Exploring Nitrous - Part Three

In this – the final part of our nitrous-oxide series – we’ll check out the difference between basic constant-flow and multi-stage set-ups. We’ll also discuss avoiding detonation and look at some typical nitrous system outcomes.

Constant Flow v Multi Stage Nitrous Delivery

In previous parts of this series we’ve looked at the outcomes achieved with different nitrous and fuel enrichment nozzle locations. Arguably as important, however, is a controlled rate of nitrous-oxide delivery. Most aftermarket nitrous systems are constant-flow type but some offer multi-stage progression.

Let’s look at the differences...

Constant-flow nitrous systems (which are the cheapest and most common) deliver the same flow of nitrous-oxide into the engine regardless of rpm and load. As a result, there’s more time for a greater amount of nitrous to be swallowed into each cylinder when the engine is operating at low revs. This means the low rpm torque gain is much more pronounced than at high rpm.

There are a few problems with this characteristic.

First, most people don’t want the top-end power gain to fall away. They want a huge peak power number to brag about. Second, there’s a very high chance of detonation when there’s a lot of nitrous being injected at low revs. Everyday pump fuel simply does not have the required octane to cope with such tremendous cylinder pressures. This is the reason why many aftermarket nitrous kits will activate only above certain revs – typically about 2500 rpm.

In contrast, a multi-stage nitrous system allows you to tailor the rate of nitrous delivery.

In order to control detonation and the rate of torque delivery you can use a multi-stage nitrous-system. Some of these systems use a small initial nitrous boost followed later by a second (and sometimes third) additional boost. This progressive rate of delivery helps maintain traction and minimise engine and driveline stress. Interestingly, all of the progressive controllers we’ve seen are time based – in other words the second phase of nitrous is injected a certain period after the initial phase was triggered.

It appears there are no rpm or load based progressive controllers on the market.

Avoiding Detonation

As mentioned, one of the biggest problems of a constant-flow nitrous system is the potential for low rpm detonation (aka knock). Detonation can also be an issue when a very large nitrous shot is being used.

Detonation can be reduced by feeding the coolest possible supply of induction air, adding octane booster or slightly increasing fuel enrichment. Water injection also performs well in a high horsepower nitrous-oxide application. Note that many aftermarket nitrous kit manufacturers specify that you fit colder spark plugs and/or retard the ignition timing to avoid detonation.

Nitrous Systems in Action

The first thing that draws most people to nitrous is a very high advertised horsepower increase.

Off-the-shelf kits are available to boost engine power anywhere from ten to several hundred horsepower. Unlike many other forms of enhancement, the advertised power increase using nitrous is usually very close to what you will achieve. Figures may vary slightly depending on engine tune, nitrous distribution and other factors, but if a kit is advertised at 100hp, that’s more or less what you’ll get.

In the real world, however, it’s the average power increase that is typically more impressive than the peak power gain. This is especially the case when talking about constant-flow nitrous systems.

A constant-flow nitrous system usually gives a monumental boost in everyday driving conditions. A huge low/mid-range boost is essential if a decent top-end gain is to be achieved. Depending on the size of the nitrous kit, a low-down torque increase well in excess of 50 percent is quite common!

As mentioned, the severity of the nitrous ‘hit’ depends largely on the location of the nitrous and fuel enrichment nozzles. Another factor that comes into play is under-bonnet heat soak...

When the nitrous supply line is sufficiently heat soaked, the nitrous-oxide inside the line can convert from liquid to gas. Once this happens, you get a very ‘fluffy’ nitrous hit and less outright power.

There is a tried-and-proven way around this problem.

One of the traditional drag-racing solutions is to install a purge valve in the nitrous line. Venting to atmosphere, the purge valve is opened prior to a full-throttle run and releases the gas inside the line. This makes space for a fresh supply of nitrous liquid to flow into the line – and you’re ready to go!

So what are the drawbacks of nitrous-oxide injection?

Well, most importantly, a fully functional nitrous-oxide system is illegal for road use in most (all?) countries. This is because the effect of nitrous oxide on emissions is unknown, and due to safety issues.

Certainly, nitrous oxide does require special care.

Nitrous oxide should not come into contact with skin as it can cause ‘freeze burns’. Inhalation in small doses should not cause any ill-effects – a high concentration is required before there is a significant anaesthetic effect.

It’s also important that the nitrous solenoid forms a perfect seal. If nitrous is able to leak past the solenoid, it will accumulate in the intake manifold and - when the engine is next started - can cause an explosion. This explosion can easily destroy an intake manifold and lead to an engine bay fire.

A throttle switch must not be overlooked as an essential part of the system because, without it, closing the throttle in an emergency situation will barely reduce engine power. Nitrous and enrichment fuel will still be pumping into the engine downstream of the throttle and you might still have 100hp of motivation!

How Long Will a Nitrous Bottle Last? One of the biggest drawbacks of nitrous-oxide is its limited in-car supply. A typical aftermarket kit comes with a bottle that holds about 10 – 15 pounds of nitrous oxide. How long this lasts depends on your driving style and the size of the hp shot. As a guide, however, a 100hp kit consumes around 0.8 pounds of nitrous over 10 seconds. That means a 15-pound bottle will give you just over 2 minutes of fun. Then it’s off to your local commercial gas supplier – again...

And now for the question everyone needs to ask....

Is there a limit to how much power you can achieve with a nitrous-oxide boost? Well, not really. You’re limited only by the strength of the engine, driveline and – of course – bravery!