Utilizing Nitrous Oxide to Boost/Improve Your Supercharger Performance

There comes a point in your power buildup where you may consider adding nitrous oxide injection to your supercharged car. This point typically coincides with reaching a level of performance that means increased investment and diminishing returns from your supercharger. For example, my car comes from the factory with a 5th generation Eaton MP45 supercharger. This supercharger is limited to about 230hp worth of flow rating and so no matter what I do with bolt-on upgrades on nitrous oxide engine, my peak horsepower will not exceed 230hp limit because that is the point at which the supercharger becomes the bottle neck in my system.

As we've talked about in previous articles there is still the option of porting the factory supercharger for a 10 to 15% gain in capacity (which in this case would be another 23 to 35 horsepower). There is also the option of retrofitting a larger supercharger such as the Eaton M62 to gain potential up to over 300hp depending on the final choice of a supercharger.

This modification path (porting or replacing the factory supercharger) can prove to be complex and costly, especially if the supercharger is integrated into the intake manifold (and possibly an air to water intercooler) as the case is with many factory supercharged cars.

A possible viable solution for this situation is to use nitrous oxide injection to supplement the power delivery when racing, and being satisfied with a reliable lower powered car when the nitrous is off and we're not racing.

The reason why nitrous oxide (N2O) becomes a great power adder is twofold:

1- Nitrous is cheap as far as horsepower per dollar goes, and especially in the situations where we're already supercharged and so will only be using it on the rare occasions when we do hit the track.

2- Nitrous oxide is a great 'chiller' as it comes out of the bottle at a temperature of negative 127*F and is capable of cooling the overall supercharged air charge mixture by over 100*F as reported by enthusiasts, this is an additional temperature reduction over the effects of whatever intercooler you have fitted. This in-fact makes nitrous a great proposition for cars that have already maxed out their superchargers, where the supercharger is running at peak rpms and producing very high outlet temperatures. The nitrous oxide injection can effectively boost the thermal efficiency of the supercharger when it is most stressed out and give us a nice, cool, and dense mixture.

3- Nitrous oxide fuel delivery is fairly straight forward to setup and to tune, especially on newer model cars with return-les fuel systems, or difficult to crack computers that make it difficult to upgrade (and properly tune) a much larger supercharger setup. Nitrous oxide fuel delivery can be set-up totally independently from the OEM ECU and fuel system and thus makes nitrous a possible application for German cars with stubborn computers.

4- This is a racer technique... most cars seem to perform better during the winter months because the air is cooler, horsepower is elevated, and the tracks although cold, can be prepared for traction and will heat up enough during the night to allow for traction and to give people the ability to exploit the cold dense air to post their best times of the year. As the weather gets warmer, traction increases because the asphalt is warm and sticky, but horsepower is reduced due to warmer, less dense air. Typically racers find that their cars vary in their quarter mile performance by as much as a half a second between their summer tune and their winter tune, especially if you're using a supercharger or turbocharger that compresses (and further heats) the incoming air.

The solution to on-track consistency, racers have found, is to combine the use of nitrous oxide (which is summer friendly) with forced induction (superchargers and turbochargers) which are winter friendly. In the summer time, the outside temperature is high, and so the nitrous bottle pressure is maintained at a high level above 1100 psi. This allows for a generous nitrous flow rate under the sustained pressure (even without a bottle heater) which gives great summer performance for nitrous assisted cars. While in the winter, the outside temperatures drop significantly, the nitrous in the bottle contracts and the bottle pressure drops, subsequently, the nitrous flow rate drops and nitrous assisted cars show worse performance in the winter times.

The complete opposite is true for supercharged cars that produce great horsepower in the winter from compressing cool dense air, and poor horsepower in the summer heat. When you combine these two power adders you get pretty flat and consistent horsepower production year round because the supercharger shines when the nitrous is weak, and the nitrous shines when the supercharger is weak, and thus together, they give consistent power deliver year round.

Pre-cautions:

Now we have to consider that nitrous oxide is an oxidizer and thus not only does it increase the amount of air and fuel combusting in the cylinder, but it also produces a faster moving flame front due to the oxidizer properties of the nitrous oxide. This means that additional timing retard, great octane fuel, and possibly colder spark plugs will be required to run spray on a supercharged car. 

Furthermore, because of its cooling effect, a 100hp shot on a supercharged Camaro can very easily put down OVER 120 rear wheel horsepower of additional power. This means that the 'out of the box' jetting of a nitrous kit may not be adequate on a supercharged car and you'd have to make sure to monitor and possibly increase the fuel jetting to match the final horsepower figure of your car). Last but not least, if you're running a 500hp supercharged car with an additional 120hp of nitrous oxide injection, then you must make sure that your fuel delivery (fuel pump and fuel lines) are able to flow the total amount of fuel required to deliver 620hp.

