Nitrous Oxide Injection on a Rotary Engine
By Sean Cathcart

Due to the high levels of misinformation I have seen being provided on about nitrous oxide, and its use on rotary engines, I thought it was about time to clear up a few things.

I have seen almost every question imaginable when it comes to nitrous on this forum. And really, the answer to all of them is: it depends. The use of nitrous is always dependant on goals. How much power are you looking to put out? How long do you want your engine to last? How much money do you have to spend? What kind of modifications already exist?

In this article, I will review the basics of nitrous, to building set-ups to work safely on ALL engines. 

Nitrous Basics:

Nitrous Oxide is a gas comprised of two nitrogen atoms bonded to a single oxygen atom, and has the chemical formula N20 thusly. It is common among the drag scene, with many common names tossed around for it: juice, gas, NOS, laughing gas, spray, and nitrous.
***Automotive grade Nitrous Oxide is identical to medical grade nitrous oxide (laughing gas), save one point: automotive grade Nitrous Oxide contains sulfur dioxide, about 100ppm.

 Unlike medical grade nitrous oxide, the automotive grade gas cannot be "huffed" for delusional purposes. The sulfur dioxide is added for the soul purpose of preventing abuse of the gas, and attempting use of auto-grade nitrous oxide will result in extreme sickness. The additive does not affect performance in an engine.

First off, it is important to understand the basics behind horsepower. Think of an engine as an air pump: The more air you can move in, and out, the more power you'll make. As well, the easier it is to move the air in and out, the more power you'll make. This makes sense, as removing exhaust and intake restrictions with higher flowing replacements increase engine power, as does adding more air to the engine via a turbocharger.
Nitrous oxide can be thought of as "liquid air".

When Nitrous Oxide is heated to around 574oF, it breaks apart into two nitrogen atoms and a single oxygen atom. This extra oxygen allows for more fuel to be added to the combustion chamber, and thus more power to be made. In our air pump analogy, it makes more power by adding more air, like a turbocharger does. Nitrous Oxide itself is not flammable. However, when heated, it breaks apart and separates its molecular bonds, and gives up its oxygen atom, allowing for gasoline combustion.

The spare nitrogen atoms acts as a buffer to the combustion process, slowing down the combustion reaction, and preventing detonation. Detonation damages engines. This buffering effect is the reason pure oxygen can not be added.
Nitrous Oxide is stored in bottles at approximately 950 psi as a liquid. When released from this pressure, it quickly drops temperature to approximately -127oF and comes out in a gaseous form. This cooling effect does the same thing as an intercooler: it allows for a more dense air charge, and thus more power.

For every 10 degree reduction in intake charge temperature, a 1% increase in power can be had. A nitrous oxide injection typically lowers air temperature by 60 to 75 degrees. Thus, on a 200 hp rotary, the mere change in intake temp level would net 14hp.

Nitrous oxide systems are available in two different forms: wet systems, and dry systems. Since nitrous is simply a method of adding more oxygen to an engine, more fuel must be added with the nitrous to prevent the engine from running lean and destroying itself, as well as to make the extra power. A wet system adds the extra fuel by mixing fuel with nitrous oxide gas through a "fogger" nozzle and spraying the mixture into the intake tract. A dry system adds the extra fuel via the injectors, by using a computer or a fuel pressure regulator increase supply.

Nitrous itself will not increase the wear on your engine. As with any increase in horsepower, however, they will come some extra wear and tear on your engine. This is increase in wear would be the same no matter where it comes from. In fact, one of the advantages of nitrous oxide is that you only use it when you want, and thus, only put strain on the engine when you feel it is necessary.
Typical nitrous kits come with 10 lb bottles. Nitrous is measured by mass, not by volume, and a 10 lb bottle holds 10 lbs of nitrous oxide. Other common bottle sizes are 2.5 lb, 15 lb, and 20 lb. The length of time a bottle will last depends on your set up. The formulae for calculating nitrous use is: 0.8 lbs of nitrous x 10 seconds = 100 horsepower.

The Button:

We'll get more to nitrous activation later. A system can theoretically be activated for as long as there is nitrous in the bottle. It is recommended, however, to never get on the system for more than 15 seconds at a time. I abuse that rule.
Nitrous should only be used at WOT. It is load-dependant in that rpm point usage depends on the amount of load on the engine. You could engage nitrous at 1500 rpm in first gear, but never below 3000 in fourth. It should never be used above redline, or until fuel cut occurs. Failure to follow these rules can result in nitrous backfire, where the nitrous/fuel mixture actually ignites and backfires through the intake manifold, commonly causing intake manifolds and related houses and systems to explode. This can be very dangerous, and cause bodily harm.

