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 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
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.
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 WALBRO 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.
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
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
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
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.
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
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
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 vapors. 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
vapor 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 vapor 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
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 The Fast and The Furious 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 micro switch, 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
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 micro switch.
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
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.
Safety Blow off Valve.
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.
Safety Blow-off valve.
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. Safety Blow-off valve.
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 problem for these things. If you have specific
questions and think you can handle it, ask me.
Anything bigger… you shouldn’t need my help.
For any basic kit... add $500 for *required* accessories.
Planning on a set-up? Run it by me, either here, or at firstname.lastname@example.org,
and I’ll make sure you do it safely. I’ll need to know: Budget, horsepower goal,
car, and modifications. Give me the part number of a kit you're interested in,
and I'll give you the part numbers of everything else you will need to build a
safe system, from the same manufacturer.