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How Do Nitrous Nozzles Work?

By: Jeff Smith | 10/23/2020 < Back to Motor Life Home

Everybody knows how nitrous works. It’s pretty simple. Compress the gas to between 850 and 900 psi, direct it into the engine with a specific nozzle size, add a measured amount of additional fuel so it doesn’t run lean – and whammo – you have instant horsepower at the touch of a button.

Okay, it’s a little more specific than that generic description, but most get the general idea. So, we thought we’d dive into a few specifics about nitrous nozzles. Carbureted engines will most often employ a plate using a pair of spray bars. EFI engines can use similar technology, but more often the EFI guys, and plenty of carb guys as well, prefer more sophisticated nozzles to introduce nitrous into the engine.

Let’s start the fact that there are two kinds of nozzle configurations – wet and dry flow. Wet nitrous uses a nozzle that combines nitrous and fuel right at the exit. This can be in the form of a single nozzle located either in front or behind the throttle body or multiple nozzles placed at the base of each intake manifold runner.

A dry system employs a nozzle that introduces only nitrous into the airstream. Since additional fuel is still necessary, the fuel is supplied by the fuel injectors commanded by the ECU. There are no dry carbureted nitrous systems because there’s no provision for the additional fuel. Dry EFI systems generally require an aftermarket ECU that has nitrous fuel enrichment control features. We won’t go into all those details but all of the Holley stand-alone EFI controllers like the HP, Dominator, Terminator X, and some Sniper units offer dry nitrous fuel enrichment capabilities.


The simplest nitrous system to plumb on an EFI engine would be with a single outlet dry nozzle placed ahead of the throttle body. This would have to be used in conjunction with an aftermarket EFI controller so the fuel can be added through pulse-width control of the injectors. Of course, this also assumes that the fuel delivery system is sufficiently robust to handle the additional fuel requirements.


The first and foremost concern is to ensure that the fuel pump and injectors can supply the additional fuel required. For example, if you want to add a mild 100 hp dry nitrous package to your EFI engine, the injectors will need to be capable of delivering an additional 50 pounds or more of fuel per hour (lbs/hr). This means on a V8 engine, that each injector would need to be able to contribute another 6 to 8 lbs/hr more fuel. Many OE EFI injectors are not capable of supplying that additional fuel flow, so that’s something you need to consider before investing in a dry flow system. Adding larger injectors can trigger a chain of tuning requirements since the system must then be custom tuned to accommodate the increased fuel flow.

Even on EFI engines where aftermarket ECU control is not available or desirable, there are still many ways to add nitrous. As one example, a single wet Fogger nozzle works very nicely. A Fogger combines fuel with nitrous with two separate inlet ports each of which is tuned by a jet to balance the fuel enrichment with the horsepower bump metered by the nitrous jet. Fuel for the nitrous system is generally tapped from the existing fuel delivery system.


Nearly all OE EFI systems offer a Schrader valve in the fuel rail to be used as a tap for checking fuel pressure. If this valve is removed (it looks just like the tiny valve inside a car tire) this creates a perfect tap into the engine’s fuel delivery system for nitrous enrichment. Of course, this assumes that the fuel delivery system can supply the additional fuel required to feed the nitrous. This is true with both dry and wet nitrous systems.


Generally, these single outlet nitrous kits (either dry or wet) are used with conservative horsepower ratings of 75 to100 hp so as not to outrun the factory fuel supply.


Here’s an example of a wet Fogger system installed on a single plane intake manifold. Each nozzle can be individually tuned if desired. The advantage of port nozzles is that each cylinder will receive the desired nitrous and fuel mixture which eliminates variations in cylinder filling due to manifold design.


If a more serious nitrous application is in your crosshairs, the most popular ploy would be the multi-port NOS Fogger nozzle. Here is where you will begin to rub elbows with the serious nitrous users. This approach applies a separate nitrous nozzle for each individual cylinder. This allows the tuner to custom tune the amount of nitrous and fuel delivered to each individual cylinder. These are generally reserved for serious drag race applications and can often involve multiple stages. In this case, you may see either multiple nozzles in the same port, or a single nozzle accompanied with an additional plate system positioned under the carburetor(s) or throttle body.


An alternate approach to a staged system can be created using one large system capable of 300 hp or more but controlled by way of either a separate mini-computer controller (for carbureted applications) or by an EFI ECU that has the capability of pulse-width modulation (PWM) control of the solenoids. PWM control (as the name implies) pulses the solenoid open and closed at varying rates to create a continuously changing rate of nitrous delivery.