The RX-7 and nitrous:
Wet Vs. Dry:
Which is better on an RX-7? The stock fuel injection system on RX-7's is being pushed pretty hard even at the stock ECU usage. Attempting to push more fuel through the stock injectors can prove fatal when the injectors simply cannot provide enough fuel for the engine. On the other hand, dry nitrous kits are much less likely to nitrous backfire.
So, in this case, the answer is dependant on user needs and existing modifications. For situations where 50 hp and lower tuning is being used, the stock injectors should provide adequate fuel for nitrous use. However, if your motor is severely ported, and extremely modified, you already may be approaching your injector delivery capacity. As well, a clogged fuel injector may just slightly lean out stock performance, but could absolutely destroy a motor running on nitrous.

Overall, I completely recommend wet kits. It offers easier tuning, does not rely on the already sketchy stock fuel injectors, and offers a wider amount of power. A dry system should really only be used on a minimally modified N/A RX-7 looking for no more than a 50 hp increase, and have verified a perfectly working fuel injection system, complete with recently professionally cleaned injectors. Turbocharged vehicles should never run a dry system.
Seeing as how most users have already modded their cars and are looking for 60-75 hp shot, it seems to make the most sense to concentrate on wet kits from this point on.

Kits:
All nitrous manufacturers offer "complete" kits for our cars. Never, in any circumstance, should these kits be considered "complete". They provide the minimal amount of items to get a system running, but hardly ever enough accessories to make a safe system.
A safe system:
First and foremost, a nitrous inducted engine needs fuel. Every RX-7 running nitrous oxide must first upgrade their pump. The Walboro drop-in pump is more than apt in this situation. In NA's, this can, however, present a problem:

The stock fuel pressure regulator is unable to compensate for the added flow of a higher volume pump at idle and low load situations. With this upgrade, the stock fuel pressure regulator cannot flow enough fuel out as is coming in, and the fuel rail pressure increases, causing the injectors to add more fuel than the ECU registers. Hence the car runs rich at idle and low load throttle. This can cause hesitation and a large lack of power on the bottom end of the cars, as well as compound a carbonizing problem already existing in the lower rpm range.

Turbo cars already have upgraded pumps, and although they run the same fuel pressure regulator, use a resistor pack to lower voltage to the pump at idle and at low rpm points.
In order for an N/A car to run properly with an upgraded pump, either the Turbo fuel pump resistor pack must be wired in, or an aftermarket, higher flowing fuel pressure regulator must be added. The latter is obviously better for overall tuning, but there is the added cost, not to mention the change in fuel system plumbing. Regardless of method chosen, this problem must be addressed.
 

Ignition:
The stock ignition on RX-7's is already exceptional compared to other stock cars. An upgraded ignition is obviously better, but not required for nitrous oxide use. However, the ignition system should be retarded under activation. For smaller shots, up to 75 horsepower, the stock Timing maps should be fine. Above that power level, timing becomes risky, and retard should be used to avoid detonation and harm to the motor.
The amount of detonation depends on your horsepower jetting, current set up, and various other factors, but as a well of thumb, the timing should be retarded 1 degree for every 25 horsepower of nitrous jetting. Thus, a 100 hp shot should be retarded 4 degrees, and a 200 hp shot retarded 8-10 degrees.

Methods to retard are questionable. You can retard the entire ignition at the crank angle sensor, but the power will suffer the entire time during non-activation.
A better idea is to use a retard box. MSD is known to make a few, and I believe even one set-up to work without a CDI box. They are wired in to retard the timing a preprogrammed amount set via retard "pills", and only retard when the nitrous system is activated. This is the best system, but again can add cost to a system build-up. A retard unit should be considered essential above 125 hp shots.

The Bottle:

The bottle should be maintained at a temperature around 70 degrees F. Normally, a bottle heater is required to keep the temperatures this high, however, in hot weather the temperature can actually increase too much, and will need to be cooled with either a wet rag or some ice. Too low a temperature and the system will run too rich and make little power, too high a temperature and the system will run too lean and detonate.
In the end, a nitrous pressure gauge should be used to monitor the system. The system should be kept between 900 and 1050 psi.

Any higher and you should cool the bottle, and lower and you should heat it. However, never at any point should flame be used to heat a bottle, nor have the bottle left unattended with a bottle heater on. A pressure gauge should be a required accessory to any system, but is rarely included in kits.
As well, a safety release valve should be employed with any kit. This safety valve should be plumbed to the outside of the car, and when the bottle pressure gets to high, the valve will dump all of the contents of the bottle. It prevents the bottle from exploding from excess pressure.

