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How It Works - Nitrous Oxide Systems

February 2014

Stephen Kim Jan 24, 2014
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Nitrous oxide needs no introduction. Despite the hoards of racers jumping on the forced induction bandwagon, nitrous remains one of the most devastatingly effective and incredibly affordable performance modifications in existence. For a measly $600, a basic nitrous oxide kit can add 150 hp to a typical small-block V-8. That's enough to push a 12-second street car deep into the 11s and potentially into the 10s.

Dollar per dollar, no other performance modification comes close. With the potential for these tremendous performance gains, however, come certain risks. Although nitrous kits are completely safe if installed correctly, the greedy and uninformed racer can get himself into a lot of trouble in a hurry as well. To make sure that doesn't happen, we recently had a chat with Kyle Smith of Nitrous Oxide Systems.

Like Coke and Xerox, the company has been around for so long that "NOS" has evolved into a generic name racers often use to describe all nitrous systems. In addition to offering tips to stay out of trouble, NOS divulged heaps of helpful information for maximizing nitrous performance including what types of fuel to run; whether a plate system or fogger is right for your application, selecting the right pistons and rods for heavy-duty nitrous use, and how to manage power coming out of the hole at the track.

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Nitrous Basics

Every hot rodder knows that nitrous oxide injection kits add silly heaps of horsepower for not a lot of money, but why is it so effective? A nitrous oxide molecule contains two parts nitrogen, and one part oxygen. "Nitrous oxide by itself is non-flammable, but when combined with gasoline, its oxygen content accelerates the combustion process and increases horsepower," Kyle explains. Furthermore, the nitrogen content acts as a buffer to dampen and control the combustion of oxygen. Nitrous oxide does not break down into nitrogen and oxygen until it reaches 572 degrees Fahrenheit. This means that it absorbs a tremendous amount of heat from the incoming air charge, reducing inlet air temperature by 60-75 degrees, further increasing power.

Serious Plates

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It wasn't too long ago when plate nitrous systems were limited to 300 hp. Thanks to steady improvements in nitrous technology; plate systems can now move 500 hp worth of nitrous to a hungry engine. "The increased horsepower capability of plate nitrous systems in recent years stems from plates being built with multiple sets of spray bars or diffusers combined with larger orifice solenoids. The NOS Big Shot system employs orifices that are strategically placed in the plate for improved atomization and efficient nitrous/fuel mixture distribution," says Kyle. Nitrous Oxide Systems' Double Cross system ups the ante even more with a plate design that incorporates spray bars intersecting at a 90-degree angle. This creates a highly efficient distribution of fuel and nitrous, which is particularly advantageous in engines with intake manifold that have long runners or divided plenums.

Plates vs. Foggers

As its name implies, a plate nitrous system utilizes a plate that bolts between the intake manifold and carburetor to deliver nitrous oxide and fuel into an engine. A direct-port nitrous system, on the other hand, relies on injector nozzles plumbed directly into each individual intake runner. Plate systems are much easier to install, but in the past, were limited to 250-300 hp of nitrous. In contrast, while direct port systems can handle in excess of 600 horsepower of nitrous, they require drilling and tapping the intake manifold for fogger nozzles, and bending up hard lines for both the fuel and nitrous side of the system. Considering the latest crop of plate systems are now capable of adding 500 hp, what are the advantages of a direct-port system? "No matter how big a plate system is, it is never as good a direct-port system," Kyle explains. "A plate system lacks the cylinder to cylinder tuning ability of a direct-port system, and does not distribute the nitrous/fuel mixture as evenly. Likewise, direct-port nozzles put the mixture directly in the path of the intake valve for quicker response on the bottle. This is very important because uneven distribution can load the crankshaft and accelerate engine wear."

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Wet or Dry?

In both a plate and direct-port nitrous system, a mixture of nitrous oxide and fuel is injected into incoming air intake charge. As such, they're referred to as wet kits. Conversely, dry nitrous systems inject nitrous oxide only, relying on the factory fuel injectors to provide the additional fuel flow. This is accomplished by applying additional pressure to the factory fuel pressure regulator using the nitrous solenoid, which in turn increases fuel flow. Alternately, the EFI system's computer can be re-tuned to increase fuel flow as well. Since dry systems rely on the factory fuel injectors, whereas wet systems supply their own fuel through a dedicated fuel solenoid, the volume of nitrous they can handle are limited by injector size. However, dry systems offer several advantages. Considering that EFI intake manifolds are not designed to distribute fuel, dry systems are said to distribute nitrous more evenly to the cylinders than a plate system. Dry nitrous systems offer tuning advantages as well. "The biggest advantage of a dry system is that the operator has ultimate control of the fuel delivery at nearly any point in the run. This allows you to lean out or richen up the nitrous/fuel mixture point during a pass at the dragstrip, depending on track conditions," says Kyle.