Applications scenarios:

1- You have a car like mine, a 2005 C230 kompressor that comes with a 230hp limited Eaton MP45. ECU on the car is a Siemens ECU that very few people know how to tune, and the fuel system uses a return-less setup with an in-tank fuel pressure regulator. With this kind of setup all forms of dry nitrous injection are out of the question because we can neither compensate for fuel through flashing the factory ECU, nor can we elevate fuel pressure during the nitrous injection because the fuel pressure regulator is in-accessible....

Recommended kit:

A wet nitrous injection kit that injects both fuel and nitrous oxide from the injection nozzle.

Injection location:

After the supercharger, after the intercooler, and into the intake manifold of the car.

Maximum recommended injection:

25% of the original total power figure which corresponds to around a 50 hp shot of nitrous on our example.

Expected final horsepower:

60 to 65 wheel horsepower and possible about 130 ft-lbs of additional torque!

2- You have a car that has an accessible fuel pressure regulator, or an ECU that can be re-flashed for nitrous oxide or a 'dual tune' setup. In this case it is recommended to use a dry nitrous kit for two reasons:

First: Dry kits are safer on supercharged cars (as long as the fuel delivery through the injectors or raised fuel pressure is adequate) because they hold a reduced chance of intake backfires because the intake manifold is dry of fuel.

Second: Dry nitrous injection contains no fuel, and so we don't need to worry about fuel falling out of suspension from the injected air. This means that we no longer have to spray the nitrous right before the intake manifold and we now have the option to move the point of injection much farther back. Spraying nitrous BEFORE the intercooler, right after the supercharger gives the nitrous stream more time and more contact with the compressed air coming out of the supercharger which results in more cooling and further increased horsepower.

Recommended kit:

A dry nitrous injection kit that injects only nitrous oxide from the injection nozzle.

Injection location:

After the supercharger, before or after the intercooler and not necessarily right at the intake manifold of the car.

Maximum recommended injection:

25% of the original total power figure which corresponds to around a 50 hp shot of nitrous.

Expected final horsepower:

70-75 wheel horsepower and possible about 130 ft-lbs of additional torque!

3- You have a car that has an accessible fuel pressure regulator, or an ECU that can flashed for nitrous oxide or a 'dual tune' setup. You also want to make as much horsepower as possible from your nitrous...

In this case it is recommended to use a dry nitrous kit injecting before the supercharger. As we mentioned in our articles on twin charging (combining turbochargers with superchargers for added performance), when two 'chargers' are chained in series where one charger feeds the next, then the two pressure ratios of the charger combine because the second charger compresses air that is already compressed by the first. For example two turbochargers set for a 1.5 pressure ratio (or 7 psi of boost), running in sequential mode will result in a final pressure ratio of 2.25 bar (or 18psi of boost) which is more than the 'expected' 14psi that is the sum of the two boost levels.

Similarly, injecting nitrous oxide before the supercharger, delivers already compressed air. This is true weather we are talking about nitrous being compressed because it has twice the oxygen concentration as normal air or we're talking about the nitrous cooling and compressing the incoming air. The final amount of compression observed by the supercharger inlet will vary depending on the ratio of incoming air to the size of the nitrous shot, and can result in an increase in boost of between 0.5 to 2.5 psi!

This boost increase is in addition to the power increase of the nitrous oxide injection and so it can be an additional 5 to 25 hp.

Recommended kit:

A dry nitrous injection kit that injects only nitrous oxide from the injection nozzle.

Injection location:

Before the supercharger inlet.

Maximum recommended injection:

25% of the original total power figure which corresponds to around a 50 hp shot of nitrous.

Expected final horsepower:

75-100 wheel horsepower and possible about 160 ft-lbs of additional torque!

Things to avoid:

1- No matter where you setup the nitrous injection, make sure not to spray nitrous into your MAS air flow sensor or your intake air temperature sensor. These temperature dependent sensors, tell the ECU to advance the timing in colder conditions. As we mentioned earlier, nitrous is an oxidizer that increases the speed of travel of the combustion event and thus requires maintained (if not retarded) ignition timing compared to a supercharged only setup. Avoid spraying on these temperature sensitive sensors to prevent accidental timing advance from occurring.

2- Avoid spraying a wet kit (fuel) before your supercharger, as the wet fuel mist will damage the supercharger rotors and strip their coatings.

3- Make sure you check your air fuel ratio on the nitrous and don't stick to the 'out of the box' air to fuel settings with the kit. For example an extra 2.5 psi in your intake may or may not be compensated by your stock ECU and so depending on how well the ECU reacts you will have to adjust the fuel jetting on the nitrous kit. 

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