The bottle should be mounted with the valve facing towards the front of the car, and mounted at a 15 degree angle. The bottle should be mounted as far back as possible in the hatch, or in the trunk.
It is possible that the nitrous solenoids could leak, and while the car is shut off, fill the engine with nitrous. Upon start-up, this would cause the engine to run extremely lean and detonate. Thus, when not in use, nitrous oxide bottle valves should be kept in the closed position. Most kits include a manual knob to open the valve, and most companies offer a remote bottle valve that can be opened and closed electronically at the flip of a switch. At around $150, this accessory is not necessary, but a nice convenience, especially for those with trunks.

When the bottle is close to being emptied, a surging effect is felt. This is the time to bottle refilled. It can be tiring to be without nitrous, so many users keep several bottles either swap in a spare filled bottle, or get your single so that they always have nitrous available.

System Solenoids:

Each wet system has 2 solenoids which opens and closes to allow the nitrous and fuel to flow into the engine. These solenoids are typically rated at around 1000 psi operating range, but usually can withstand up to 1500 psi without worry.
Different flow rates are available for different needs in both fuel and nitrous solenoids, typically rated by horsepower values. Try to select a solenoid most suited to your range of horsepower shot. Most solenoids included in kits will flow enough to support up to 250 horsepower levels of tuning.

Usually not included in kits, but should be considered a necessary accessory, are filters. Nitrous filters should definitely be used to prevent the nitrous solenoid from sticking in the open position (which would allow nitrous to fill the engine after you turn the system off, running the system extremely lean and blowing the motor). Fuel filters are also a great idea, but not near as necessary.

Injection:

Typical wet system plumb into a "fogger", which is just a nozzle with two lines running into them, one a nitrous feed and one a fuel feed, whose purpose is to mix the two to form the nitrous oxide-fuel atomized "fog".
Most basic system use one fogger. This setup should be mounted 4-6 inches in front of the throttle body, with the fogger outlet nozzle pointing in the direction of airflow.
More advanced systems plumb a separate fogger into each intake manifold runner, and such is called "direct port". These systems allow for more even distribution of nitrous oxide per combustion chamber, but are usually only necessary when extremely large horsepower shots are being used. A direct port setup usually costs 50% more than a single fogger system, and involves a lot more plumbing and installation time.

Supply system:

All nitrous systems should use quality braided stainless steel hose to supply both the nitrous and the fuel to the system solenoids. Every kit includes the necessary lines, and every manufacturer sells these lines. These lines are typically rated to 2000 psi, and come in pre-determined lengths.
A purge valve is an excellent accessory to your supply system. After system use, the nitrous line is typically filled with nitrous oxide vapours. When the bottle is opened, this gas is compressed, but still exists at the front of the nitrous feed line. When the system is activated, this gas, which is nowhere near as concentrated as the nitrous liquid that is fogged in, causes the car to run momentarily rich, and make a small "bog" until the nitrous liquid reaches the fogger.
 A purge valve is a separate nitrous solenoid that allows the nitrous vapour in the feed line to be "purged" out of the system. This allows for more of a 'hit" feeling when the system is activated. The spare vapour is vented outside the engine, typically in front of the windshield. An activated purge valve is what is causing the "steam" to shoot over the windshield typically seen at the drag races.

Electronics:

As we all know, driving is nothing like the fast and the furious. Nitrous oxide will not just come on when you push a button and then hang on. Unless you use an on/off push button (forget what its called, push once on, push again and its off), the nitrous would only be on as long as the button is depressed. The method of just tapping the button in TFATF would only allow the nitrous to come on for those few milliseconds while the button was tapped. As well… the "too soon Junior" line makes no sense. As they are claiming them to be 10 second cars, hitting nitrous for 10 seconds at a time is fine, and thus, the person hitting the shot sooner would be in a better position.

Most systems actually don't use a button.
Every system should be wired into some form of WOT-only activation switch.*** Most kits include a microswitch, which is simply a button which completes a circuit whenever it is depressed, mounted to the throttle body. Push down on the gas pedal to WOT, and the circuit is complete. Let off the gas, and the system is deactivated. This makes it impossible to blow your engine by accident with a part-throttle activation, and turns off the nitrous oxide injection during shifts. As well, it is possible to use a switch that uses the TPS to activate the system, but knowing how finicky the TPS is already on our cars, I don't recommend this.

