Car Modding, Car Tuning and the Science of Horsepower: Nitrous Oxide
Nitrous: Fun at Your Fingertips
It seems like nitrous and cars were made for each other. I mean, how awesome is it that you can push a button and make your car suddenly take off like it's on steroids? It's addictive just thinking about it. And it doesn't take some sort of complicated technology or hours of professional installation to enjoy either. It's fun in a bottle and I think appeals to how every man's most basic reward system works: Something doesn't work? Hit it. Something looks good? Eat it. Car not fast enough? Push button!
What is nitrous oxide exactly and why is it so awesome? Nitrous oxide is the same stuff the dentist uses to dope you up before a procedure. It's the stuff used to pressurize whipping cream canisters. It's the molecule N20, a colorless, odorless gas at room temperature.
Tell Me More...
As you may or may not know, the air your car breathes has 21% oxygen. That's the only part that matters because the rest doesn't support combustion. The oxygen in the air limits the power of an engine because the car's computer has to mix just the right amount of gas to burn with the oxygen. It's all chemistry. You can't just add more gas without adding more air and the intake system of the car can only draw in so much air. Wouldn't it be nice if the air had more oxygen in it? What a waste at only 21% right? Well, N20 clearly has 3 atoms in it, 2 nitrogen and 1 oxygen. That means that 1/3 or 33% of every nitrous molecule is oxygen. That's 57% more oxygen than air has! What if we replace air with nitrous oxide in a car engine? Well, nitrous by itself is worthless because it isn't flammable but if you heat it up it becomes unstable and splits apart! 2N20 + Heat --> 2N2 + O2. The O2 still represents 1/3 of the molecular fraction of the products but now it can combine with gasoline in the same way atmospheric oxygen would! What's more, since nitrous doesn't decompose until heated to the sort of temperatures you'd see during the engine's power stroke, it helps prevent premature detonation.
The idea of a nitrous oxide kit is to carry around with you a supply of nitrous oxide in a tank and when you want more power, you inject it into your engine via a hose, which burns with extra fuel to produce significantly more power for however long you're injecting the nitrous. Now nitrous oxide can be compressed into a liquid fairly easy so in order to make storage practical, it's compressed to about 1100 psi or so, which transforms it into a liquid. You may be aware from high school chemistry that when a liquid changes state back into a gas, it must absorb a certain amount of "latent heat" before becoming a gas. That latent heat is usually quite significant so a secondary benefit of nitrous oxide, which nets you even more power, is that by injecting it in liquid form, it soaks up heat from the intake air in order to become a gas and in so doing, it cools the intake air significantly, increasing its density. So nitrous has a 1,2 punch effect of adding more oxygen AND more air mass to each power stroke. That means more gasoline and more power!
A Summary of Nitrous Kit Types
Tell Me About Nitrous Kits
There are 2 main categories of nitrous kits. There are DRY nitrous kits and there are WET nitrous kits. The dry kind are so named because you're only injecting nitrous oxide into the engine. You have to rely on some other means to supply the extra gasoline to go with it. The wet kits all have provisions to inject EXTRA gasoline along with the nitrous oxide so that the car's computer can worry about the atmospheric air alone, leaving you to figure out how much extra gas is needed to go along with the nitrous and squirting that along with it. The fact that you're squirting gas into the intake tract will wet the intake runners with gas and sometimes all those gas fumes in the intake can lead to an intake backfire, resulting in a dangerous explosion erupting from your air filter or manifold. Sounds cool but the main idea is that "wet" goes with extra gas wetting the intake runners.
Deciding on a dry or wet kit comes down to a couple of questions. How much power do you want to add and how complicated do you want your system to be? Nitrous is designated by how many additional horsepower it adds when you're spraying it. Therefore a "100 shot" kit will add 100 horsepower. If you want to add a lot of power, you are pretty much limited to a wet kit because it becomes exceedingly difficult to figure out how to trick the car computer into supplying that much extra gas when you're spraying. You could configure it to supply the extra gas every time you are at wide open throttle but that will cause drive-ability problems, knock and reduced engine life in the long run. There are some tricks and hacks to make the computer compensate for nitrous only when you're spraying it but generally speaking they aren't worth the hassle when you start dealing with serious horsepower so dry kits are almost always pushed as entry level kits. They are however very simple to install so if you're adding a small shot only, I'd recommend them. They're no worse in quality and are a pleasure to install and maintain. All you have to do is connect your nitrous to your intake tubing after the MAF sensor and you're good to go! Wet kits have a lot more plumbing and require that you either tap into your car's fuel rail to draw extra gas or create an auxiliary fuel tank/pump system to draw fuel from. So the rule of thumb is, if you want a lot of power, go wet. If you want a little power, go dry.
