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Everybody has their favorite euphemism for it – fast gas, throttle-in-a-bottle, chemical supercharging, or just simply NOS. Nitrous oxide has become a favorite boost to engine performance that is easy to install, delivers an instantaneous horsepower and torque increase, and is among the least expensive ways on the planet to make horsepower. For those unfamiliar with nitrous oxide or N2O, let’s take an abbreviated dive down the rabbit hole and see what we can learn about getting a running start on this power-adder.
It might be a good idea to first address why a nitrous system could be a better power-adder rather than a turbocharger or supercharger. Much of this comes down to personal preference, but nitrous systems are generally less expensive than a supercharger and far less complex than the selection process for turbochargers and certainly far less cumbersome to install compared to a turbo’s custom fabricated plumbing demands. Another advantage to a nitrous system is that it does not affect engine operation when not in use, unlike big cams or monster cylinder heads added to a normally aspirated engine. Perhaps the only disadvantage to nitrous is that a 10-pound nitrous bottle for a 150-horsepower kit is generally only good for about three or four quick quarter-mile passes before it will need to be refilled. That’s why you’ll see serious nitrous users with multiple bottles.
In a plate system, like this Crosshair unit, the plate fits between the carburetor and the intake manifold. While nitrous is often associated with a single plane manifold, a plate system will work just fine on a dual plane intake manifold. When it comes to making connections to the plate, fuel connections are always red and nitrous connections are always blue.
To begin, let’s describe a simple nitrous kit bolted to a carbureted small-block and use that as our base from which to build our knowledge of how nitrous works. The kit consists of a high-pressure nitrous bottle, two solenoids (one each for nitrous and gasoline) that will be electrically actuated by a driver-operated switch. The bottle is connected via a high-pressure hose to the nitrous solenoid while the gas solenoid taps into the fuel delivery line. Short lengths of tubing connect the nitrous and gas solenoids to a thin plate positioned underneath the carburetor. This plate contains a spray bar fitted with multiple small holes that introduce both the nitrous and the gasoline into the manifold when the system is triggered.
If your interests lie with late-model, fuel injected engines there are nitrous kits for those applications as well. Some kits introduce both nitrous and fuel from a plate behind the throttle body and are referred to as “wet” kits which introduce the required fuel in the manifold. An example of this is a Holley Sniper system for ’97-2012 GM LS truck engines.
Conversely, NOS also offers kits designed as “dry” applications where nitrous alone is introduced behind the throttle body. Additional fuel is then added by an innovative device that applies an adjustable amount of nitrous pressure to the engine’s fuel pressure regulator during nitrous operation. This increases the fuel pressure the desired amount to maintain a proper air-fuel ratio. Other more sophisticated dry nitrous systems rely on changes to the EFI software to add the proper amount of fuel through the injectors. These are generally used in higher horsepower applications and are especially useful when combined with an aftermarket EFI controller such as Holley’s HP or Dominator system.
So far we’ve covered what could be called centrally-located or single position nitrous systems employing a plate under the carburetor or the throttle body. Another method of controlling the amount of nitrous and fuel entering each cylinder is to upgrade to a multi-point or direct port injection systems. These systems use what NOS calls fogger nozzles that direct fuel into the high-pressure nitrous spray, causing the fuel to disperse more evenly, which is generally beneficial for power.
On a V8 engine for example, the system will employ eight individual fogger nozzles, one for each port. Each of these nozzles will include both a nitrous and fuel jet, allowing the user to individually tune the amount of fuel and nitrous for each cylinder. These fogger nozzles are generally plumbed with individual fuel and nitrous hard lines leading to a larger nitrous and fuel solenoids. Most often, one pair of solenoids is used for each side of the engine, requiring a total of four solenoids. With this increased flow capacity, direct-port kits can deliver upwards of 500 horsepower and often builders create separate stages with two fogger nozzles per cylinder – this is where the pro’s play.
