Nitrous oxide injection is an easy and highly cost effective performance modification, yet it is also one of the most misunderstood. And these misunderstandings usually result  in very expensive engine damage. Vendors selling nitrous systems tend to gloss over the details and some of the systems lack basic safety features. 

How Nitrous Oxide Increases Performance   

Nitrous oxide (N2O) is a colorless and almost odorless gas. Joseph Priestly, an English scientist and clergyman, discovered nitrous oxide in 1793.  Following Priestley's discovery, Humphrey Davy of the Pneumatic Institute in Bristol, England, experimented with nitrous oxide and noted its anesthetic effects. Today nitrous oxide is widely used as a dental anesthetic, an engine performance enhancer, and for processing in the semiconductor industry.

Only limited information is available on nitrous oxide relative to automotive performance. Recommended books available on Amazon.com include: Nitrous Oxide Injection by Dave Vizard (1987), Nitrous Oxide Injection Guide by Joe Pettitt (1998), Supercharging, Turbocharging and Nitrous Oxide Performance by Earl Davis (2002), and The Nitrous Oxide High Performance Manual by Trevor Langfield (2006). The early book by Dave Vizard, who is a knowledgeable automotive engineer, includes extensive background information on nitrous oxide chemistry but is somewhat out-of-date as far as applications.  

Nitrous oxide was first used as an engine performance enhancer on World War II aircraft. The original NACA (predecessor of NASA) wartime report, Nitrous Oxide Supercharging of an Aircraft-Engine Cylinder, is interesting reading but not directly applicable to modern practice. Tests were done on a single cylinder from a V12 Allison aircraft engine. Nitrous oxide was injected as a gas in all tests, not in the liquid form commonly used today.    

One of the major misconceptions about nitrous oxide relates to whether it is an oxidizer or fuel. While nitrous oxide is an oxidizer from a chemical reaction standpoint, it can also be used as a monopropellant fuel. An recent paper on this subject, Nitrous Oxide Catalytic Decomposition for Space Applications, discusses the use of nitrous oxide in satellite propulsion systems. Energy release from thermal decomposition of nitrous oxide is used for a satellite thruster. The paper discusses problems encountered due to temperatures in excess of 2700°F generated during thermal decomposition. This sheds some light on what happens to the pistons and spark plugs in an automotive engine when a nitrous system runs lean (not enough fuel).   

In an automotive engine, thermal decomposition of nitrous oxide at combustion temperatures releases significant energy and creates free oxygen. The free oxygen can then react with supplemental fuel and release further energy. Injecting the nitrous is generally the easy part - the practical difficulties lie with matching the correct amount of supplemental fuel and controlling the system in a safe manner. 

Dave Vizard worked out the overall chemical reactions and optimum (stoichiometric) ratio for combustion of nitrous oxide and gasoline. While gasoline actually consists of many hydrocarbon fractions and additives, equivalent reactions of iso-octane provide a close approximation for practical purposes. Iso-octane is a single component fuel often used for lab tests. The overall combustion reaction of iso-octane and nitrous oxide is:

C₈H₁₈ + 25N₂0 -> 8CO₂ + 9H₂O + 25N₂

From this reaction, one can calculate an ideal ratio (by weight) of 9.649:1 nitrous oxide to iso-octane. In practice, a considerably richer 6:1 nitrous to fuel ratio is commonly used to prevent detonation. 

The thermochemistry of the nitrous oxide and iso-octane reaction can be better visualized by breaking down Vizard's original reaction into two stages: the thermal decomposition of the nitrous oxide and the combustion of the iso-octane. On a gram mole basis:

 25N₂0 -> 25N₂ +12.5O₂ + 2040 kJ heat (from thermal decomposition)

 C₈H₁₈ + 12.5O₂ -> 8CO₂ + 9H₂O + 5112 kJ heat (from combustion)

The second reaction is the same as if the iso-octane was combusted using oxygen from air. As we can see, for a given amount of fuel (iso-octane), using nitrous oxide as a source of oxygen yields 40% more energy due to the additional energy from thermal decomposition.

Another common misconception is that nitrous oxide displaces the air in the intake system. All modern applications inject liquid nitrous oxide into the intake manifold. The latent heat of vaporization of the nitrous oxide significantly cools the intake air. Our analysis shows that for a wide range of nitrous oxide injection flow rates, the cooling effect is such that the mass airflow is not significantly reduced. Tests were conducted on a vehicle with a mass airflow sensor. Nitrous oxide was injected into the air stream after the mass airflow sensor and the sensor monitored with an OTC scan tool. The tests showed that over a range of nitrous oxide flow rates, the mass airflow through the sensor did not appreciably drop. The impact of the cooling effect is significant. 