A Typical Wet Kit
All kits have the same basic components. You'll have 1 or more "bottles", which are nothing more than compressed air tanks to hold the nitrous. You will have to get these refilled regularly at a performance shop that supplies nitrous. Since nitrous is stored in liquid form, these bottles you buy are rated by how many pounds of liquid they hold. You can get anything from 12 oz. all the way up to 20 lb bottles. Inside the bottle is a siphon, which is like a straw that sucks the liquid from the bottom of the bottle first so you'll want to position your bottle in the car so that gravity draws the liquid towards the siphon rather than letting the siphon suck gas. After the bottle comes the solenoid. The solenoid is a high pressure valve powered by an electromagnet. You apply car voltage to it and it will open. Therefore it runs on a safety switch and push button. When you hit your nitrous button you're actually opening the solenoid valve by closing an electrical circuit and supplying power to it. After the solenoid you will have a bunch of plumbing in the form of hoses and fittings. These carry the nitrous oxide and gas in liquid form all the way to where they are sprayed. Somewhere down the line you will also have what are called "jets". Jets are very very important because they are the only means you have of controlling the dosage or flow of these liquids. If you've got a dry kit, the jets are sized to allow a certain "shot" that will give you a certain horsepower gain. Without the right size of jet you'll get a weak shot or you'll blow something up. In wet kits the jets are even more important because you need to jet the nitrous according to how much power you want and then you need to look up the corresponding fuel jet so that you supply enough fuel to go with it. If you get the fuel jet wrong and the mixture is lean, again, you will blow something up. Some wet kits have "fogger systems" which are plates or multiple sprayers that distribute the fluids from deeper inside the engine, like inside the manifold or ports. The logic here is to shorten the fluid's path to the combustion chamber so that the heat scavenging effect of the liquid nitrous isn't lost through the very hot metal walls of the intake passages. Wet kits will also have extra plumbing, a fuel solenoid, Y connections and possibly an auxiliary fuel tank and pump.
Nitrous Done Right!
Dry Kit Considerations
With dry kits, the installation is the easy part and your main concern will be where and how to get the extra fuel into the engine. If you have a really small shot you can mess around with pointing the sprayer at your car's Intake Air Temperature sensor, if it has one, and seeing if the extra fuel supplied by the car is sufficient. You'll want to tune the car a bit rich before doing this and ease it back later.
Another idea, again exploiting the temperature sensor is to bypass the sensor with a resistor. IAT sensors work by measuring the voltage drop across a resistor that changes resistance according to temperature. With a scan tool and a handful of cheap resistors from an electronics shop you can see which resistances correspond to which temperatures reported on the scan tool. Simply disconnect the IAT and jam a resistor into the wire harness. Then turn the car on and check the scan tool to see what temperature it reads. Anyway, find out which resistor corresponds to the lowest temperature the car is capable of registering. On my old car it was -40 degrees. Once you find the right resistor you want to create a relay circuit which can flip between "resistor only" and "IAT sensor only". You'll have to rig the relay into the same circuit as your nitrous push button so that when you hit the nitrous button, the car suddenly things it's -40 outside. You could stop here and see how much fuel the computer adds but if it's not enough, take it one step further!
Go into your tuning software and locate the IAT vs Spark vs AFR table or whatever table you can find that relates IAT to fuel (and spark if you can find it). The idea is to find the column in that table corresponding to -40 degrees or whatever you chose your resistor for and change all the values to add extra fuel and reduce the timing by a couple of degrees. Play around with the numbers until you get what you want but this is your best bet at making a dry kit work for you without too much hassle. It's a really great idea because your car will almost never see -40 anyway so this column is almost "reserved" for nitrous use only and will only activate when you push your nitrous button and trick the car with your phony resistor into accessing that column in the IAT table.
