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NOS
How a colourless, odourless and non-toxic gas can capture the imagination of street racers

It's a scene straight out of a Hollywood cliché: the young street racers of The Fast and the Furious hitting the button that says 'NOS' and blasting off into celluloid glory. That button, typically shown connected to a gas cylinder hidden under the seat, should perhaps be more appropriately labelled 'YES!' For it can, in the 20-odd seconds it takes to empty a standard ten litre cylinder, propel your average street car into the stuff of screen legends.

In the world of muscle cars, souped up stock vehicles and the general dare devilry of street racing, reality seems to blur into legend.

In a world somewhere between shoving your foot down hard and buying a ticket to the cinema, anything seems possible. But while films might allow us to cross the line, the technology and skills they glorify are sometimes very real, and very often misunderstood. Like the special gas injected quickly into a cylinder, the equilibrium rests between combustion and failure.

That much sought after gas is nitrous oxide, not NOS, which stands for Nitrous Oxide Systems, the largest marketer of nitrous oxide injections system for automotive use. Nitrous oxide is a colourless and non-flammable gas, slightly sweet in taste and odour. It is non-toxic and non-irritating and when inhaled in small quantities can produce mild hysteria and laughter. This is where the nickname 'laughing gas' comes from. When inhaled in pure form it will cause death by asphyxiation because at atmospheric temperatures and pressure the oxygen in nitrous oxide is not available to the body.

Three reasons it wins races
At about 300ºC, it breaks down into nitrogen and oxygen. Oxygen, of course, is of value to the driver because it is used for combustion within the engine. So when you inject nitrous oxide into the cylinder you have more oxygen. And this gas has a higher percentage of oxygen content than does the air in the atmosphere. Nitrous has 36 per cent oxygen by weight, while the atmosphere has 23 per cent.

It gets even better: the gas is 50 per cent denser than air at the same pressure. Thus, a cubic foot of nitrous oxide contains 2.3 times as much oxygen as a cubic foot of air. This means that if we substitute some nitrous oxide for some of the air going into an engine and then add the appropriate amount of additional fuel, the engine is going to produce more power.

Nitrous oxide has another effect that improves performance even more. When it vapourises, it provides a significant cooling effect on the intake air. When you reduce the intake air temperature you increase the air's density, and this lets even more oxygen inside the cylinder.

Trying not to blow up
So why aren't we all bundling cylinders and blazing away? The use of this gas in engines is a complex process that is perhaps best left to the cinema or laboratory. Balls of fire might look cool in the movies, but are best avoided on real world tarmac.

When you significantly increase the cylinder pressure in the engine, you also increase the engine's tendency to detonate. This is why almost all nitrous motors require retarded spark timing during nitrous oxide operation. The cylinder pressure increase is also why, when misused or improperly installed, operation with the gas causes problems with the head gasket seal and failures of the rings or pistons. Any number of things that put an engine into severe detonation such as too much boost from a supercharger, low octane fuel, an excessive compression ratio or an overly lean air-fuel ratio will also cause the same kinds of damage.

Another challenge with the system is getting the delivery of nitrous oxide and additional fuel at the correct proportions. If you feed nitrous to the engine without enough extra fuel, the lean air-nitrous to fuel mixture will make the detonation problem even worse. Combustion temperatures will skyrocket and catastrophic failure is certain to occur. If the proportion is such that too much fuel is delivered, the power advantage degrades rapidly.

Small doses of nitrous oxide can be used in engines to gain up to 35 per cent more power. Any more nitrous than that with a stock engine compromises durability too much. This is not only true of nitrous but any modification. Once you pass that figure you need to look at things like forged pistons, better connecting rods, better quality bearings, etc.

Lastly, after you're done with the intricacies of staying alive, you'll have to deal with space constraints. The only problem with nitrous oxide is that it is fairly bulky, and the engine needs a lot of it. Like any gas, it takes up a fair amount of space even when compressed into a liquid. A five litre engine running at 4,000 rotations per minute (rpm) consumes about 10,000 litres of air every minute (compared to about 0.2 litres of petrol), so it would take a tremendous amount of nitrous oxide to run a car continuously. Therefore, a car normally carries only a few minutes of nitrous oxide, and the driver uses it very selectively by pushing a button. Is all the effort, money and danger enough to justify a few seconds? That sort of decision separates real racers from moviegoers.

How it was, and can be used
The injection of nitrous oxide into the chambers of an internal combustion engine as a way to increase power output was discovered by the German aircraft industry early in the Second World War. Thousands of German fighter and reconnaissance aircraft were equipped with the so-called GM-1 system that added nitrous oxide to the intake charge to compensate for reduced air density and less oxygen at high altitude. The British Royal Air Force also used aircraft engines with performance enhanced by nitrous oxide.

During the 1950s, the American racer Smokey Yunick discovered nitrous oxide injection as one of his many schemes for winning races, until discovered and outlawed by NASCAR. Nevertheless, there have been several nitrous oxide cheating scandals over the years and it is probably still used today on the sly. By 1980, nitrous oxide was rediscovered by drag racers and hot rodders.

In the end, nitrous oxide is like any other power increasing modification: when used wisely and installed properly, it works well. When used foolishly or installed incorrectly it can significantly reduce the reliability and durability of your engine. But the best conclusion to this foray into the very fast and very dangerous is this: some things are best left for the movies.