- A chemical form of forced
- Provides instant torque.
- Push button operation
or full EMU control.
Another way to increase air flow is to use
nitrous injection. Atmospheric air contains 23.3% oxygen by weight. Nitrous
contains 36.4% oxygen – this is a 56% increase in oxygen content. No other form
of supercharging can provide the instant torque and low speed rush of power that
a nitrous system can give.
Care must be taken to properly prepare the engine for nitrous installation. It
is easier to overlook the needs of the engine because nitrous installation
appears to be easier than a blower or turbo installation. But the same care must
be taken to prep the engine for nitrous as is taken to prep the engine for other
forms of forced induction that provide similar levels of power.
There are three main areas that must be considered in order to guarantee a
reliable nitrous install… the correct nitrous to fuel ratio, equal distribution
of nitrous and fuel to every cylinder and the correct amount of ignition retard. Lastly, the pure mechanical considerations must be considered similar to any
Wet and Dry Nitrous Systems.
- Wet systems are most
popular… inexpensive and easy to install.
- Dry systems are complex and
- A wet system introduces the
nitrous into the petrol mix earlier than the dry.
- Wet systems don’t
require advance mapping techniques to safely operate.
Originally all nitrous systems were wet systems.
A fogger nozzle was used to inject fuel and nitrous mix into the inlet tract.
A dry system uses a nitrous metering nozzle to discharge nitrous only into the
inlet tract and relies on the EMU to meter the correct fuel for the additional
nitrous content. The EMU increases the duty cycle of the existing injectors or
uses additional injectors to supply the needed fuel. Some dry systems change the
fuel pressure in order to supply the required additional fuel. These systems
should be avoided because they are impossible to tune properly and will lead to
minimal power increase at best and will lead to eventual engine destruction at
The dry system is the best all round system available. However, it is expensive
and time consuming to install and often involves changing several parts of the
fuel delivery system such as the fuel injectors and the EMU. For more extreme
power additional injectors have to be added and very careful mapping must be
carried out. It is also time consuming to alter any of the settings after you
initially install the dry system. If you decide to go for another power upgrade
then the full system must be carefully remapped etc.
But with professional setup, the dry system can actually be much safer than a
wet system. Nitrous in itself doesn’t represent a problem, but when it is mixed
with petrol the situation changes. With a dry system, the nitrous and fuel are
mixed together at the last possible time and in a very controlled way and is
therefore, safer. The volume of potentially explosive vapour is much smaller,
reducing backfire to a subdued pop.
However, despite the above points, most people still opt for a wet system
because of the following: Wet systems are much cheaper, you can alter the power output in a simpler and
much quicker way without the need for advanced mapping knowledge. A simple
change of metering nozzles can do the trick. This is handy for anyone who wants
an easy way to lower the power for road use and up the power for drag or
Also, the dangers of a wet system over a dry one are minimal when a good install
is carried out. In the wet system the petrol and nos get mixed in the ducting
before the inlet tract so t he volume of explosive mix is higher, and with a
backfire, an explosion is a possibility, but is practice it rarely happens
(Note: when talking about danger, I normally refer to the danger to the engine
rather than the driver, there is rarely a real risk of injury to a person
because of the layers of bulkheads etc).
- Mount bottle upside down
for inverted systems.
- Otherwise, mount at
10-20deg making sure the pickup pipe is at lowest point.
- Orient the bottle front to
back… nozzle facing front.
The nitrous cylinders are usually made of
aluminium and contains between 5-15lbs of nitrous. To ensure that liquid rather
than gas comes from the bottle, a good bottle position must be used. If an
inverted bottle arrangement is used then the bottle should be mounted upside
down (this is rare). The normal situation is that the bottle is mounted 10-20deg
up from the horizontal position.
The orientation of the bottle is also important because, inside of the bottle, the pickup pipe goes from the outlet valve at the
top of the bottle to an offset position at the bottom of the bottle (off to one
side). The bottle should have a marking on it to indicate where the pick-up pipe
is located inside the bottle. Usually this means that the sticker should be face
up. The bottle should be oriented with the front of the bottle facing forward.
This ensures that nitrous in liquid form keeps flowing under heavy acceleration… ie nitrous surge doesn’t occur.
- Use braided steel line…
different to fuel and oil lines.
- Avoid heat sources and
surfaces vulnerable to abrasive damage.
Braided steel line should be used to carry the
nitrous from the bottle valve. This line is not the same as braided fuel or
oil lines. It is required to carry greater pressure and to operate at much
Make sure the line doesn’t rub against anything that will scrape or damage it
over time. Also note that the line itself will damage surrounding aluminium or
light gauge steel. The line should also be kept away from any heat source so
that the nitrous stays in liquid form until is erupts from the nozzle. If its
necessary to run close to a heat source (exhaust) then use a heat shield and
Nitrous Solenoid and Jet. - Solenoid must operate in
high pressure environment.
- Extreme loads can
reach 30amps. Incorporate into stock loom carefully.
The nitrous solenoid is used to start and stop
the nitrous flow. It operates differently from a fuel solenoid (on a wet
system). It has to operate at very high psi and at very low temps. It requires a
powerful coil to pull the valve open against 800psi. Additionally, the rubber
seals must resist chipping or perishing at freezing point.
Because of the solenoid current requirements, load may exceed 30amps in some
setups. Careful design must be carried out to avoid excessive load on the stock
electrical system. The original schematics need to be studied to ensure the
voltage drop doesn’t slow the fuel pump or rob the ignition system of its
potency… resulting in hard to trace misfires.
- Use top quality
fogger nozzles to atomise the fuel.
