Nitrous Explained
How Nitrous works
It is a colorless, odorless gas composed of two (2) nitrogen atoms bonded to one (1) oxygen atom. The scientific abbreviation for nitrogen is N, and O for oxygen. The proper abbreviation for one nitrous oxide molecule is N2O.
Nitrous Oxide is an oxidizer that is used as a carrier for oxygen. Mixed with the right ratios of fuel, and fed into the intake, it provides additional combustible material into the cylinders, creating more power. There are many ways to get the nitrous and fuel into the engine, the following describes typical applications that have proven successful.
The Basics
The nitrous is compressed to high pressure (900-1100psi) in a tank, in liquid form.
From the bottle (typically fastened down tightly in your trunk or on the swingarm), a hose runs up to the engine. From there, an electrically controlled valve called a solenoid is used to release the nitrous into the motor when you request it. At the same time, a fuel line in a "wet system," is controlled by another solenoid, and releases fuel into the motor. This provides the basic mechanism for the nitrous system.
Wet -vs- Dry
You may have heard the terms "wet kit" and "dry kit".
A "wet system" is a nitrous system that mixes both nitrous and fuel, and feeds it into the intake.
A "dry system" only feeds nitrous into the intake. Additional fuel is added through the fuel injectors via a seperate nitrous map or a nitrous compensation table through the ECU/fuel controller.
As mentioned, there are several ways to feed the nitrous and fuel into your motor. Here are brief descriptions of them.
Fogger Nozzle
A nozzle can support either a single line for nitrous, or a pair of lines for nitrous and fuel, and sprays a fine mist into the intake.
Direct Port
The ultimate setup- each port is tapped and threaded specifically for a nozzle at each cylinder. Nitrous and fuel lines to spray directly into the cylinders. This setup typically provides the most horsepower for extreme race applications.
Triggering the System
Of course, you don't want the system to be running all the time - a 10lb bottle will last you less than a minute, if it's open. Typically, you want the system triggered on while you're at the track, at WOT (wide open throttle), and at relatively high rpm's (see "Safety" for why). To make that happen, you'll typically want to wire, in sequence, several switches. I won't describe the specific wiring here, but you'll have some or all of the following:
1) Arming (On/Off ) switch.
2) WOT switch is a micro switch installed on the throttle system, that activates the circuit only when your foot is on the floorboard.
3) Activation switch in the car
4) "Window Switch" (see "Safety" for details) that closes the circuit only when the engine RPM is between a certain range (like 6000-11000) that you decide is acceptable
5) Fuel Pressure Safety Switch
Nitrous Controllers
The system to trigger described above is a basic "single stage" setup. The nitrous is either on or off, and when it's on, the full volume dictated by the jets is sprayed into the engine. There are other applications that are full race or multiple stage nitrous system that require more detailed management at higher rpm, with time-based systems, which delay the nitrous flow for some time after you launch, etc.
These Nitrous controllers are a great addition to any nitrous system and can help to safeguard the engine from Lean-Condition.
Safety
Use all the safety mechanisms you have available. They are cheap and very effective. Components such as Fuel Pressure safety Switch, Rev limiters, EGT sensors, Window switches, etc. are relatively inexpensive ways to protect your investment.
What Can Go Wrong?
Well, a lot can go wrong, but hopefully you'll have adequate safety mechanisms built in to protect your motor when it does. The main thing that can go wrong is adding nitrous into your engine without compensating fuel. This extreme lean condition is disaster for the engine, and you're not likely to get a second chance - at least with the same engine. Conversely, adding extra fuel without nitrous is not particularly bad for the engine, so you can imagine, it's safer to start with the car running rich (too much fuel), then lean it back from there.
Ignition RPM limiter
The rev limiter is implemented by cutting the signal to the fuel injectors so the cylinders have no combustion. If you're running a dry system, which depends on the fuel injectors to provide compensating fuel for the nitrous, losing fuel this way is the ultimate disaster. An after market ignition will typically implement the rev limit by cutting off spark rather than fuel, which is a much safer implementation of the rev limit.
Window Switch
This electrical device provides an open or closed circuit based on the engine being between two RPM values (hence "window") that you chose, so that you'll only flow nitrous in this range. Why would you do that? Well, for two very different reasons.
1) At low RPM, think about what's going on: you're spraying nitrous into the intake at a constant flow. That is, the nitrous bottle and solenoids have no idea what RPM you're at, and they're just pushing it into the intake at a constant volume. Inside the engine, though, the nitrous and fuel combination is being sucked into the cylinders during every stroke. The net result is that at low RPM, you're getting far more of the mixture into the cylinders. At 6000 RPM, for example, you're getting twice the amount as at 12000 RPM. So, you can imagine that running nitrous at, say 3000 RPM, is far more stressful on the motor as at 6000 RPM, and typically causes a "nitrous backfire" - meaning that the nitrous/fuel combination can explode in the intake manifold (rather than the cylinders) - a bad thing. So that's why you don't want the system triggered at low RPM.
