Mike Mavrigian has written thousands of technical articles over the past 30 years for a variety of automotive publications, in addition to writing nine automotive technical books for four different publishers. Mike also owns and operates Birchwood Automotive in Creston, Ohio, where he builds custom engines, street rods and performs vehicle restorations. Mike can be reached at 330-435-6347 or . Birchwood’s website is

What is nitrous oxide? Nitrous oxide is a cryogenic gas composed of nitrogen and oxygen molecules, and is comprised of 36 percent oxygen by weight. Nitrous oxide alone is non-flammable and is stored as a compressed liquid. Two distinct grades of nitrous oxide exist-U.S.P. and Nitrous Plus. U.S.P. is medical-grade nitrous oxide commonly used in dental and veterinary anesthesia, and is not available to the general public. Nitrous Plus is intended for automotive applications and contains trace amounts of sulfur dioxide to prevent substance abuse (this stuff will make you happy when you whap the throttle, as opposed to inhaling it). In automotive applications, a combination of Nitrous Plus and fuel is injected into the engine's intake manifold, which lowers the intake air temperature, producing a dense inlet charge. Nitrous Plus/fuel also increases the inlet charge oxygen content (air alone is only 22 percent oxygen by weight). This nitrous/fuel induction increases the rate at which combustion occurs. Basically, it's a steroid to boost combustion, elevating cylinder pressure and cylinder temperature while increasing the combustion rate. Direct-port style injection kits are intended to provide maximum performance and tenability. Horsepower increases from these kits will vary with engine displacement and configuration. Approximate power increases can be estimated based upon the massflow of nitrous into the engine. The preceding table provides the power increases that can be expected on a typical engine. The system shown here is the NOS Pro Fogger kit (P/N 04430), a single-stage system intended for maximum-effort competition engines only. NOS (the maker of this system) suggests several engine upgrades if you intend to increase engine output by more than 40-50 percent. These include the use of forged pistons, forged or steel billet or aluminum connecting rods, main support (girdles, sleeves) and cylinder head studs to resist cylinder head lifting. A high-output ignition system is also strongly recommended.


Adequate fuel delivery and pressure is an absolute must. Fuel pumps and lines should be able to flow at least 0.1 gallon per hour, per horsepower at 6 psi (for carbureted applications) or at system pressure in a fuel injected engine. For example, at 42 psi flowing, a motor that produces 450 horsepower while the nitrous system is activated will require at least 45 gph at 42 psi flowing at WOT (wide open throttle). Note that aftermarket pumps may be rated under free-flowing conditions and at system pressure their flow rates may be reduced. Check the fuel pump maker's specs carefully or have the pump checked before using it. EFI jetting is applicable to vehicles that operate at 40-40 psig at WOT.


While many intake manifolds feature already-tapped holes to accept nitrous nozzles, or feature blank bosses that can readily be drilled and tapped, some intakes require welding bungs onto the runners. The specific intake manifold shown here is a "sheet metal" welded aluminum manifold with no provision for tapping (the manifold runners were not thick enough to accommodate tapped 1/16" NPT holes), so a set of aluminum bungs were welded onto the runners. The bungs feature 1/16" NPT female threads to accommodate our nozzles.


