after the pressure pump, provided the re-lief valve is set well below the 100 lb. per square inch point, the advantages of so doing being evident on a little thought.
While on pump pressures there is also the question to be considered when using carburetors fed from float chambers, of the actual fuel cutoff valve lifting under pressure.
In some cases some four pounds pressure will do just that and cause flooding, so a check will have to be made to establish just what line pressure can be utilized without this taking place.
It is most important to check the oil at frequent intervals and if the amount has increased, as a result of the fuel getting down the bores past the rings, essential to change when the increase is 25 per cent or more for two reasons.
In the first place such a mixture of fuel and oil is no longer a good lubricant, and in the second place there will now exist a danger of sump fires and even explosions, since the oil mist plus the oxygen-rich atmosphere is very liable to catch fire or explode.
The only way to overcome such a fire is by the use of C02 type or "OnBoard" Freon type fire fighting equipment, the nozzles directed into the sump itself.
The first signs of such a fire are lazy, yellowish flames, seen possibly at one or more of the sump breathers or rocker cover outlets.
It is important when using over 20 per cent nitromethane mixtures to check the engine over after shut off for at least two minutes by visual examination for such possible fires