Exploring Nitrous - Part Two

In Part One of this series we explained how injecting nitrous-oxide – along with enrichment fuel - gives your engine more power. We also looked at the basics of an off-the-shelf aftermarket nitrous system. Now it's time to look at the different types of nitrous set-ups...

Types of Nitrous Systems

There are three categories of nitrous-oxide systems – dry manifold, wet manifold and direct-port.

Dry manifold systems are becoming more popular as the bulk of today's cars employ electronic fuel injection. In a dry manifold system, the nitrous-oxide can be injected anywhere between the intake ports and upstream of the throttle butterfly (or just after the air filter, as seen here).

The enrichment fuel is delivered through the existing fuel injectors or dedicated fuel nozzles near the mouth of the intake ports. As you've probably guessed, the dry manifold name is derived from the lack of fuel in the intake manifold.

In some EFI nitrous set-ups the existing injectors can be used to inject the normal fuel quantity in addition to the enrichment required for nitrous operation. If the OE injectors are being used you will probably need increased fuel pressure to provide the necessary extra fuel flow. A high-flow fuel pump may also be required.

Alternatively – if the existing injectors are large enough – you can take the option of increasing injector duty-cycle when the nitrous system is activated. A programmable management system with switchable maps is ideal for this purpose.

Note that mounting the nitrous nozzles closer to the intake ports gives a noticeable nitrous 'hit' but provides less intake air cooling and mixing.

A wet manifold nitrous system employs a flange plate where the nitrous-oxide and enrichment fuel are injected together. The flange plate is sandwiched beneath the throttle body (or carb) and intake manifold. In some applications, however, the nitrous and enrichment fuel can be injected about 10 – 30cm upstream of the throttle to give slightly better mixing and a greater cooling effect. As the name implies, a wet manifold system uses an intake manifold that's wet with fuel.

A wet manifold system is very well suited to turbo and supercharged engines. By injecting the nitrous and enrichment fuel a long way upstream of the intake ports, it has plenty of time to mix and to cool the compressed intake air. A wet manifold nitrous system is said to reduce manifold air temperature up to 18 degrees Celsius.

This configuration does not provide the sudden jolt that you get with systems where nitrous is injected near the intake ports.

The final system is known as direct-port nitrous injection. This set-up involves injecting the nitrous and enrichment fuel directly into each intake port. This means that, for a six-cylinder engine, you'll need six nitrous/fuel nozzles. The advantage of this approach is perfect cylinder-to-cylinder nitrous and fuel enrichment distribution with a firm 'hit'.

On the downside, there is only a short distance for the nitrous and fuel to mix. This generally requires fogger-type injection nozzles that help to mix the nitrous and enrichment fuel. Individual jetting of the nitrous and fuel enrichment nozzles for each cylinder also allows you to compensate for any intake airflow variation between cylinders.

Nitrous System Tuning

There are a few key points when it comes to tuning nitrous injection.

Control of Nitrous Temperature and Pressure

Nitrous systems are very sensitive to temperature and pressure variation. At atmospheric pressure and temperature nitrous oxide exists in a gaseous state. However, when compressed to 760 psi and at around 21 degrees Celsius, it changes to a liquid.

Nitrous Gas v Nitrous Liquid When nitrous-oxide is injected into the engine in gaseous form it gives less power than it does in liquid form. This is because nitrous-oxide gas is less dense and, therefore, a given mass of nitrous displaces a larger amount of induction air. This is wasted oxygen. When nitrous gas is delivered into an engine you also lose the cooling effect of nitrous liquid. Liquid nitrous has a super-cooling effect during the transition into a gas – this tremendous cooling effect lowers the temperature of induction air and leads to even more power.

Nitrous-oxide delivers peak performance when the nitrous-oxide is delivered into the combustion changer in liquid form. The challenge, therefore, is to keep the nitrous as a liquid – this means maintaining a bottle pressure of more than 700 psi and keeping it at a relatively cool temperature.

And there's a second major reason to keep tabs on nitrous temperature.

An uncontrolled increase of nitrous bottle temperature causes an increase of pressure. When nitrous pressure in the bottle increases, a greater quantity of nitrous will be forced into the engine when the system is activated. This upsets the existing nitrous/fuel enrichment calibration causing lean mixtures and probable engine damage.

Most aftermarket nitrous kit manufacturers suggested that the bottle should be kept between 18 and 27 degrees Celsius and at a pressure between 750 and 1000 psi. Most systems will perform near their best across this range; some systems incorporate automatic pressure compensation.

Depending on temperature variation in your area, you may need to employ measures to cool the nitrous bottle or to warm it above ambient. There are many ingenious ways to address temperature control, but the neatest approach is to use a thermostatic control system that is sold by some manufacturers.

Fuel Enrichment Tuning

In addition to keeping an eye on nitrous temperature and pressure, the importance of adequate fuel enrichment cannot be overstated.

Nitrous-oxide is a form of supercharging and – just like bolting on a turbo or blower – you must increase fuel flow into the engine to maintain safe mixtures.

The amount of fuel enrichment required to maintain safe mixtures varies from engine to engine. As a rule of thumb, however, it is suggested that a naturally aspirated engine will provide near optimal power and reliability with a slightly richer mixture than you'd run without nitrous. Some sources suggest these rich mixtures are necessary to compensate for the increased flame temperature associated with nitrous – other sources disagree!

Whatever the case, don't go overboard with fuel enrichment or else you'll lose power and cause "bore wash", which leads to bore wear.

How is fuel enrichment calibrated, you ask?

Well, in all nitrous systems - except those that rely on the injectors - enrichment can be varied by changing small jets inside the fuel delivery nozzle. These jets are effectively just a restriction in the fuel delivery line - a large jet allows a greater amount of fuel into the engine, thereby delivering a richer mixture than you'd achieve with a smaller jet.

Thankfully, most of the guess work has been removed from this 'black art', which is reminiscent of carburettor jetting. The majority of kit manufacturers now provide the appropriate fuel nozzle to accompany a given nitrous 'shot'.

Nitrous Jet Tuning

The horsepower boost with a specific nitrous kit can be fine tuned by changing the jets that fit inside the nitrous nozzle (similar to the jets in the fuel enrichment nozzle). Again, a larger diameter jet allows a greater flow into the engine - a bigger nitrous jet means more nitrous and more power.

Note that the nitrous jet must cause the biggest flow restriction in the nitrous supply line. If there is not enough restriction near the nozzle there's a chance that the nitrous solenoid might freeze open. This allows nitrous-oxide to continue into the engine, even when the system is electronically deactivated. The result is invariably a catastrophic engine melt-down...

Hang around for Part Three (the final) of this series. We'll look at a couple of other nitrous set-up considerations and compare how certain systems perform in the real-world.