The 12' -6 AN hose (supplied) connects to the bottle, using the bottle valve adapter and sealing washer. The -6 hose then connects to the nitrous solenoid side inlet. The nitrous solenoid connects to and feeds a distribution block. Stainless steel hard lines route nitrous to each fogger nozzle (in our Honda's case, we have one nozzle per cylinder), which is threaded into each intake runner. In addition to the nitrous charge, the system also requires an additional fuel feed since a combined nitrous and fuel charge is delivered when the system activates. The "Cheater" fuel solenoid is fed by a fuel source (preferably a separate and dedicated fuel tank), and connects to and feeds another distribution block. Stainless steel lines route from this fuel block to each nozzle. The fogger nozzles feature two inlet ports (one for nitrous and one for fuel, each of which are labeled) and a single outlet nozzle that outputs the combined nitrous/fuel charge into the intake runner. The nozzles may be of the annular or soft-plume type. Annular nozzles output at the tip of the nozzle. Because of this directional output, the nozzle must be installed at an angle to aim the charge toward the intake port. The soft-plume nozzle features a directional (90-degree) output, allowing the nozzle to be installed 90 degrees to the intake port. The nozzles feature male 1/16" NPT threads. If the intake manifold runner walls are thick enough (for example on a cast aluminum unit), simply drill a 1/4" hole and use the kit-supplied 1/16" NPT tap to create threads. If the runner walls are not thick enough, custom bungs can be welded to the runners. In either case, be sure to tap the 1/16" NPT holes just deep enough to allow the discharge orifice to protrude into the runner. If the hole is tapped too deep, the NPT nozzle threads may not seal. When soft-plume nozzles are used, pay attention to nozzle orientation (clock position) when installing, making sure that the discharge orifice aims toward the intake port. Once the nozzles are installed into the manifold (and you've decided where the solenoids and distribution blocks will be located), you'll need to make the nitrous and fuel feed tube assemblies. The kit includes a batch of 12" long stainless tubes, connection features flares, tubing sleeves and barrel nuts where the lines connect to the nozzles, and straight-end (no flare) and brass compression fittings where the lines connect to the distribution blocks. The nozzle-end tube sleeves and barrel nuts are color-coded-red is for fuel and blue is for nitrous. The tube lines must be cut to length, flared at the nozzle end and bent according to the individual application, both for fit and for aesthetics. For this task, you'll simply need a tubing cutter, a flaring tool and a tubing bender. No surprises here. Since appearance is an important consideration, it may be best to first mock fake lines using scrap tubes or coat hanger wire to finalize your lengths and shapes. Just remember to slip the tube sleeves and tube nuts onto the tubing and butt them up against the flared ends before any sharp bends are made, otherwise you won't be able to slide the tube sleeves and nuts over the bends. Note: The brass compression fittings are supplied with the distribution blocks. They're held captive inside the distribution block's tube nuts, so be careful when you remove the nuts to avoid dropping the compression fittings. As you're fitting the lines, be sure to select and install the supplied flare jets into the feed ports of the nozzles for both fuel and nitrous. All jets are marked for size, but the markings can be very difficult to see. I suggest having a magnifying glass handy to identify each. With the jets installed into the nozzles, you can design and install the tubing assemblies. While many installers rely on the stainless nitrous and fuel lines to support the distribution blocks and solenoids, we opted to fabricate a mounting bracket that provides support from the intake manifold plenum. The bracket (a wing-shaped piece cut from 1/8" high-temper aluminum) is mounted directly to the intake manifold at the extra-thick welded-on base plate that the Venom intake features for vacuum connections. This plate thickness is about 0.185". We drilled two 0.1360" blind holes and tapped these at a depth of 0.170" using an 8 x 32 bottoming tap. The plate is then attached using a pair of 8 x 32 stainless socket head bolts with a shank length of 0.340". The individual nitrous and fuel blocks and solenoids are attached to this bracket. Just for giggles, we also drilled a series of "lightening" holes in the bracket for added visual appeal. Whenever given a choice, we tend to always choose the more time-consuming approach, mostly because we're just plain goofy. Note: When you assemble the solenoids to the distribution blocks and the tubing to both the blocks and nozzles, pay attention to thread sealing. Apply Teflon-based thread paste to all of the fuel and nitrous fittings that screw into the solenoids, to the feed fittings that connect the solenoids to the distribution blocks and to the nozzle threads where they screw into the manifold. Do not use Teflon tape, since there's a chance that small pieces of tape can accidentally enter the system. Always use Teflon paste instead. Don't use any additional sealer at either end of the tubes. The compression fittings and nuts will seal the distribution block-to-tube connections and the flared AN connection at the tubing-to-nozzles will take care of that seal. Only apply thread sealant to pipe threads.