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Overview of Nitrous Oxide

 - A chemical form of forced induction.
 - 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 power upgrade.

Wet and Dry Nitrous Systems.
 - Wet systems are most popular… inexpensive and easy to install.
 - Dry systems are complex and expensive.
 - 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 worst.

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 trackday use.

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).

Cylinder Mounting.
 - 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.

Supply Line.
 - 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 lower temperatures.

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 insulation jacket.

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.

Discharge Nozzle.
 - 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.

This breaks the fuel down into droplets of uniform and easily combustible size.

Click To Enlarge
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.

Nozzle Locations.
 - One nozzle per runner is best.
 - Usually a single nozzle is adequate.
 - 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.

Fuel Delivery
 - 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.

Nitrous Activation.
 - The simplest system uses a push button.
 - 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 temperature.
 - 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.

Ignition Timing.
 - 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.

Compression Ratio.
 - 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.