The most critical aspect of the nitrous setup
(apart from the mapping), is the discharge nozzle. The simplest EFI systems use
a plate which is sandwiched between the inlet manifold and the throttle body. In
wet systems, there are two thin brass tubes stretched across the plates. The
discharge holes in the nitrous tube cause a high pressure/high velocity nitrous
blast into the fuel flowing at 40 to 70 psi from orifices in the fuel tube.
breaks the fuel down into droplets of uniform and easily combustible size.
No matter how much fuel and air we can squeeze into the cylinders, we won’t get
the expected power return unless the mix is properly broken down and thoroughly
blended. This can be made difficult in a nitrous system because the mix is being
super chilled. This is desirable because it increases air density and cools the
chambers and valves. However, all that cooling makes it that bit more difficult
to vaporise the fuel. All this means that only the best fogger nozzles should be used.
- One nozzle per runner is best.
- Usually a single nozzle is
- Position the nozzle
300mm away from hot-wire air sensors.
The number of nozzles required is the next
consideration. Ideally we want a nozzle down each runner as close as possible to
the injector. This ensures that every cylinder receives an equal amount of
nitrous and fuel. With good balance and mix in each cylinder we can run as close
a possible to the ideal air/fuel mix and with an ignition advance giving max
power and economy. However, for more economical systems we might want to use
less nozzles than the ideal. Sometimes a single nozzle installation will give us
the required power without the expense of installing multiple nozzles.
The best position for the nozzle(s) will now depend on the inlet tract design.
In a turbo or blower engine, position the fogger at the intercooler outlet.
For NA engines, the position will depend on the length of the inlet duct and on
the type of air flow meter being used. Speed density and alphaN systems don’t
use an air flow meter, so the nozzle can be placed just down stream of he air
box. Vane type air flow meters aren't much affected by nitrous so the nozzle can
be placed in the flexible trunking immediately after the air vane. Hot wire and
hot film meters are affected by nitrous so the nozzle must be placed about a
foot away from the meter.
- Ensure there is enough fuel
supply for the nitrous install.
The most mistakes are made in setting up nitrous
systems because the fuel delivery needs are not properly addressed. Most engine
damage occurs because there isn’t enough fuel delivered to maintain the correct
air/fuel mix. Fuel lines can be too small, metering jets are too small, deformed
bends restrict flow, rubber lines get swollen internally fuel pumps aren't
adequate to maintain flow. Of course with a fuel injection system none of this should ever occur, but it
regularly does occur in high output nitrous systems.
- The simplest system uses a
- More practical systems are
the norm, from RPM cuts to full EMU control.
As mentioned there are two systems involved, the
nitrous and fuel delivery systems. Different control systems are used to
determine when the nitrous gets injected… all of them electronically controlled
to one degree or another. The simplest systems contain only an rpm activated on and off arrangement. The
nitrous kicks in at the low rpm point and shuts down at the high rpm limit, only
while the throttle is fully depressed.
In turbo cars, you can use a pressure switch in series with the low/high rpm
cuts to allow the nitrous to cut in when the car is off boost. The great
advantage of this system is that turbo lag is greatly reduced and the risk of
detonation is minimised because of the cooling affects of the nitrous and
because the nitrous is cut as the boost builds past a set point. Also, in lower
gears, the nitrous is cut to protect against an over rev.
Ever more sophisticated nitrous controllers are available right upto MoTeC and
Gems systems. Jacobs, NOS and Schnitz all make good standalone electronic
nitrous control. They can provide progressive nitrous control with gear/boost
etc input and highly controlled nitrous OP control. They can also me set for
traction and transmission friendly output.
Nitrous Fuel Ratio.
- Maintain a constant
- A full dyno tune is
required for correct setup.
- NEVER use a generic,
manufacture supplied map without dyno testing it.
The first step here is to maintain a constant
nitrous bottle temperature. When the temperature in the bottle fluctuates, so
does the pressure of the nitrous. Variable pressure means variable nitrous flow
which must be minimised to allow accurate mapping. The use of a temp controlled
bottle jacked should be used if temp variations are occurring.
Multiple maps can be written for each temperature situation be this complicates
things and increases the cost of mapping and the cost of the nitrous ECU. Its
works out cheaper to just buy a jacket for the nitrous bottle that is
automatically thermostatically controlled.
With the flow rate stabilised we can start the process of mapping the nitrous
supply. Nitrous engines prefer to run rich similar to alcohol engines. Best
power will be achieved at a ratio of about 6:1, equiv to 9.5:1 AFR. However, the
only way to map for max power is to run it on the dyno and give it a full dyno
tune. AFRs will vary depending on the cars manifold.
A lot of nitrous suppliers suggest that nitrous engines should be tuned to a
particular brake specific consumption number, but this idea has no practical
merit. You simply have to give the engine the fuel it needs for best power.
Efficient engines in excellent condition will give better power and require
different inputs than a similar engine with poorly atomised mix, inefficient
exhaust, high frictional losses. Therefore, the BSFC number can only be used as
a yardstick to make a base map for further tuning and at best is can only save
time in tuning, but can’t be used for specific mapping.
- A fulldyno ignition map is
required along with the fuel map.
As with all forms of supercharging, the ignition
will have to be retarded to protect the engine. More nitrous means more spark
retard. Again a full ignition map is the only real option here. Any kind of
guestimating will result in lower power or in a wrecked engine.
- Generally no need to modify
the compression ratio.
Unlike other forms of supercharging, there is no
need to reduce the compression ratio of a nitroused engine, providing the fuel
and spark maps are professionally carried out. Top class competition engines may
be an exception to this.
Piston requirements are the same as other supercharged vehicles. High silicon
content hypereutectic pistons must be avoided. They are too brittle and will die
with mild detonation. Pressure cast solid skirt pistons are a little better, but
forged pistons are the only way to go for bigger power applications.