Nitrous oxide systems make large amounts of torque by allowing an engine to burn more fuel at a lower rpm range than normal. Burning more fuel this way creates a longer burn period (and slightly higher cylinder pressures, if the timing is not corrected), that will push down on the pistons with greater average force. When the nitrous is injected into an engine and the initial combustion takes place, it creates enough heat to separate the nitrous oxide into its two components, nitrogen and oxygen. Once separated, the additional oxygen is then free to allow combustion of the additional fuel, while the released nitrogen acts as a buffer against detonation and damps mechanical loads.
To run nitrous successfully and safely, you have to introduce precise amounts of additional fuel with precise amounts of nitrous oxide. All of the extra oxygen provided by the nitrous oxide must have fuel with which to burn or you may damage your engine severely. When the amount of nitrous and the amount of supplemental fuel is controlled precisely, your engine can safely and reliably generate exceptional power increases.
Combustion
Nitrous oxide does not burn, it is an oxidiser. It provides more oxygen, so more fuel can be burned, and the result is more power. The atoms in a nitrous oxide molecule are bonded together. The oxygen is not free, but fortunately the bond breaks down as temperature rises. At 565° F, the bond is broken and the oxygen is then free. Combustion temperatures are much more than 565°, so it's not a problem. By adding nitrous oxide to an engine, the total amount of oxygen is increased while the volume of nitrogen is decreased (as a percentage of the whole). This speeds the burn rate and requires less timing advance for peak output. It is hard for many people to grasp gaining power with less timing, but it's a fact. Peak cylinder pressure must occur at approximately 20°ATDC to make peak power. If you speed the burn rate, peak cylinder pressure will occur too soon. It is easy to run too much ignition advance with nitrous, but too much will not only hurt power, it can quickly bring a nitrous engine into detonation and destroy it.
Detonation
Large power increases achieved by using nitrous oxide can increase the chance of detonation. To keep the engine out of detonation, you must control the extra heat that nitrous can make. The easiest way to do this is to add more fuel. All nitrous systems come with rich jetting to give you a safe starting point. The extra fuel takes away heat and raises the detonation limit. If you don't try to over do it, and keep the hp levels within reason, running slightly richer should be all you'll need to control detonation. Running richer will reduce the power output, but raising the detonation limit will allow more nitrous to be used to get more power.
Nitrous-to-fuel Ratios
The chemically correct nitrous to fuel ratio is 9.649:1. If a nitrous engine runs lean, it can destroy the engine in a matter of seconds. There must be enough fuel to maintain this correct ratio, if there isn't, temperatures rise rapidly. The oxygen that was left over from burning the limited amount of fuel will result is a lean burn situation raising cylinder temperatures and melting components. So don't run lean.
Cooling Effects
Cooler intake air is denser and contains more oxygen atoms per cubic foot. So cooler air will allow more fuel to be burned and in turn, make more power. A 10 degree drop in temperature can add 1 to 1.5% power to an engine. Nitrous oxide boils at -129°F and it will begin to boil as soon as it is injected. This can cause an 80° or so drop in manifold air temperature. Now if we are dealing with say a 400 hp engine, we can see a gain of well over 30 hp from the cooling effect alone. This cooling effect also helps the engine deal with detonation.
Average Power
If you were to build a 350 hp 3.5 Rover V8, it would have to rev to 7000+ rpm to make that kind of power and only make power over a narrow rpm range. A nitrous injected 3.5 Rover V8 making 350 hp would make that power at a much lower rpm with a higher average horsepower. So the nitrous engine will out perform the normally aspirated engine by a healthy margin. The reason is that nitrous flow remains constant no matter what rpm the engine is running at. At lower speeds there is more time for the nitrous to fill the cylinders, so you get more nitrous in the cylinders per power stroke at lower rpm. This will boost torque and consequently power more at low rpm. As rpm increases, you will get less nitrous per power stroke, but the engine will start making more normally aspirated power. This really flattens out the torque curve and widens the power band.
So Why Not Pure Oxygen?
Air has only 23.6% oxygen by weight, the rest is made up largely of nitrogen. That nitrogen does not aid in combustion at all, but it does absorb and carry heat away. When you add nitrous, it has 36% oxygen with the rest being nitrogen. So the more nitrous oxide you add, the less percentage of nitrogen is available to absorb heat. That is why nitrous increases engine heat very rapidly. If we were to add pure oxygen (which has been tried), the percentage of nitrogen would fall even lower as more oxygen was added. We would not be able to add much oxygen before heat was a problem to control. Also compressed oxygen is in a gaseous form, so adding oxygen takes up more room and reduces normally aspirated power, and the amount of nitrogen from it. To put it simply, using nitrous oxide, we can get more oxygen atoms in the engine and have a lot more nitrogen as well. Nitrous can make much more power before heat is uncontrollable.