As well, a master switch should always be used, otherwise the system will activate any time you floor the car. This is usually just a toggle that is thrown to activate the system.
It is possible to wire in a button into the system, or as a replacement for the master switch, but it must be wired inline with a WOT microswitch.
Really, a toggle master switch "is" a button. Just throw the toggle in whatever gear you want to activate the nitrous system while driving, and as soon as you floor it, the system will become active.

A good accessory to add is a Hobbs switch. A Hobbs switch is a switch that opens only a set pressure. In our terms, you would use it to deactivate the nitrous system should the fuel pressure drop.
Another available accessory is the Nitrous Express (I think) air/fuel system. Should the air/fuel ratio become to lean or too rich under nitrous activation, this computer will deactivate the nitrous system. I have no experience with this system, and hope it would employ a better unit than the stock O2 sensor, but I doubt it. In this case, I imagine the damage would already be done by the time the computer deactivated the system. I'll look into it further.

Getting really hardcore… progressive computers. This was the *** from activation systems above. These allow you to program your nitrous settings, such as rate advance, and part throttle activation. Only should a system be activated at part-throttle if you have a progressive controller. They are pretty expensive, but allow for the ultimate in tuning. I'm in the process of getting one.

Building your system:

-50 hp and less: If your injectors are verified as perfect, go dry. I recommend Venom.
If not, use a single fogger mounted 4-6" in front of the Throttle plates. Add an upgraded pump, plumb your feed line out of the banjo bolt. Use a nitrous filter, and get a bottle heater and a nitrous pressure gauge. 4-AN nitrous feed line.

75 hp and less: same set-up as wet kit as listed above.

-125 hp and less: Wet only. Single fogger mounted 4-6" in front of the Throttle plates. Manifold porting, TB mod, upgraded pump, upgraded fuel pressure regulator. Nitrous filter, bottle heater, 4-AN nitrous feed line, plumb fuel from a y-splitter off of the secondary rail. Upgraded Clutch. Run a retard unit.

400 hp and less: Wet only, upgraded external fuel pump and filters, aftermarket fpr, y -splitter to feed nitrous solenoid. Nitrous and fuel filters, 6-AN feed line, purge valve. Hardened Stationary gears, oil bearing mods, higher rate oil pressure regulator. Direct port only, consider using a progressive computer. Large engine porting, all manifolds ported. Upgraded clutch, and probably transmission if your are hitting a "high" shot. Consider Staging shots. Bottle heater, and retard unit.

Nitrous on a turbo: don't do it if you need to ask. Even with a massively ported S5 wastegate, you'll still experience crazy boost creep. Sorry to sound rude, but blown motors are already a prob for these things. If you have specific questions and think you can handle it, ask me.

Anything bigger… you shouldn't need my help.

 

My system: (Aug '02)

I have built a very extensive system for nitrous oxide injection on my 1990 TII. My mods are as follows:
- 7000 miles on Street Ported Rebuild
- electric fan
- electric water pump
- sumped stock tank to 100- micron stainless steel filter to 1200 hp aeromotive fuel pump into 10-micron paper filter into braided feed lines into Y splitter into rails, Y splitters off of each rail to aeromotive fuel pressure regulator, with a base setting of 40 psi. Each Y-splitter off the fuel rails feeds one nitrous solenoid.
- Completely mirror polished and ported intake manifolds, mirror polished and Modded Throttle body, polished intake elbow, all emissions removed.
- no crank driven accessories save alternator

- MSD 7-AL3 CDI box with addition 2-stage retard on leading coil.
- intake, 3" downpipe and midpipe to horrible stock catback.
- Stock turbo with massive ported S5 wastegate.
- Boost gauge, fuel pressure gauge, nitrous pressure gauge

- 15 lb nitrous bottles with a remote bottle valve into a 6-AN feed line to a purge valve, purging excess vapour out through the stock intercooler, nitrous and fuel filters.
- First stage: Single fogger, currently jetted to 120 horsepower, in intake elbow.
- Second stage: Direct port, one fogger per intake runner, each jetted for 40 horspower.
- Bottle heater, separate circuits per stage wired to covered toggles mounted in front of the shifter (where the ash tray used to be).

- Combined stages: 280 horsepower. However, using the fuel jettings to compensate for added boost from the turbo creeping. I'm sure its running rich… Its time for an upgrade.

Planning on a set-up? Run it by me at shutup_billy@hotmail.com, and I'll make sure you do it safely. I'll need to know: Budget, horsepower goal, car, and modifications