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The Nitrous FAQ


DSM Wiseman
Apr 22, 2002
Cape Coral, Florida
Moderators: I've put this together to minimize the repetition of questions asked in the nitrous forum. I've been hesitant to reply lately because I keep seeing the same questions posted over and over again, and it gets daunting to answer them all again and again. If we could make this a sticky so every user can get some basics, we may be able to avoid a great majority of identical situations since this will likely answer most questions.

Some basic facts about Nitrous Oxide

-Nitrous oxide consists of two nitrogen atoms combined with one oxygen atom (N2O) Nitrous oxide is an oxidizer however it will not burn if subjected to a flame by itself. However, it does an excellent job of contributing to the combustion process and making horsepower.

-When compressed in a tank to a pressure of 760 pounds per square inch or above, the gaseous nitrous turns into a liquid. When the liquid nitrous is released into a low pressure area, it transforms from a liquid into a gas again, but that process reduces it's temeperature at the same time to -127 degrees F. This is the first advantage of nitrous oxide. Before it even reaches the combustion chamber, just releasing it into the intake manifold reduces inlet temperature as much as 65 degrees which can be worth about 6% in additional power.

-Once nitrous reaches the combustion chamber, and is heated to a temperature of more than 572 degrees, the chemical bond that holds the oxygen and nitrogen atoms together disassociates and the oxygen is released to aid in combustion.

-Basic nitrous kit consists of a Bottle, a nitrous solenoid, a 'fuel' solenoid (more on that later) nitrous feed line, a relay, an arming switch, a momentary (full throttle, etc.) switch, an optional purge solenoid and button, and a slew of other options.

-nitrous oxide is 36% oxygen by weight. The air we breathe is 78% nitrogen, 21% oxygen, and 1% other gases. The additional volume of nitrous oxide accounts for the added power.

Wet kits and Dry kits - What does it all mean?

The most basic definition of a dry kit is that it sprays only nitrous oxide, and not fuel. Don't think that just because it sprays only nitrous that it doesn't have provisions for additional fuel, it simply adds it in a different manner; fuel pressure. A dry kit adds fuel by spraying a controlled amount of nitrous into the fuel pressure regulator's pressure source, thus bumping fuel pressure in the same manner boost adds fuel pressure. Another important thing is nozzle placement. With a dry kit, you simply have a cloud of nitrous to carry, which is very light. You can take advantage of that by giving it a longer path of travel to atomize. Plumb the nitrous nozzle on a dry kit right at least 12 inches from the throttle body to give it time to atomize. Dry kits are going to be better for a Naturally Aspirated vehicle for a few reasons. Number one, it will atomize better than a wet kit because you simply have to carry gaseous nitrous and not heavy liquid fuel. This will result in better distribution within the intake manifold; a more even mixture between cylinders. Number two, you don't need as rich of a condition with a NA application as you do with a turbo so you can get away with bumping fuel pressure as a means of adding fuel with much more leniency than with a turbo car. Number 3, it's an easier install, and you don't run the risk of a nitrous backfire which occurs when fuel puddles up in the intake manifold and can be ignited by high temperatures. When it comes to jet sizing, the size of the nitrous jet for a wet and a dry kit work the same respectively. On the fuel side of things however, they are opposite. On a dry kit, the smaller the 'fuel' jet, the richer the kit will be because it's squeezing the same amount of nitrous through a smaller hole thus increasing fuel pressure accordingly. The larger the jet, the leaner it will be because the nitrous is not exiting the jet at quite the same velocity. When tuning a dry kit, I recommend experimenting with the 'fuel' side jets first to get a feel for how the car is going to respond rather than trying to swap nitrous jets. If you have a 50 shot in there, and you run lean, you can always richen up the 'fuel' side. If you have a 50 shot in there, and the car runs too rich, you can always lean it out on the 'fuel' side. Get familiar with that side of it first before moving on to larger nitrous jets. One more thing just for knowledge, is that when you heat your bottle up to operating temperature, such as 900-1100 psi or whatever pressure you choose to run, don't think 1100psi is being sprayed into a rubber vacuum line. That 'fuel' solenoid has it's own nitrous pressure regulator that sprays a controlled amount of nitrous into the vacuum line that goes to the FPR. You further control the pressure by selecting a jet size to determine the amount of nitrous pressure the kit will actually bypass into the the FPR thus bumping your fuel pressure accordingly. The AMOUNT of pressure is really dependant on your fuel pump, and on a smaller extent, your fuel injector size, smaller injectors, if run at the same milisecond (duty cycle) as larger injectors, will run a bit higher pressure due to less drop off in the rail.