2) At high RPM, the situation is easier to explain. Given the discussion of the rev limit above, you may just want the nitrous system to cut off before hitting that rev limit.
Fuel Pressure Safety Switch (FPSS)
This is a device that's plumbed into the fuel system, and provides an open or closed circuit based on availability of fuel pressure. It can be used in the triggering circuit to make sure the system isn't on when you've got a fuel problem.
Timing Retard
A nitrous/fuel mixture increases the burn rate in the cylinder, and typically adding a few degrees of timing retard is recommended for safety. A rule of thumb is two degrees per 50hp of nitrous, but this will also reduce the power generated.
High Octane Fuel
High octane gas (e.g. 100 or more, unleaded) will also slow the burn rate in the cylinder. This will provide another way, similar to retarding timing, to avoid knock and detonation
By the way, watch out for Octane Boost claims. Typical claims are "8-10 points of octane boost for a tank of gas." You should be aware that these "points" are tenths of a point of octane as you'd purchase at a gas station. So the above example will raise your octane from 92 to 92.8 or 93, not 100-102 as you might think.
Don't assume that if high octane fuel helps on nitrous motors, that it'll help your naturally aspirated motor too. A naturally aspirated motor is tuned for a particular octane of gas; adding more doesn't help one bit. Save your money.
Nitrous Filter
A simple part, but essential in any nitrous system. This filter is added in-line to your nitrous line, between the tank and the solenoid. Install it as close to the solenoid end as is convenient. It will trap any small particles that may come through the line, much like a fuel filter. A common solenoid failure is due to some particle jamming it open.
Fuel Systems
Your fuel system is the most important part of the system. The worst scenario in a nitrous system is a lean air/fuel mixture. The solutions to a good fuel system depend on the type of nitrous system you're using.
On a wet system, you simply need to ensure that your fuel pump and solenoid can supply adequate fuel.
On a dry system, you need to make sure you don't over work in the injector above 80% duty cycle. If you are increasing fuel pressure with your stock injectors, ensure your fuel pump can handle the extra volume
Spark Plugs
Generally you want to use copper spark plugs. You also want to reduce the gap from the stock 0.030" down to 0.022"-0.018". The reason you want a smaller gap is because of ionization. If you change from the typical air (78%nitrogen, 21% oxygen)/fuel ratio, a given gap requires more energy to ionize the mixture, resulting in less energy in the spark, if you even get a spark. You could also increase the coil voltage instead of decreasing the gap, but I think using a smaller gap would be preferential since the spark time will be smaller. It also raises the cylinder pressure, much like a supercharger. Therefore "blowing" the spark out. When you close the gap it cannot put out the spark as easily.
Testing Solenoids
Mentioned previously was failed fuel or nitrous solenoids doing damage. Some of the issues here may be hard to cover with only other safety devices. It is recommend you wire your solenoids with spade clips, so you can easily disconnect them, and test them on a regular basis. Simply disconnect them from the rest of the wiring, then ground one side, and connect the other side to 12V, and listen for the click-click to make sure they open and close
Tuning
All of the kit systems will come with a couple tuning setups, labeled "50-shot", "100-shot", etc. These are tuned to provide 35, 50, 75 or other horsepower amounts, usually measured at the crank (i.e., measured on a chassis dyno, you'll get a bit less). Consider these a starting point, and certainly good for your first passes (hopefully you'll make these with the lowest power, until you tune the system up). Once you've got the system installed and functional, though, tuning it is paramount, before running any serious power through it. It is recommend you do this tuning right away, even though the temptation will be strong to just go out and enjoy the power. This is the time you're very likely to do some serious damage to the motor, it's important to get it set up right.
Jets
All nitrous systems use "jets" inserted in the fuel or nitrous lines to limit the flow. These jets have openings of a specific size, measured in thousandths of an inch. So a "35 jet" is usually a jet with a hole drilled 0.035" through it. ZEX and HOLLEY use a numbering scheme for the jet sizes, so you will have to use a chart to do a cross reference.
Increasing a nitrous jet size will make the system run more lean, increasing the fuel jet size will make the system run more rich.
Purge
Most nitrous systems are built with a purge feature. The purpose of a purge is to get liquid nitrous oxide up to the nitrous solendoi, filling the hoses with nitrous oxide rather than air. To do this, another solenoid is used, but rather than shooting the nitrous into the motor.
Bottle Heater
It's virtually mandatory that you install your nitrous system with a bottle heater, which is used to raise up the temperature of the bottle, and therefore increase the pressure at which the nitrous is delivered. If you don't use one, your pressure will quickly drop on a cool day. You want to keep pressure between 900-1000 psi.