Nitrous oxide is an oxygen bearing compound. Its chemical designator is N2O, so we know each nitrous oxygen molecule has two nitrogen atoms and one oxygen atom. Nitrous oxide is sometimes incorrectly known as "NOS". That is an acronym for the company, Nitrous Oxide Systems, which is the largest marketer of nitrous oxide injections system for automotive use.

Injection of nitrous oxide into the combustion chambers of an internal combustion engine as a way to increase power output was discovered by the German air craft industry early in the Second World War. Thousands of German fighter and reconnaissance aircraft were equipped with the so-called "GM-1" system which added nitrous oxide to the intake charge to compensate for reduced air density and less oxygen high altitude. The British Royal Air Force also used aircraft engines with performance enhanced by nitrous oxide. Interestingly, there was no use of nitrous oxide injection by the American military air forces other than very limited experimental use. It is interesting to ask oneself that, if nitrous oxide injection was so dangerous to an engine's reliability, why would so many airplanes have used it?

In this country during 1950s the famed stock car racer Smokey Yunick rediscovered 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 in NASCAR over the years and it is probably still used today by the slowest of backmarkers. In the late-70s/early-80s nitrous oxide was "rediscovered" by drag racers and hot rodders.

Today nitrous oxide injection, like many other modifications such as more aggressive camshafts, bigger carburetors, higher compression ratios, more free flowing intake and exhaust systems, can be a practical way to more horsepower. and like any other modification...perhaps even more so because it so easily lends itself to misuse...there can be a reliability and durability price to pay.

Nitrous oxide is a colorless, non-flammable gas. It has a slightly sweet taste and odor. It is non-toxic and non-irritating and when inhaled in small quantities can produce mild hysteria and giggling or laughter. This is were the nickname "laughing gas" comes form. 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.

A property of nitrous oxide is that at about 565 degrees F., it breaks down into nitrogen and oxygen. When it is introduced into the intake tract of an internal combustion engine, it is sucked into the combustion chamber and, on the compression stroke, when the charge air temperature reaches 565 deg., a very oxygen-rich mixture results. If we add extra fuel during nitrous oxide injection, the effect is like a super charger or increasing the compression ratio of the engine. Automotive nitrous systems work like the automotive equivalent of a jet's "afterburner" and is used for short duration extra bursts of power.

Nitrous oxide has this effect because it has a higher percentage of oxygen content than does the air in the atmosphere. Nitrous has 36% oxygen by weight and the atmosphere has 23%. Additionally, nitrous oxide is 50% more dense 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. Just do a bit of math in your head and you can see if we substitute some nitrous oxide for some of the air going into an engine than add the appropriate amount of additional fuel, the engine is going to put out more power.

Simply stated, nitrous oxide injection is very much like a supercharger or a compression ratio increase in that, during combustion, it can dramatically increase the dynamic cylinder pressure in the engine.

Of course, when we significantly increase the cylinder pressure in the engine, we 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 nitrous causes problems with head gasket seal and failures of the rings or pistons. I should point out that any number of things that put an engine into severe detonation, such as too much boost from a supercharger, low octane fuel, excessive compression ratio or overly lean air-fuel ratio will also cause the same kinds of damage.

Another challenge with a nitrous oxide 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.

As you can see, nitrous oxide is like any other power increasing modification in that, when used wisely and installed properly, it works well. Then used foolishly or installed incorrectly it can significantly reduced the reliability/durability of your engine.

Small doses of nitrous oxide can be used in stock engines to gain 25-35% more power. In my opinion, any more than nitrous than that with a stock engine compromises durability too much. This is not only true of nitrous but any modification. Take a stock 82 or 84 engine, up the horsepower to 300hp and do nothing to improve durability and your engine will eventually suffer. Once you pass the 35% power increase mark with nitrous oxide you need to look at things like forged pistons, better connecting rods, better bearings, etc.