This allows a 300 hp system for example to start delivering around 100 hp worth of nitrous where the solenoid is only open approximately one-third of the time. The system then gradually increases the rate of open time until the solenoid reaches full open or what engineers call duty cycle where a solenoid at open third open would be a 33 percent duty cycle and fully open would be at 100 percent.


Annular discharge nozzles like this Fogger are designed to exit the nitrous parallel to the fitting. According to NOS engineers, there is no power difference between right- angle Foggers and 180-degree annular discharge styles. It becomes more of an application question of which is a better fit for the manifold.


With all of the different design approaches covered, there’s more to the story. Through the years, NOS has come up with several unique design variations on the classic Fogger nozzle. Even in the earliest days, the Fogger was intended to create an extremely fine mist of fuel. Every student of combustion science knows that fuel in a liquid or droplet form does not burn very efficiently, if at all. Conversely, vaporized fuel burns quickly and efficiently.


The wet Fogger nozzle is designed to inject gaseous nitrous at between 900 and 950 psi across a stream of fuel leaving the Fogger nozzle. The high-pressure nitrous then helps to break up the fuel droplets, creating a vapor that will quickly mix with the nitrous and burn more efficiently in the combustion process.


While the original Fogger nozzle works very well and has a loyal following, racers and horsepower seekers are always looking for a better mouse trap. This original Fogger was intended to be inserted into the intake manifold runner, usually near the manifold base. The nozzle intrudes slightly into the intake port and is designed to spray nitrous and fuel at a 90-degre angle.

But as in all things mechanical, the Fogger may not be ideal for all applications. This pushed the development of what NOS calls the annular discharge nozzle. As a wet nozzle design, this configuration sprays fuel out of six smaller discharge holes in line with the nozzle body with nitrous exiting from a larger center hole. As with the original Fogger nozzle nitrous is used to break up the liquid fuel into a fine fog of fuel and nitrous. This spray is designated as a 180-degree exit, meaning that it exits in line with the nozzle. The annular version is intended to not intrude as far into the port and is usually intended to enter the port at more of a 30 degree or smaller angle.


Beyond the physical plumbing, there are also some tuning aspects that are worth at least a casual introduction. The previously described nozzles are all generic and are designed to be used in conjunction with specific NOS tapered jets. These perform a similar duty to main circuit jets in a carburetor by establishing either fuel or nitrous flow for a specific application.


Earl’s also makes a slick billet fuel line adapter fitting for GM 3/8-inch quick-disconnect fittings. This fitting slips over the GM fuel hard line but offers a separate tap that can easily route fuel to the fuel solenoid. Note also the separate NPT fitting that can be used to monitor fuel pressure.


As you can imagine, in a single point delivery system like a single nozzle system, the jet size will be rather large to accommodate the total system flow. For example, a single dry Fogger nozzle for a 150 hp hit will be generally (individual kits may vary) a 62 nitrous jet. This means the diameter of the jet is 0.062-inch. This will obviously change when converting to a multi-point system with eight individual jets. For example, the NOS tuning jet recommended for a multi-point 150 hp Pro Fogger system uses eight number 22 nitrous jets.


It all comes down to the flow area of the jets. We’ll skip showing our work on the math but if we calculate the area of a 22 jet and multiply by 8, this produces a number only slightly larger than the single orifice are for a 150 hp system.

In addition to nitrous systems, NOS offers multiple controllers that allow for progressive nitrous actuation and can command multiple nitrous stages.


This can put system tuning into perspective, with an eye toward perfection since a slight flow variance with relatively small (0.022-inch) flow jets can produce different results when it comes to air-fuel ratio. Among the variables a good tuner must deal with is bottle pressure, fuel pressure, ignition timing, fuel octane, individual cylinder differences or preferences, and of course the actual flow rate of each individual component within the system. Keeping control over these variables has multiple benefits including consistent engine performance, easier predictions of results, and a happier engine that can live a long life under high output.


So now you have a working knowledge of the different wet and dry flow nitrous systems and also an introductory perspective on nitrous tuning. Nitrous is a great way to make big horsepower very easily but it also does require some tuning experience once you get into the higher horsepower systems. But no one will argue that is great fun when you hit the button!

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