Face it, chemistry is only fun for those who prefer to dine around the periodic table. For the rest of us, chemistry and all those confusing terms can be a bit intimidating. But bear with us as this geeky tech stuff has useful purposes. Nitrous oxide (N2O) is a molecule consisting of two parts nitrogen and one part oxygen. If we squeeze it to a pressure of 760 psi, it will begin to transform from a gaseous form into a liquid. Now we have a much more dense and powerful form of nitrous.
The beauty of this gas is its one part oxygen. A common belief is that the atmosphere that we breathe is mostly oxygen, but the reality is that 78 percent of the air we breathe is really nitrogen and only 21 percent is oxygen with that last one percent a gaggle of other chemistries. This means that N2O contains significantly more oxygen per unit than our atmosphere.
This is important because oxygen is the oxidizer that supports combustion. Cover a candle with a glass jar and eventually the fire goes out due to a lack of oxygen. So if we combine the right ratio of fuel and air, squeeze it real tight and light it with a strong spark, we will have combustion. To enhance that normally-aspirated combustion process, we can inject a measured amount of N2O into the cylinders along with the proper ratio of additional gasoline, we can make a lot more power – hence nitrous’ nickname – chemical supercharging.
Note how this NOS pressure gauge offers a somewhat narrow green (optimal) range between 900 and 1,000 psi. The yellow areas represent either too high or too low pressure for best performance. As mentioned in the text, heat is sometimes needed to raise bottle pressure to between 900 and 1,000 psi. A 12-volt heater blanket like this one is the best way to accomplish this in the car. Never apply open flame to a pressurized bottle to increase pressure.
The easiest way to store this nitrous gas is in a spun aluminum tank designed to handle extreme pressure. The best way to store and use nitrous is under sufficient pressure to turn the gas into a liquid. That happens above 760 psi. Some nitrous fans think that more pressure always equals more nitrous. But this is not so because nitrous is a fickle beast. It begins to transform back into a gas if we allow the pressure to exceed 1,069 psi. So this places the optimal operating pressures of nitrous between 850 and 1,000 psi. According to our chart, this means that bottle temperature should be between 80 and 90 degrees F.
Nitrous temperature and pressure are directly related as our chart illustrates. The best way to monitor nitrous pressure is with an accurate gauge. But for the casual user, if you create a small chart listing temperatures and pressures, you will have a close approximation of operating pressure by reading the bottle temperature using an inexpensive infrared thermometer. This won’t be as accurate as a quality pressure gauge, but it will get you close.
We’ve emphasized bottle temperature and pressure because nitrous pressure plays a huge part in ensuring the system will deliver its maximum horsepower potential. But beyond pressure there are other variables that also demand attention. Let’s take a typical carbureted plate system as an example. Among the most popular systems is the NOS Cheater system that includes changeable jets for the plate that allow you to tune the system for multiple horsepower levels from 150 to as much as 250 horsepower. But as the horsepower levels increase, this also requires some additional changes to the engine.
This is a NOS Fogger nozzle up close. Fuel and nitrous are introduced in the body of the nozzle and mixed at the tip creating a fuel and nitrous fog that will easily combust. Nitrous and fuel can be Individually tuned with jets at the nozzle.
We won’t get into specifics, but nitrous horsepower levels beyond 100 horsepower will require modifications or changes to the tune. Some of these include increasing fuel octane, an adequate fuel delivery system to maintain fuel pressure, retarded ignition timing for safe combustion, colder spark plugs, and other changes that the tuner will need to perform to ensure optimal performance. NOS offers tuning charts that will help you determine the safe application of increased nitrous horsepower. These tuning changes are not that much different from mods required on other power adders to maintain safe engine operating conditions.
We’ve only just skimmed the surface of the wild world of nitrous oxide. Depending upon your specific goals, there are hundreds of ideas for using nitrous to get where you want to go. For example, you may want to not announce exactly your car’s true potential. It’s impossible to hide a 6-71 or a centrifugal blower on an engine. But you can hide a nitrous system. There are some very creative ways to make that work for you and slide it right past even the most observant on-looker.
When it comes to the most fun-per-dollar, you can’t beat a simple-to-install nitrous system. When you hit the button for the first time, you’ll know what we’re talking about!