Nitrous Fuel

In recent years, Q16 has become very popular for naturally aspirated and forced induction race engines. According to NOS, however, that doesn't always make it a good choice for heavy-duty nitrous applications. "Octane is not the only consideration when choosing a race fuel, but also the formulation of the fuel is very important as well. For a nitrous application, you want to use a higher octane fuel that has a slower burn rate," says Kyle. "Oxygenated fuels, such as Q16, are generally not suggested for nitrous motors, except possibly for jet-limited classes. Nearly every fuel company these days has a fuel designed specifically for nitrous, and that is the type of fuel that should be used if possible."

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Auxiliary Fuel Cells

Many street/strip nitrous applications utilize a separate fuel cell that feeds the nitrous system. In this arrangement, the engine runs off pump gas, while the nitrous system runs off race gas. In addition to a bump in octane, this arrangement provides a secondary fuel system that dramatically increases fuel volume. Some hot rodders feel this type of setup is completely safe while others think it's risky, so who's right? "While mixing different types of fuel in the combustion chamber is not ideal, it has become common," says Kyle. "That said; it's important to keep in mind that the pump gas is going to dilute the race gas and reduce overall the octane rating of the mixture. As long as you keep this in mind with the tune-up you shouldn't have any issues."

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Although mild doses of nitrous oxide are perfectly safe to use in a stock engine, NOS recommends loosening up internal clearances for race engines built specifically for large volumes of nitrous. "While there are no hard and fast rules about what needs to be done, high levels of nitrous require certain concessions to be made," says Kyle." In nearly all cases where nitrous amounts are more than entry-level, we recommend wider ring gaps and increased quench clearance. How much those clearances need to be changed is directly related to the amount of nitrous you intend to spray and something the engine builder will have to take into consideration when they are assembling the engine."

Pistons and Rods

In a typical V-8 application, upgrading to a forged rotating assembly is very good idea when nitrous levels exceed 150 hp. Additionally, since the pistons are literally on the front lines of combustion, it must go beyond the specifications of a standard forging when nitrous levels exceed 300 hp. "There really is no such thing as a ‘nitrous piston' per say, but the real question is whether or not the piston is durable enough for the power level you are about to subject it to. Along that vein, you could say a nitrous piston will likely be a little heavier, have the top ring moved down a little, and have more spacing between the ring lands if possible," Kyle explains. He also points out that the proper selection of ring material will be just as important as piston design. As far as connecting rod selection is concerned, some engine builders claim that aluminum connecting rods in a nitrous motor help absorb the shock on the rotating assembly when the nitrous system activates, thus improving engine durability. While the virtues of aluminum rods in a street engine are debatable, they're very common in race engines that run nitrous. "I agree with that premise and once heard a engine builder say, ‘Would you rather hammer on your crank and block with a steel hammer, or an aluminum hammer?'"

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Compression Ratio

In an all-out naturally aspirated race engine running race fuel, compression ratios of 15:1 or slightly higher are common for maximum performance. Although nitrous oxide works well in high-compression applications, NOS recommends dialing back the compression just a bit to provide a margin of safety. "In a max effort race motor, most engine builders will shoot for a compression ratio of between 13- and 14:1. Higher compression motors can get a lot more timing sensitive when the nitrous power levels get really high, so having slightly lower compression increases the tuning window," says Kyle.

Timing Advance

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Since nitrous oxide injection increases the rate of combustion, it's often necessary to retard ignition timing to prevent detonation. However, determining how much the ignition timing should be reduced is based on multiple factors, including compression ratio, nitrous volume, spark plug heat range, fuel type and compression ratio. "There is no simple formula for this based on the amount of nitrous you are spraying. The ideal timing for a given combination is dependent on many things," says Kyle. "These days, the cylinder head used and combustion chamber efficiency is one of the largest factors in that equation, which makes it impossible to come up with a one-size-fits-all formula. This is an area that most any inexperienced nitrous user is going to need help. That said, the timing chart is included with every new NOS brand systems and that is also available on our website has a very good timing chart that explains recommended timing requirements, and how you go about determining what is best for your combination."

