NOS Nitrous Oxide Guide

EVERYTHING YOU NEED TO KNOW ABOUT ADDING NITROUS OXIDE TO YOUR POWER-STARVED CAR.

EVER STOPPED TO THINK about what you're actually doing when you add a bigger carb, port a cylinder head, add bigger valves, a larger lift and duration cam, or indeed a turbo or supercharger? Quite simply, you're trying to improve the flow of air into the combustion chambers.
And the reason you do this is? The more air you get in, the more you can mix it with fuel. And the more fuel you burn, the more power you make - it's as simple as that.
But the more highly-tuned your motor is, the more of an inconvenience it is to live with. Let's face it, a cam that comes in at 4500 revs means you're going to be constantly slipping the clutch trying to keep engine speeds up. In traffic, that's a complete pain.
But there is a way of introducing monstrous levels of oxygen so you can burn more fuel, instantly And it's called nitrous oxide.
Now before we wade in too deep, nitrous is not a fuel. Neither is it called nitro (that is a fuel and a very volatile one). If you want to avoid looking an arse, don't call it nitro, OK?
Nitrous oxide (N20) is a gas made up of two parts nitrogen to one of oxygen. It's stored as a liquid under pressure in a bottle (usually in the boot). But as soon as it's injected into the engine it turns to a gas, heat breaks the chemical bond down, the inert nitrogen disappears into the atmosphere, while the engine is fed a nice rich dose of oxygen. So the role of nitrous oxide is that of a carrier of oxygen.
What we have now though, is a thoroughly dangerous situation. The ratio of fuel to oxygen is too low, meaning, in short, detonation, which spells instantly melted pistons.
So as soon as you inject nitrous oxide into your motor, you must also introduce an equal (in proportional terms) quantity of fuel too.
Now this sounds like all you have to do to gain more power is forget about carbs and big heads, just inject nitrous all the time. Nice, but unfortunately not practical. The levels of oxygen, and therefore fuel, are so instantaneously great that a massive strain is put on the engine - you can literally double the horsepower in some cases So nitrous injection is limited to quick bursts, and the ideal place for that is drag racing. Build your motor right and you'll have the wheels lifting skywards in no time.

Getting Nitrous In Your Motor

A nitrous system is actually very simple. Both fuel and nitrous oxide are controlled using separate solenoid valves, which open and close at set periods to meter specific amounts of either substance.
Very similar to fuel-injection, the amount of fluid that passes through either solenoid depends on how long it's open, the pressure, and the size of the hole.
The latter is measured in simple bhp terms so you might, for example, have a system fitted with 50 bhp jets. What will be metered out is gas and fuel sufficient to create an extra 50 bhp.
But, and this needs saying before we get too carried away, the net effect you'll get will not be 50 bhp extra - if you've got a 100 bhp engine and add 50 bhp of nitrous, don't expect 150 bhp Why? Well, what actually happens is, if you were able to measure the flow of air coming into the engine, you'd see a drop as soon as you introduced the nitrous This is because this gas carries a greater quantity of oxygen than air does and by simply filling the ports with air, nitrous, fuel and yet more fuel, it can't physically fit it all in. Therefore, some of the air's oxygen is displaced in favour of the nitrous' richer source.
The effect is that our 100 bhp engine will actually only produce about 135 bhp when nitrous is added, due to the gas displacing some of the normally-aspirated oxygen. So don't expect by adding more and more gas, you'll get more and more power - popular misconception that one, so you're not alone.

Nitrous Oxide Systems

Introducing nitrous oxide into an engine is very similar to the various systems of fuel-injection - there are two major types Very much like throttle-body fuel-injection, a spray-bar system feeds a common plenum chamber with charge, which finds its way into the combustion chamber as it's needed This is the simplest way of adding nitrous - a thin aluminium plate is sandwiched between the carb/airflow meter and the manifold. Running through the plate is a tube containing spray holes, which is fed from one end by fuel (coded red) and the other by nitrous (coded blue).
But this system is only simple if there's a plate kit for your car, and since most of the systems available come from the good old US, they've never heard of a 38 DGAS! If you've got a Rover with a four-barrel manifold and Holley, then you're sorted (and those of us lucky enough to have an Essex V6 Swaymar/Holley manifold are laughing, too).
This leads us on to the system most suitable for the majority of classic Ford owners, even more so if you've got sidedraught carbs. This again is similar to fuel-injection, only the direct port/sequential-type.
What you have here is combined injectors for both fuel and nitrous, individually plumbed into each - inlet tract or port. This is known as a Fogger system (although that is a trade name of NOS - Nitrous Oxide Systems).
This system is probably the most versatile and is the one mostly used in professional drag racing. It's probably most apt for our use, since a simple Weber DCOE usually requires its own separate manifold, feeding a dedicated port Fitting fogger nozzles isn't actually that daunting.
All you need to do is drill the appropriate size hole, thread it with the tap provided in the NOS Fogger kit and screw the injector into the manifold. Then it's just a case of simple plumbing.