Nitrous oxide is also a great value on a dollar-per-unit-power increase when installed and operated properly. The downside, of course, is the fun ends quickly. The power boost lasts as long as the nitrous. The average bottle is a 20 pounder and with a street V8 that might be worth 20 seconds of use.

So, nitrous oxide is not the instant-engine-failure many people think it is. When used properly and when dispensed by a properly designed and installed system nitrous oxide can be responsible for some phenomenal increases in power.

Article by John Erb
Chief Engineer,
KB Pistons

Nitrous oxide can double the horsepower of most engines with less effort and money being spent than any other modification. Even the "smog people" are usually happy.

A nitrous engine can be built as a stock rebuild or it can be a dedicated effort to maximize the total performance package. As more power is generated, more waste heat, exhaust air flow and combustion pressures push the limits of engine strength. Often more beef is needed in the drive train and tires.

All stock factory engines are built with a safety factor when it comes to RPM, HP produced, cylinder pressure, engine cooling, etc. If you are only going to use a 100 HP nitrous setup on a 300 cubic inch or larger engine, built in factory safety factors are probably sufficient. As power output levels are raised engine modifications are usually prudent.

The most common mistake made when using nitrous oxide injection concerns ignition timing. A normally aspirated engine makes its best power when peak cylinder pressures occur between 14 and 18 degrees after TDC. KB Pistons usually require 34 degrees BTDC ignition timing at full mechanical advance to achieve proper ATDC peak cylinder pressure. The total time from spark flash to the point of peak pressure is typically 48 to 52 degrees. If an engine is producing 30% of its power from nitrous, the maximum cylinder pressure will occur too close to TDC to avoid run away detonation. If ignition does not get retarded, good-bye horsepower and head gaskets. The key to getting max HP from a max nitrous engine is to shift the maximum cylinder pressure event progressively further after TDC.

Cylinder pressure of 1000 PSI at TDC, (FIG.1) , can drop to 500 PSI with less than 3/8" of piston travel, (FIG. 2). If you can manage to get 1000 PSI in the same engine after the 3/8" travel, (FIG.3) , the pistons will have to travel an additional 3/4" to lower the cylinder pressure to 500PSI, (FIG.4). Work is defined as a force times distance. An average pressure, (750 PSI X 12-1/2 sq. in.), times distance in feet, (3/8"divided by12), equals 293 foot pounds of work. Our second example, because it has twice the chamber volume above the piston location, must move twice as far to lower the cylinder pressure by 1/2. Since all the other numbers, by our own definition are the same, the force multiplied by a distance twice that of the first example will equal twice the work done, 586 foot pounds of work. There is no free lunch in horsepower equations because to get 1000 PSI above the piston in the second example takes twice as much fuel and energy as the 1000 PSI in the first example. What this offsetting of the peak pressure does is allow us to use the extra fuel mix available to a nitrous engine without breaking and melting things. The system that allows us to postpone maximum cylinder pressure is ignition timing retard. To a lessor extent short rod ratios, lower compression ratios, high RPM, aluminum heads, a tight quench, a rich fuel mixture, a small carburetor and hotter cams tend to delay maximum cylinder pressure.

Understand that, in our quest to delay cylinder pressure’s peak time, more is not necessarily better. Instead, consider that the ideal cylinder pressure would be just short of detonation pressure and this pressure would be maintained from top dead center, and as long as possible after TDC. If timing is really late, you won’t build enough cylinder pressure to start the car, let alone drive it. The 1000 PSI pressure in the example is not the maximum allowable combustion pressure but, rather, a comfortable pressure for illustration of the work principle.

Some nitrous manufacturers recommend, "retard the timing two degrees for each fifty horse power of nitrous". Other nitrous kits have the flame speed artificially slowed by the intentional use of a rich fuel to nitrous ratio. The maximum performance engine with a heavy nitrous load must achieve peak cylinder pressure progressively further after TDC. The heavy load engine will have the fuel and oxygen mix to make high cylinder pressures, with the combustion chamber size being drastically increased due to the piston being on its way toward bottom dead center. The strongest engines have less compression ratio, less spark advance, and more nitrous.