Another consideration with dry kits is that you don't point the nitrous jet directly at any sensors or BEFORE the MAF sensor. Nitrous is cryogenic and is so cold when it vaporizes that it will either drive a sensor haywire or will physically damage it so spray indirectly or in the case of a MAF sensor, not at all. Only spray downstream of the MAF sensor.
Just a Friendly Reminder
Wet Kit Considerations
Fuel, Fuel, Fuel. Unlike a dry kit, where you're aware of the need for fuel from the very beginning and are already worrying about it, wet kits lull people into complacency because the kit comes with all the parts and they figure they don't need to worry as long as they install everything correctly. Not so. Your car's fuel system has a certain margin for supplying extra fuel. It could be a lot or a little. If your fuel pressure regulator can't keep up with the extra fuel demand, the pressure may collapse while you're spraying and you'll get a lean spike followed by an expensive explosion. The bottom line is that you have to check and possibly upgrade your fuel pump and fuel pressure regulator if you're using the existing fuel system. If you're using an auxiliary fuel system, just make sure it supplies enough pressure for the jetting you've selected.
Get colder spark plugs. What do I mean by colder? Spark plugs have heat ratings which are sort of a measure of how hot the tips get, which is a function of tip material and geometry. Copper spark plugs are not as durable and are somewhat more prone to misfires than platinum or iridium but they have the colder heat ratings. You'll want to select plugs that are 2 heat ratings colder than stock if you're adding serious power from your nitrous kit or maybe 1 rating colder for weaker kits. Dry nitrous guys may need to replace their plugs too but as I said before, dry kits tend to be weak by design so it's less of an issue. Don't use platinum plugs with nitrous. The nitrous will eat the plugs in no time. Copper is the best according to many kit owners. The colder plugs will prevent premature detonation.
A Warning For Both Kit Types
Never spray nitrous at any time except when the car is at wide open throttle. More specifically, don't spray if the car isn't in power enrichment mode because you could go lean and blow up your engine. Don't spray if the engine is bogging, as it would in a manual if you've selected too high a gear and your RPMs are low. The best way to deal with the former is to install a limit switch on the throttle body so that when the throttle cable pulls the throttle plate to 100%, it activates the limit switch mechanically and enables your solenoid circuit. Some cars have electronic throttle and you can't do this mechanically but there are other ways. You shouldn't rely on your own reflexes to arm the solenoid circuit manually because if you make a mistake, it'll be an expensive and potentially dangerous one.
Never store nitrous bottles inside the passenger area. Make sure a firewall separates you from the tank for your own safety in case it explodes.
Mount the bottle on an angle so that the siphon picks up liquid nitrous no matter how empty the bottle is.
Try not to use the kit when the bottle is almost empty. Better to waste a bit of nitrous than get an erratic air-fuel mixture and possibly damage the engine. Also, adding a purge valve can be a good upgrade for your kit because it will get rid of all the gas and fill the line with liquid just before you're about to spray. This makes performance consistent and maintains an even AFR.
Don't spray more than your engine can handle. Most stock engines should be able to handle 25% extra power without modification but that's by no means a rule. It's just an educated guess. You could have head gasket failures, cracked pistons, burnt piston rings, you could throw a rod, damage valves... don't risk your engine over a 2 dollar jet. Jet your nitrous according to the limits of the engine. For this you need to do some homework. Talk to other nitrous users who own your car. Look up the manufacturer specs of the engine. Ask around in the forums. Start by jetting low and go up in small increments. For 20 bucks you can get quite a few jets which will make adding power very quick and easy without adding huge risk. If you have enough jets you'll see signs and symptoms of problems before they become serious and then you'll know to switch back to a lower one.
Juice it Up!
Nitrous is incredibly fun, relatively cheap for what you get and most kits are quick and easy to install with basic mechanical skills. With great power comes great responsibility though so make sure you get it right the first time and you can relax later on and enjoy your shiny red push button :D