A wet kit is similar in execution, however instead of bumping fuel pressure, it adds additional fuel by injecting it into the intake tract, similar to an additional injector. First and foremost, I want to comment that a wet kit is the best choice for a turbocharged vehicle. Your turbocharger adds fuel the same way a dry kit does; by bumping fuel pressure at the ratio of 1 pound of fuel pressure for every pount of boost. What this is doing is taxing the pressure capapbility of your fuel system. For example, if you run 40psi of fuel pressure, and add 20 pounds of boost, you will be running 60psi of fuel pressure. You don't want to introduce an additional means of adding fuel pressure on an already taxed fuel system (remember that your injector duty cycle goes up with fuel pressure) because you never know when you are going to overdo it and lock up and injector and cause major problems. Another point with a wet kit is that nozzle placement is different than that of a dry kit. Since you have to carry heavy fuel along with the light gaseous nitrous, it's beneficial to inject it closer to throttle body for a more even distribution of nitrous and fuel. With a wet kit, you want to place the nozzle close to the throttle body to help best acheive this effect. On the fuel side, you will need to tap into the car's existing fuel feed line. There are a few ways to do this. The easiest/cheapest way is to simply buy a brass or stainless T tap with two barbed ends on it, and a -4 male AN fitting on the third end. You cut your factory rubber fuel feed line after the fuel filter, slide the barbed ends into the respective fuel feed line(s), and thread the nitrous line on to the AN fitting. You would need to get these fittings from a hydraulic supple store such as Aeroquip for instance. Obviously, make sure you use high pressure clamps on the fuel feed ends that you have just cut. The other way to do it is a bit cleaner, and that is to purchase the fuel pressure gauge kit from B&M, and use their NPT fitting. Then you must buy a fitting from there that also has a -4 AN for which to thread the nitrous line to, which could be a screw on affair if you luck out, but will most likely end up being a weld-on affair. This way is harder, a bit more expensive, and doesn't have any advantages other than it looks more like a factory install. As far as jet sizes, a wet kit is a little different than that of a dry kit. Remember, the nitrous jets are relative to eachother, however the fuel jet is opposite. With a wet kit, the larger the fuel jet, the richer the shot will be because you aren't dealing with pressure here, but rather volume. The larger orifice jet will allow more fuel to pass through it thus allowing a richer condition. Again, when tuning, I suggest tuning the fuel side first before moving on to larger nitrous jets. A little hint; all nitrous systems are intentionally jetted on the rich side as a safety precaution, so there is additional power to be made by experimenting with different fuel jets to find out which is best for your particuar application.

A quick note on a direct port kit is that it is a wet kit, however instead of one nitrous nozzle and one fuel nozzle, there is one of each for each cylinder and it is plumbed directly into the intake manifold rather than before the throttle body. The idea and result of this type of system is better atomization for each cylinder.

Now that you know some basic facts about nitrous, and how each kit functions, and even which is better for which particular application, I'll get a bit more technical for the next couple sections. They may seem kind of random, but I'll do my best.

Speed of Combustion, and Timing

You will often hear people ask "why not spray pure oxygen into the engine instead of N2O?" You will learn why right now. The advantage of nitrous oxide is that it will actually act as a compression buffer slowing the resultant pressure rise that occurs when the oxygen enhanced air/fuel charge is ignited. This is why the molecule N2O is a better choice than injecting pure oxygen; with pure oxygen, the resulting combustion process would be so violent that it would immediately destroy the engine; think bomb. Even with the buffering effect, it will still cause a much quicker cylinder pressure rise than a turbocharged engine. Combine the two, and pressure can get huge. With nitrous, you can actually make more power at lower timing than with just a turbocharged engine. You want to start off small, and work your way up, but don't work too far. Set your base timing back to 5 degrees BTDC. Ideally, in a nitrous application, you want peak cylinder pressure to occur with the piston roughly 10-15 degrees ATDC. Picture hitting a punching bag. Your fist is spark/explosion/combustion, and the bag is the piston. Punch the bag, and start it moving. If you hit the bag while it's coming at you, it will likely stop because the force of it swaying is meeting the force of your fist punching. Apply this to what's going on in the engine. The piston is coming up, and then combustion is forcing the piston back down the hole. The only thing driving the piston upwards is the inertia from the crankshaft. This can have adverse if not detrimental affects on your rod, and rod barings, not to mention even the main bearings when you are dealing with high rpm and cranskshaft deflection. Same scenario, you have the punching bad moving, now hit it at the peak of it's travel. The bag will sway away pretty well. You are timing combustion to occur at TDC, which is okay, but there is more power to be had. Now, back to the bag; this time, hit it just as it starts to swing away from you. The bag goes flying! Imagine combustion occuring at 10-15 degrees of crank rotation ATDC. The crank is already pulling the piston back down, and combustion is going to force the piston down even faster. You have the pulling force of the crankshaft, AND the force of combustion both working for you in the same direction. You are basically taking advantage of the leverage offered by the crankshaft as cylinder pressure pushes down on the piston. This is why over-advanced ignition timing even in a normally aspirated engine makes less power because the combustion process is so early that the engin is working against cylinder pressure as the piston approaches TDC. Nitrous is going to greatly increase the burn rate in the combustion chamber so retarding the timing can actually yield more power. Start off slow, and work your way up. Make a pass, look at the plugs. Any hot spots? Any flecks of detonation? Your timing may be too advanced if so, and you may want to retard it a bit. never underestimate the power of reading your spark plugs, as they are the only true window into the combustion chamber.