Among the most affordable and popular NOS kits is the Cheater kit designed to be used with a 4150 Holley four-barrel carburetor. The kit includes a 10-pound bottle, two solenoids, nitrous plate, gaskets, tuning jets, and all the brackets, nitrous plumbing, and hardware needed to install the system. All fuel fittings are colored red while nitrous is blue.
Most, but not all, NOS kits come with jets that allow the user to tune the horsepower level. In the instructions, NOS will offer jetting recommendations that combine a recommended fuel jet with a given size nitrous jet. In all systems, the jets are easily changed by removing the AN fitting at the plate or the nozzle. Older NOS kits employed brass jets but all current NOS systems come with stainless steel jets as shown here.
Plate systems introduce nitrous on one end of the plate and fuel in the other that fill these small tubes. Each tube has a series of strategically placed holes that allow the nitrous and fuel to enter the manifold. The fuel rail is located underneath the nitrous tube so the nitrous (in purple) can break up and help atomize the fuel (in red) as shown in this video image.
This is a plate for a wet nitrous for a 90mm LS3. A dry system plate will look similar except there will be no fuel connector. Fuel enrichment for dry systems is supplied by the injectors. This plate can support up to a 250 rear wheel horsepower increase if the car has a sufficient capacity fuel delivery system.
While buttons are okay for simple systems, throttle-mounted switches are easier to operate. This NOS micro-switch bracket is designed for mechanical secondary Holley carburetors and mounts to the passenger side of the carburetor. With a minor tweak to make sure the mechanical secondary opens fully, we have the perfect WOT nitrous switch.
All high-pressure bottles must include a pressure relief valve. This small valve is located on the bottle valve and is intended as a safety device to prevent overpressure in the bottle. The valve is designed to open at approximately 3,000 psi.
For most high capacity nitrous systems, the nitrous solenoid (left) will be larger than the gasoline solenoid because with nitrous pressure upwards of 1,000 psi, it requires more electrical power to open the valve against this pressure.
If you pay attention to nitrous drag strip performance claims, you may wonder why some vehicles seem to perform better than others running the same kit. To explain this, let’s use the scenario where we’ll install the same 150 horsepower nitrous kit on two different cars. The first is a mild 350ci-powered ’72 Chevelle while the second will help a ’67 454ci Rat Chevelle. For this example, let’s say the mild 350 runs 13.60’s at 99 mph, while the big-block is substantially quicker running 12.50’s at 109 mph.
After installing the same 150 horsepower Cheater nitrous kit on both cars, the small-block runs a really quick 12.60 at 109 mph. The big-block is much quicker with an excellent 11.90 at 116 mph. In this example, the small-block car gained one full second. The big-block car improved by 0.60-second.
The explanation offers some insight into the hidden advantage of nitrous. The accompanying graph shows a normally aspirated power curve we created as TQ1 and HP1 while the nitrous is graphed as TQ2 and HP2. The peak horsepower improvements are slightly more than the advertised 150 horsepower. But careful examination of the torque curve shows a massive amount of torque gain in the mid-range rpm around 4,000 to 4,500 rpm. This assumes the nitrous was engaged at 3,600 rpm.
A typical small displacement street engine like the mild 350 does not make much torque at low engine speeds. We’ll assume 420 lb-ft of peak torque for this engine. The main portion of the elapsed time improvements for both cars is achieved closer to the starting line. By energizing the nitrous at 3,600 rpm (below peak torque), the small-block generates a significantly greater percentage of torque improvement compared to the big-block. This nitrous torque spike is responsible for the car’s quicker e.t.
The big-block, which is renowned for its normally-aspirated torque, gains the same amount of torque as the small-block but because it is already accelerating quicker, the gain is not as beneficial. Plus, this may also create greater traction issues with the big-block. Peak horsepower gains for both cars are still 150+ horsepower but the smaller displacement engine will usually generate a substantially greater e.t. improvement.
Of course, there are a couple of dozen variables that we did not address in this example like gear ratio, converter stall and the like, but the point is still valid. Smaller, milder street engines generally see greater improvements in acceleration (assuming good traction) compared to larger, more highly-tuned engines.