I have tried to explain the reason for a spark retard system in a Nitrous engine. However, many people just don’t like the idea of any retard. They say, "retard timing and exhaust heat goes up". It usually does in a stock nonnitrous engine because lower peak cylinder pressure slows the burning. If the timing is retarded in a non-nitrous engine, the exhaust opens before the fuel mix is finished burning and exhaust temperatures go up. Piston temperatures usually go down and exhaust valve temperature goes up. In the nitrous engine, exhaust temperature goes up for several reasons. The first is that the power output has gone up considerably. More power usually produces more waste heat. Second, the need to keep maximum cylinder pressures within reason has dictated that the biggest part of the fire happens closer to the exhaust valve opening time. There just isn’t enough piston travel to extract all the energy out of the charge before the exhaust valve opens. Now, we could and sometimes do, open the exhaust valve later so more combustion pressure energy can be used to turn the crank. The trade off is negative torque on the exhaust stroke. If we still have significant cylinder pressure in the cylinder as the piston moves from BDC to TDC on the exhaust stroke, your net Hp falls drastically. A real problem at higher RPM.

You can improve maximum power stroke efficiency and minimize exhaust pumping losses by running the engine at lower RPM and/or improving the exhaust valve size, lift and port design. A big nitrous engine likes everything about the exhaust to be big. If it flows good enough the cylinder will blow down by bottom dead center, even at high RPM with relatively mild exhaust valve timing. There are many variables in the design and development of an all out nitrous engine. A mistake will cause the melt down of any brand of piston. The high strength of the KB piston will withstand detonation and severe abuse. Unfortunately, all pistons will melt and when cylinder pressure limits are exceeded, run away detonation can occur. The excess detonation heat makes the plugs, valves and piston so hot the ignition system alone can not be used to shut the engine down. Continued operation worsens the situation to the point of a total melt down. Designing a maximum performance nitrous engine is more of an exercise in heat management than it is in engine building.

A lack of a sufficient fuel supply is probably the most common killer of the nitrous engine. If you add a 300 HP kit to your present 300 HP engine, your fuel requirements roughly double and a shortage doesn’t just slow you down, it melts things. An electric fuel pump and fuel line devoted entirely to the nitrous equipment is recommended. Some people add a small "race fuel" tank just for the nitrous. If you are using a diaphragm mechanical pump to supply fuel to the carburetor, it is worth while to increase the fuel line I.D. If the carburetor goes lean while the nitrous is on, the pistons can melt even with a rich nitrous fuel jetting. The large fuel line trick (1/2" dia.) only makes a major improvement in the operation of diaphragm mechanical fuel pumps. It is a waste of time on most electric applications. An electric pump pushing a mechanical pump is not recommended and does not do well at high engine RPM. A large size line is effective with a mechanical pump, even if you use smaller fittings at the tank, fuel pump and carburetor. The advantage of the 1/2" large line is not related to the steady state flow rate of the line. The advantage relates to the acceleration time and displacement of the pulsating flow common to the mechanical pump.

High compression ratios can be used with nitrous but shifting the maximum pressure after top dead center becomes more and more difficult. I prefer to use street compression ratios and then just work with adding more nitrous to get desired horsepower levels.

We are currently testing some pistons specifically designed for Nitrous use. Current "off the shelf" pistons have been successfully run with a 500 HP nitrous kit combined with a Dr. Jacob's nitrous control system. Most of our effort has been to develop new ideas that will push the limit of nitrous technology. More testing is planned with a piston especially plated to reduce detonation.

A beginner would do well to build a reliable high performance engine first, then advance to nitrous, turbo or supercharging. This makes for more fun, more education with less head ache and money spent. The book titled "Nitrous Oxide Injection" by David Vizard, published by S-A Design is stocked in any good speed shop and should be required reading by anyone wanting to run nitrous successfully.

Good luck!

John Erb
Chief Engineer,
KB Pistons