Fuel-Injected Engines

So far we've been talking mostly carbs, but more modern engines now use fuel-injection, which needs special consideration. There are two methods of fitting nitrous oxide to fuel-injected cars - wet or dry.
By wet, we mean that the manifold is wet with fuel/nitrous oxide The extra gas and fuel is added after the air meter and simply enriches the system.

Dry means that the manifold contains nothing but air. Most modern engines use dry manifold fuelinjection because air alone has very different characteristics to one charged with fuel. For a start, the fuel tries to sink to the bottom so the intake runners have to be more upright. Air doesn't, so its path can be made tighter, flat and compact. What you achieve then is an enormous saving in space.
For these systems the nitrous is injected before the metering flap, which tricks the ECU into thinking it needs to inject more fuel. In these systems then, there isn't a separate solenoid for fuel, it's done by increasing fuel pressure.
What you need though, is a system with a vacuum-operated fuel pressure regulator attached to the fuel rail. The increase in air sensed by the ECU makes the fuel pump drive faster.

Coping With Nitrous

You are limited to the amount of nitrous that you can feed into your engine without it going bang. Combustion temperatures rise so dramatically that too much nitrous can melt pistons.
So to cope, if you're planning on using larger amounts than say 50-75 bhp {which is the most common recommended manufacturers' amount), you're better off fitting stronger forged pistons. In some cases you'd also need to lower the compression ratio to allow more fuel/air charge to physically fit in.

Also when you inject nitrous, very much like turbocharging, the increases in combustion pressure tries to lift the head off the block. So if you've got stretch bolts fitted, you'd need to swap these for an ARP head stud kit {typically about £90).
The gasket also needs looking at too. When the head's lifted {which it will), gas can escape through the gap between head and block, effectively rending a normal gasket useless. If the block is O-ringed - a round groove cut into its face - this can be filled with a wire ring-type head gasket, which will take up the gap and block the gas's exit.
Due to the effect coming in with a bang, the clutch might not withstand it either. So you need a heavy-duty one at the very least or a paddle-type.

Ignition

The heat built up inside the combustion chamber can be so great that you'd need to run colder plugs. But at the same time, the pressure rise can be so great that it blows out the spark from the plug. In this case you need a stronger spark, which you'll get with a specific coil and capacitor discharge ignition.
This stores up loads of current, then whacks it across the spark plug gap giving a high voltage spark, but more importantly, one loaded up with amps A system such as MSD's Digital 7 will also control twin coil distributorless ignition systems and has built in rev limiters that you can set externally.

You might not need such flash equipment. just a simple ignition boost-retard system will get you by.
This has a pipe on its side, which senses and retards ignition timing according to the level you set via a remotely mounted control knob.
Once you've prepared the engine specifically to run with nitrous, levels of increases of 400 bhp are not uncommon in drag racing circles!

Costs

Nitrous oxide can be very cost effective. If you want to keep your motor fairly standard and restrict the amount of nitrous you inject to 75 bhp jets, then a plate kit for a Rover V8 will cost £436.18.
A universal Fogger kit for a four-cylinder engine that you plumb into your manifold will set you back £502.15, while a dry manifold fuel-injection kit works out at £439.13.
These might sound a lot. but when you consider you can almost double the horsepower at the touch of a button they are very good cost effective horsepower. Obviously the cost is going to be considerably more with the greater amount of nitrous you intend to inject.
Finally, how long does it last? Well, typically, a bottle of nitrous oxide costs £35 and will take about 10 passes of a drag strip to empty (depending on your system). Once you've hit the arming button though and felt that extra power, be warned, it's addictive!