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Jun 26, 2023

Demystifying nitrous oxide, Part II

In the previous installment of this series, we discussed the function of

In the previous installment of this series, we discussed the function of nitrous oxide on a theoretical level. Now it's time to take a look at the hardware that is required to successfully employ nitrous as a power adder.

Most nitrous users will start with a kit, the thoroughness of which varies with manufacturer and the intended application. Let's take a look at a direct-port kit for a 4-cylinder application from Nitrous Oxide Systems that we happened to have laying around the garage.

The nitrous is stored in liquid form at a relatively high pressure (around 1000 PSI, depending on temperature) in a tank, which is more commonly referred to as a "bottle". It's constructed of high-quality aluminum, and also has a high-flow valve assembly to shut off the flow of nitrous when not in use, as well as a blow-off valve to allow the nitrous to escape if an overpressure situation occurs.

It's important to mount the bottle securely in the vehicle and to treat it with extreme care when outside the vehicle, as damage to it or the valve could cause an explosive release of pressure.

Insulating blankets or heaters are often used to keep the bottle at a consistant temperature, which in turn provides a predictable pressure.

The high pressure of nitrous requires robust lines and fittings. Teflon hoses, protected in braided stainless-steel sheathing, are the norm. A separate set of lines is used to bring fuel into the system.

Most nitrous systems will use "AN" fittings (designed by the US Army and Navy, hence the abbreviated name) to join hoses to other components. Compression fittings and tapered pipe thread are also commonly encountered. Information on the sizing of AN and pipe fittings can be found here; most basic nitrous systems will employ AN-4 and 1/8" NPT fittings.

Hoses can be purchased in pre-selected lengths with the proper fittings assembled on the ends, or the materials can be obtained in the bulk form and custom fabricated using normal hand tools and some patience.

Solenoid valves control the operation of the nitrous system. When 12V is applied to the electromagnet coils, the valves open, which allows fuel and nitrous to flow into the engine's induction path.

In this case, the solenoids for the nitrous are identified with a blue label, while those for the fuel have a red label. It's important to use the appropriate solenoid, as the valve seats are different. When running alcohol (methanol racing fuel, not E85 pump fuel), most manufacturers will require the use of a special corrosion-resistant fuel solenoid.

To a large extent, the quality and flow characteristics of the solenoids will determine the pricing of a kit. Lower-end kits will use smaller solenoids which are often adapted from fountain soft-drink dispensers, while higher-end kits use custom assemblies that are more robust and capable of higher flow rates.

The fuel solenoids shown above have a fuel filter installed to protect the valve seat from contamination. A filter is also sometimes employed on the nitrous solenoids. The likelihood of getting contamination on the nitrous side is minimal, but the consequences of having a piece of debris holding open the valve would be quite severe.

There are several ways in which to inject the nitrous and fuel into the intake tract; the nozzles shown here are among the most versatile and economical, requiring only a hole that is tapped with 1/8" NPT threads and approximately 3" of clearance in the mounting area. Plates that go under a carb or throttle body are also commonly employed, and some applications use custom throttle bodies or mass airflow sensor housings that contain the necessary flow paths. Direct-port systems place a nozzle, such as these, in the intake manifold port. This allows for more even fuel distribution, but complicates the install.

The amount of additional power provided by the nitrous system is determined by one or more jets, shown above. These have a small and accurately-machined orifice that will pass a particular mass flow rate at a given pressure. With the nitrous jet selected to provide the desired amount of additional horsepower, a fuel jet is chosen to supply sufficient enrichment to maintain a survivable air-fuel ratio (typically in the range of 12:1 or thereabouts).

Since the jet orifices are so small (the ones above are 0.020" to 0.030"), it's extremely important to keep them clean, which means it's probably not a good idea to set them on a dirty tractor fender.

A basis kit will use a momentary switch to activate the nitrous system upon the driver's comment. A well-protected "master arming" switch is highly advised, and the use of a keyed switch is recommended if the vehicle is operated by other drivers. Relays are used to provide high current to the solenoids, which allows the use of lighter-duty (and consequently smaller) switches, as well as somewhat simplifying the wiring.

Since nitrous should only be used at wide open throttle (WOT), a switch (shown to the left in the above shot) is usually mounted to the throttle linkage and closes only when the throttle is fully applied. Some modern systems replace this switch with a piece of electronics that monitors the signal from the throttle position sensor; indeed, on a vehicle with electronic throttle control (throttle-by-wire), such a system is the only practical solution. For vehicles with manual transmissions, it's also advisable to use a clutch-activated switch to shut off the system during shifts, especially for those that like to powershift.

Not shown is a fuel pressure cutoff switch, which is a normally-open switch that closes only when sufficient fuel pressure is present. This will help protect the engine from fuel starvation if a pump fails or a filter gets clogged.

There are other precautions that are wise to take into consideration when setting up a nitrous system. For example, the use of nitrous at low engine speeds (roughly defined as lower than halfway to redline) is not recommended, as excessive cylinder pressures and damaging backfires can occur. Additionally, it's also usually desirable to turn off the nitrous system before the shift point, or before a rev limiter is encountered (especially if said limiter is of the fuel-cutoff variety). To accomplish this, an RPM activated switch is used to control the on and off points (a device incorporating both a low and high cutoff is referred to as a "window switch").

It's also desirable to retard the timing, due to the increased cylinder pressures. This can be done via an adjustment to the PCM (or distributor, if you're old school), but a better method is to use a retard box that only pulls out timing when the nitrous system is activated. Do not depend on a vehicle's knock sensor to pull timing during the use of nitrous, for it cannot pull enough timing or do it quickly enough to save the engine if detonation occurs.

Finally, if your vehicle is equipped with the type of rev limiter that shuts off fuel at redline, it's a great idea to eliminate that system and go with an ignition-cut limiter. While it's a bad idea to hit any sort of limiter while spraying nitrous, it's especially bad to cut off part of the fuel flow (which is exactly what happens when the PCM intervenes during the operation of a wet-type system).

The above features can be incorporated via discrete modules, or an all-in-one ignition box - such as the Mallory HyFire VI shown above - can be used. Some vehicles will also benefit from the hotter spark that is provided by an aftermarket box, although modern ignition systems are indeed difficult to improve upon.

The combination of a high-quality kit, modern electronics, and the right engine and vehicle modifications provides enthusiasts with the ability to implement a nitrous system in a safe and effective manner. Obviously, the optimal set-up will vary from one vehicle to another, so we suggest doing some serious research before handing over your hard-earned money.