The basic way to look at nitrous is that it's a turbocharger in a bottle. The air that your engine compresses is about 21 percent oxygen, which is what your engine uses to make power. Nitrous oxide is 36 percent oxygen, so when you are injecting it into your engine, you are simply adding more oxygen.
There's a second part to this equation, however. Nitrous needs to react with something to make horsepower, so more fuel must be added for extra oxygen to be burned. The more overfueled a diesel engine is, the more it will benefit from the use of nitrous. Nitrous oxide also comes into the engine at-127 degrees F and acts as a liquid intercooler, cooling the intake charge by a huge amount. This can actually become a problem because diesel engines need a certain amount of hot, compressed air to function correctly. If too much nitrous is injected, the charge can actually become too cold to burn completely and make less power than a smaller amount of nitrous. Expect gains of about 100-300 hp with nitrous, depending on how much fuel (black smoke) you are pumping out of your exhaust.
Nitrous solenoids can only flow so much nitrous, so even if you're just running the hose into the intake, the most horsepower you can get will still be in the 300hp range. For those competition guys looking for more, at least two solenoids will be used to ensure the engine can get as much nitrous as it can handle. | 
This is a nitrous nozzle that is plumbed directly into the intake of a truck. Inside the nozzle are jets that can give a 50-150hp increase, depending on which ones are used. For big-horsepower rides, two or three nozzles will be used in stages when the vehicle is under power. |
Here's a propane kit being installed in the engine of a vehicle (arrow). This is an older variable-orifice kit that works through mechanical means. Some newer kits use computer controls to make sure the correct amount of propane is being injected into the engine. | |
With propane injection, the propane acts like a catalyst and makes the diesel fuel burn more effectively. The difference between propane and an actual catalyst is that the propane is used up during the combustion process. During normal combustion, about 75-80 percent of the diesel fuel is burned, and the use of propane increases that number to nearly 100 percent. Propane is also a fuel in itself, so it can help make more power because it's extra fuel-plain and simple. As the engine becomes more efficient through the use of propane, large fuel-mileage increases can be seen in addition to power gains. You'll save some money at the diesel pump, but remember, you still have to get the propane tank filled periodically. With propane injection, gains of 75-100 hp are typical.
With a water/methanol injection, any percent combination of the two can be used-from 100 percent water to 100 percent methanol. The most popular, however, is a 50/50 mix, which acts as a cooling agent and delivers extra fuel. You can also use over-the-counter windshield-washer fluid, which is about 30 percent methanol and 70 percent water. If nitrous is turbo in a bottle, think of water/methanol as an intercooler in a bottle. The water part can cool things down and lower EGT, and the methanol will lower EGT while acting as extra fuel. The greater the percentage of water, the greater the EGT drop; the greater the percentage of methanol, the greater the power gain. With water/methanol, expect a noticeable drop in EGT and look for about a 40-70hp increase.