Spark Plugs

A common mistake for nitrous newbies is using the wrong type of spark plug. The heat range of a plug refers to the temperature of the ceramic material surrounding the electrode. Since the spark plugs must maintain a certain temperature to stay clean, using plugs with the wrong heat range can result in plug overheating or fouling. Additionally, many late-model engines run leaner air/fuel ratios than older muscle cars, which require larger plug gaps and a projected plug nose to help ignite the air/fuel mixture. This can cause misfires in a nitrous application, as the ground strap is too long to dissipate the heat produced by the nitrous oxide. "With a projected nose plug, the ground strap can act as a glow plug, retaining too much heat from cycle to cycle, which sets the conditions for pre-ignition. The solution is using a non-projected-nose plug with a shorter ground strap," Kyle explains. "The plug gap will also need to be reduced as the amount of nitrous is increased, allowing the ignition system to fire the plug against the increased cylinder pressure. The ideal heat range of a plug depends on the application and amount of nitrous used. The more nitrous an engine sees, the colder plug it needs."

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Cam Specs

Some racers argue that nitrous makes so much power on its own that cam specs aren't very important in a nitrous engine. On the flip side, optimizing the valve events for heavy doses of nitrous beyond 300 hp ensures squeezing as much power out of the nitrous pills as possible. "Nitrous allows you to burn more fuel, and that in turn makes more heat and power. If you make more heat, you must get that heat and exhaust out of the chamber," Kyle explains. That usually requires opening the exhaust valve earlier and holding it open longer with a nitrous motor than in a naturally aspirated configuration. Exactly how much the cam specs need to be changed is directly proportional to the volume of nitrous an engine uses. Many nitrous cams have wider lobe-separation angles than naturally aspirated engines, but the exhaust valve opening and closing points are more important than LSA figures.

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Bottle Pressure

Nitrous oxide is a gas at room temperature, which means it needs to be stored inside the bottle at extremely high pressure to keep it in a liquid state. Bottle pressure is directly related to bottle temperature, so cool ambient temperatures will compromise performance. "Maintaining a constant pressure is one of the most important aspects of tuning a nitrous system for proper performance, and 950 psi is considered the ideal operating range. Since bottle pressure is what controls the amount of nitrous the engine is receiving, inconsistent bottle pressure can result in inconsistent performance," Kyle explains. "If the pressure falls off too fast during a run, the system will go rich, since the nitrous system will continue enriching the air/fuel mixture even though the flow of nitrous has been reduced. This causes a car to slow, or in the worse case scenario, damages parts. Ambient temperature of 80-90 degrees Fahrenheit is ideal for maintaining consistent bottle pressure, so in conditions cooler than that, we recommend using an NOS electric bottle heater."

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Due to the heat required to convert nitrous oxide from a liquid to a gas, nitrous oxide dramatically cools the temperature of the incoming air intake charge by 75 degrees or more. Consequently, it didn't take fans of forced induction to recognize this benefit. Even in an application where a supercharger or a turbocharger functions as the primary power adder, injecting a very small dose of nitrous can substantially increase horsepower due to its intercooling effect. "In forced induction combinations where there isn't space for a traditional air-to-air or air-to-water intercooler, a small 50-shot of nitrous can be used in lieu of an intercooler with great results. It can also help mask an intercooler that has been maxed out beyond its cooling capacity," Kyle explains.

Power Management

Thanks to the dramatic increases in low- and mid-range torque that a nitrous system offers, one of the great benefits of nitrous in a street car is how hard it hits. However, this isn't always desirable in a drag car, especially in small-tire racing classes. Fortunately, there are many ways of managing nitrous flow to ensure that the tires stay hooked. One of the most effective methods is running a multi-stage nitrous system. By installing two sets of solenoids, a dual-stage system allows limiting nitrous flow off the line to enhance traction, and then increases nitrous flow once a car is farther down the track. So instead of spraying 400 hp worth of nitrous all at once, the first stage can be limited to 200 hp, with a second stage injecting an additional 200 hp later on in the pass. Furthermore, NOS offers several progressive controllers that electronically ramp up the flow of nitrous based on a user-defined period of time. They work by pulsing the solenoids on and off to increase or decrease nitrous flow. "For example, racers can set a progressive controller to flow 25 percent of the nitrous off the line, then ramp it up to 100 percent 2 seconds into a run once there's more traction available," Kyle explains.

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