Potential power gains can be estimated from the amount of nitrous oxide that is injected.  Modern dyno tests suggest horsepower gains in the range of 20 horsepower per lb/min of nitrous oxide flow. This means that a 10 lb bottle could theoretically provide a 100 HP boost for approximately 120 seconds. Since the bottle cannot be emptied completely and some of the nitrous oxide is usually used up during purging, the practical value is probably nearer to 100 HP boost for 90 seconds.  

Nitrous System Jet Calculator

A nitrous system jet calculator can help you estimate jet sizes for your system. You can use the calculated values as a starting point for tuning your system. 

All of the other nitrous jet calculators we could find on the web appear to be based on old formulas originally published by Craig Walker and share a common problem - nitrous jet size is not a function of nitrous pressure. If you change the nitrous pressure, it affects the calculated result for the fuel jet size but not the nitrous jet size. Obviously this is incorrect. 

We based our calculator on actual nitrous and fuel flow tables originally determined by Mike Wood of Nitrous Express and published in the Supercharging, Turbocharging and Nitrous Oxide Performance book by Earl Davis.  

The calculator requires five input values. The nitrous horsepower target is the increase in engine horsepower that can be expected when the nitrous system is activated. Note that some vendors, such as Nitrous Express sell kits with ratings based on rear wheel horsepower. To calculate jet sizes based on rear wheel horsepower, enter an engine horsepower value that is 15-20% higher.    

The nitrous to fuel ratio for most commercial kits is set in the 5.5-6.0 range (very rich) to provide a generous safety factor. A higher nitrous to fuel ratio in the range of 7.0-8.0 will generate more power. Before you recalculate the fuel jet size using a higher ratio, you should check your air/fuel ratio (AFR) using a wide-band sensor system. If your measured AFR values are below 11.5-12.0 when the system is activated, you can try using a smaller fuel jet. Go down one jet size at a time until your AFR values reach 11.5-12.0. 

Nitrous Bottle Pressure and Temperature Calculators

Nitrous system performance is dependent on bottle pressure. Below 97.5°F, pressurized nitrous oxide exists as both liquid and gas phases in equilibrium. As the bottle is discharged, a small amount of the liquid phase will vaporize and the resulting gas phase will occupy the free volume. The pressure will be entirely dependent on temperature. If the bottle is maintained at a constant temperature, the pressure will also remain constant as long as there is some liquid nitrous oxide. If the bottle is rapidly discharged, such as may occur in a high horsepower application, vaporization of liquid nitrous oxide to fill the free volume will cause the temperature and pressure to drop. 

There is a critical temperature and pressure, referred to as the critical point, above which a single substance cannot exist as a liquid or gas. Above this point, the substance becomes what is referred to as a supercritical fluid, with properties in between those of a liquid and gas. The critical point for nitrous oxide is 97.5°F and 1,051 PSI. Supercritical fluids are compressible similar to gases. Above 97.5°F, nitrous bottle pressure will depend both on temperature and density. At any given temperature above 97.5°F, the pressure will drop as nitrous oxide is discharged and the density decreases. Operating your system with the bottle above the critical point temperature can lead to erratic and unexpected results. 

You can use our nitrous bottle pressure and temperature calculators to calculate the pressure and temperature relationships for any values below the critical point. Our calculators are based on data published in the National Institute of Standards and Technology Database 69, June 2005 release. Most vendors list a recommended pressure range for their systems.  

Why do I need a nitrous controller?

The following is taken from a recent post on the My350Z.com forum. We couldn't have said it better ourselves:

Well last night I was getting on my car, I’m only spraying a 50 shot because I haven’t had time to get the 100 shot pills....... but anyways I hit the rev limiter because I don’t have a window switch and wasn’t really paying attention. Now the car sounds like it’s misfiring and when it decelerates it sounds real bubbly. I’m thinking I might have bent a valve or something.

What are the applications for a progressive nitrous controller?

A progressive controller pulse width modulates the solenoid valves and allows you to control the flow rate. The two primary applications for a progressive controller are matching nitrous system power output to vehicle requirements and independently controlling fuel and nitrous oxide flows for optimum air/fuel ratio (AFR). The Daytona Sensors NC-2 can be used for both applications.