The Power Factor

Everyone is familiar with a 50HP jet being synonomous with a 50HP gain. For the most part, this is pretty accurate, however, on a turbo car the gains can be much greater due to the additional intercooling affect, denser air, increased spoolup, greater exhaust energy, and the list goes on and on. What s more intersting is the torque factor. While the HP gain is great, the torque gain can only be described as phenomenal. Using the formula torque=HP x 5252/rpm, plug in an example. Say your engine makes 350 lb/ft of torque at 3000 rpm, and you are adding 100 shot of nitrous to it. torque=100 x 5252/3000 = 175 lb/ft at 3000 rpm. For that same motor that makes 350 lb/ft of torque at 3000 rpm, with nitrous, it will make an astounding 525 lb/ft of torque! Just an example, but you can see how immense the gains can be with a properly tuned nitrous system.

Quick Note about Zex

Since it is one of the more asked about kits, I will insert a quick note about Zex. The ZEX kit is a dry kit which will spray a controlled amount of nitrous into the fuel pressure regulator pressure source. I have never personally installed a dry kit on a 2G NT, so I'm not sure exactly where that vacuum line is located. I would imagine it has one as I've heard of 2G NT guys running dry kits before, and EFI cars will typically have some sort of pressure compensation unit; for example you wouldn't be running the same kind of fuel pressure at 6000ft in Colorado as you would down here at sea level in Florida. The Venom kit is about the only dry kit I can think of that adds fuel in another manner; and that is via upping the injector duty cycle/pulse width when the nitrous is engaged. One more thing is that the Zex kit will add less power for the same shot as say an NOS kit. The reason why the ZEX kits make less power is because they spray a more controlled amount of nitrous based upon bottle pressure. It's a good idea, but nothing more than a gimmick. It IS safer, but only for the simple fact that you are spraying less. However, it is nothing different than spraying a 50 shot instead of a 75 shot. I personally like the NOS and NX kits, but that's just me. They are straight forward, no nonsense nitrous kits. They have the largest amount of parts available, all sorts of different nozzles and jets, and quite frankly the most experience in the industry. Any kit you get WILL work, so it's really up to you. The bigger question isn't which kit to get, but rather what to do with it when you get it.

Nitrous and Platinum Spark Plugs

Platinum plugs actually have a piece of platinum in them, hence the name, that heats up very quickly. What this does is help ensure a more complete burn in the combustion chamber, thus reducing wasted gas and in turn, lowering overall emissions. Platinum plugs are great for a stock daily driver, and can last a long time because the platinum gets so hot that it burns up any deposits on the plug; in essence it is self-cleaning. However, on a turbocharged or nitrous application, where you are increasing cylinder pressure and thus, heat, the platinum in the plug can become the hottest point in the combustion chamber and can actually ignite the mixture before the plug even fires. This is called pre-ignition, and actually tries to force the piston back down the cylinder while it's on it's upwards travel. This is not good. This breaks rods.

I hope this helps everyone get a basic understanding of nitrous, and how it works. I usually spend a lot of time answering questions in the nitrous forum, and have no problem addressing specifics, however take the time to read and learn if you have a more basic question so we can clean up the forum a bit, and save people from having to read the same thing over and over.



DSM Wiseman
Apr 22, 2002
Cape Coral, Florida
Just a quick update---

This information was accurate at the time I wrote it. However, companies like ZEX now offer wet kits whereas they previously only offered dry kits. My information on dry kits remains accurate, however I simply wanted to inform the readers that most companies offer both wet and dry systems at this point, mostly for the reasons I have mentioned. ;)

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