A Nitrous Engine - Choosing a Camshaft
Optimum cam timing for a nitrous motor will be different than optimum timing for that same motor off the bottle, so you will have to make a choice as to whether you want the most power with or without nitrous. Obviously if you are driving the car on the street most of the time, you will want the best power off the bottle. If you find that you can spare some power to make your car faster at the track, picking a camshaft to favour nitrous can make a substantial difference when nitrous is in use. Of course it is a trade off, but usually the power that you make on the bottle, will be far greater than the amount lost off the bottle.
Pumping Losses
Nitrous oxide adds oxygen, much of which is in liquid form. So you can see that a large intake valve and port is not required or desirable. Larger intake ports cause more of the nitrous to turn to a gas and reduce the amount of normally aspirated power, if the nitrous takes up more room, there will be less room for air, reducing volumetric efficiency. Also, you do not want or need long intake duration or a very high lift, so the intake side of the cam does not need to be any different when nitrous is used. The exhaust is a totally different story. All that extra oxygen and fuel makes for a substantial increase in exhaust gas volume. How can the exhaust valves deal with this? It can't, pumping losses go out of sight. Much of the extra power made in the cylinders never makes it to the flywheel, because it is used to push out the exhaust. Since making the exhaust valve large enough and the port flow enough is impractical with most cylinder heads, we must take other actions to cut pumping losses (which is actually just a band aid fix).
Reducing Pumping Losses
The first obvious step is to use a dual pattern cam with longer exhaust duration. Opening the valve earlier will help by getting the valve open more and bleeding off some pressure before the piston starts moving up the bore. This does eat into the power stroke, but more power is freed up than would be made by holding it closed longer (the best solution would be a larger valve and better port). The blow down phase (overlap period) becomes very important in a nitrous engine, because the gas has a much greater velocity and can over scavenge, closing the exhaust valve a little earlier helps. Anytime you make more power by reducing pumping losses, you are freeing up horsepower that already existed in the cylinders. The engine will still experience the same loads, but more power will be put to the flywheel and less will be used to push out exhaust.
Camshaft Specs
As said earlier, the intake needs to remain pretty much the same, but the exhaust needs more duration, an earlier opening point and an earlier closing point. To make this happen, you need to use a dual pattern cam with more exhaust timing, and a wider lobe separation angle. Cam's with 112-116° lobe separations are common is nitrous motors. To keep the intake timing the same, you must install the cam advanced, usually 6-8° advanced. The good thing about this is that advancing a cam will bring more low-end (at a trade off of top-end) when running without the nitrous and the wider lobe centre angle will also help idle and vacuum. Even the most radical nitrous profiles are usually pretty tame on the street. Ultra high lift cams are not need to make power with nitrous. On the exhaust side, the low lift flow is the most important thing, and must be dealt with much more seriously than high lift flow.
Intake Port Work
Nitrous adds so much oxygen that getting oxygen in is no longer a problem. A large intake port is not needed or desired. The larger the port, the more surface area it has and the intake charge will have lower velocity. Slower moving nitrous has more time to turn from a liquid to a gas, so a large port will have less liquid nitrous getting in the cylinder. As nitrous turns to a gas it will expand, taking up room in the intake and reducing the amount of normally aspirated air. More surface area will give the nitrous more area to absorb heat, which will cause even more nitrous to turn into gas. The same goes for large intake valves. The intake valve is the hottest part of the intake system and when nitrous is involved you don't want excess surface area on the valve. The exhaust is a different story.
Exhaust Port Work
All the extra exhaust has to be dealt with. The exhaust valves of a nitrous engine are almost always too small. When possible it is best to reduce the size of the intake to allow room for a bigger exhaust valve. The head of the exhaust valve should not have any sharp edges. It should have a nice smooth radius to allow the exhaust to travel around it as easily as possible. The valve job on the exhaust is the most important part. There will be much more cylinder pressure when the exhaust valve opens which means there will be more burnt gasses trying to escape through the valve at low lifts. Low lift exhaust flow should be your number one concern (up to about .300" lift). A good multi angle valve job is the best bang for the buck in a nitrous engine. The short side radius will usually benefit from a straight cut to the port floor. The area directly past the seat should be as wide as possible. The valve seats should be slightly wider also (.010"-.015") to help get rid of some of the extra heat in the valves.
Combustion Chamber modifications
Usually you cannot do much chamber work without reducing compression and being forced to use a high dome that hurts power. With nitrous, a high compression ratio is not needed, so some work can be done in this area. Nitrous can make very respectable power with compression ratio up to 10:1. First step is to angle the exhaust valve as much as possible so the gasses can move around the valve easily. The next step is to polish the combustion chamber and remove any sharp edges. Sharp edges will be the first to get hot and cause detonation (as well as be the first to melt). Polishing the combustion chamber will help keep carbon build up to a minimum (a good idea for any engine).