If you have a high horsepower nitrous system in a vehicle with limited traction, you can use time or RPM based progressive control to reduce the power output in the mid-RPM range or off the starting line to eliminate problems with wheel spin. You can also use RPM based progressive control to reduce excessive strain on the engine in the mid-RPM range.

By independently modulating the fuel and nitrous solenoids, you can control the AFR. This is especially useful for late model fuel injected vehicles.    

 

Do I need a bottle heater and what is the effect of bottle temperature on performance?

Most nitrous systems are designed to operate at 900-1000 PSI. This corresponds to 85-93°F. In the winter months (or even summer if you live in a cold climate area), you will need some means of heating the bottle. Electric bottle heaters are readily available for this purpose. Depending on the manufacturer, the bottle heater is controlled by a thermostat or pressure switch. Each approach has pros and cons, but a pressure switch gives the most accurate control. Some designs use a pressure switch located downstream of the bottle valve, as this is a convenient attachment point. There is at least one documented case of a bottle explosion that occurred when this type of heater was inadvertently operated with the valve closed. A safer design approach is to attach the pressure switch to a port on the bottle valve that sees bottle pressure even when the valve is closed. 

It is fairly obvious that a low bottle temperature will result in reduced nitrous oxide flow and a drop in performance. But what happens in the hot summer months? As the bottle temperature increases, the pressure and resulting nitrous oxide mass flow rate will also increase until the temperature reaches the critical point at 97.5°F. Liquid flow through a metering jet is proportional to the square root of pressure. At the critical point, the pressure will be exactly 1,051 PSI. For a system that was designed to operate at a nominal 900 PSI, this corresponds to an increase in nitrous flow of about 8%. If the system was jetted rich, with enough extra fuel to provide some safety margin, this should not pose a problem.

Above 97.5°F, the nitrous oxide exists as a supercritical fluid. As the bottle temperature increases further, so does the pressure. But the fluid density also decreases rapidly. There are additional effects that must be considered when the supercritical nitrous oxide flows through pressure drops in the system, expands, cools, and reverts back into a bubbly mixture of liquid and gas. From tests we have conducted, it appears that as the bottle temperature increases above 97.5°F, the nitrous oxide mass flow rate drops. 

A further power loss occurs because the charge cooling effect diminishes. When liquid nitrous oxide (below  97.5°F) is injected, the charge cooling effect is the result of the both the latent heat of vaporization (heat energy absorbed when a liquid is vaporized) and the expansion of the resulting gas to atmospheric pressure. Supercritical nitrous oxide (above 97.5°F) has zero latent heat of vaporization, so the only cooling effect is from expansion of the fluid.     

Results from two dyno tests on a 2006 Nissan 350Z with a 100 HP nitrous system are shown below. The first test was conducted with the bottle at 88°F. The second test was conducted with the bottle at 106°F and resulted in a drop of 20 HP. A normally aspirated baseline run for this vehicle is about 240 HP. 

Dyno Test with Bottle at 88°F

Dyno Test with Bottle at 106°F

The conclusion is that bottle temperature must be maintained in an optimum range as recommended by the system manufacturer. In areas with hot summers, you might benefit from a combination bottle heater/cooler, such as the NX Fire & Ice Pressure Controller.

 

What happens inside the nitrous oxide bottle at the critical temperature?

When nitrous oxide transitions to a supercritical fluid at 97.5°F, distinct liquid and gas phases no longer exist. All of the nitrous oxide in the bottle becomes a homogeneous fluid phase. Above the critical temperature, orientation of the siphon tube is irrelevant. However, what happens as the nitrous oxide transitions to a supercritical fluid is very relevant. Click on the following link to the University of Leeds Supercritical Fluids site for pictures of carbon dioxide undergoing phase transition. Nitrous oxide will behave in a similar manner. Near the critical temperature, the liquid meniscus starts to disappear and the liquid and gas phases become less distinct.

If you are operating your nitrous system with the bottle just below the critical temperature, the siphon tube will not consistently draw liquid and you can expect surging as the nitrous oxide mass flow rate varies.

The conclusion is that the system should never be operated just below the critical temperature. Since temperature of the nitrous oxide within the bottle is difficult to measure, pressure measurements will give more accurate results. If you keep the bottle pressure at or below 1000 PSI, you should be safe.

If the bottle gets above 97.5°F in the hot summer months, you can still run your system. You will be feeding supercritical nitrous and the mass flow rate will probably drop. Unless you re-jet the system, performance will decrease and the engine will tend to run rich.