BOTTLE PRESSURE

At standard atmospheric temperature and pressure, nitrous oxide is only a gas; but when the gas is sufficiently compressed, it will transform to a liquid. However, the pressure will depend on the temperature. To convert nitrous oxide at standard atmospheric pressure of 14.7 PSI, the temperature must be reduced to -127.6 degrees Fahrenheit. Conversely at 70 degrees (F) a pressure of 760 PSI is required to maintain nitrous oxide in a liquid form. Therefore, when compressed nitrous-oxide is stored in a closed vessel at standard temperatures, the internal pressure created when a portion of liquid reverts to a gaseous state will create a pressure balance (760 PSI) that retains the balance of the nitrous as a liquid; the pressure generated in this way is known as vapor pressure.

The best way to visualize the situation is to first imagine this liquid inside a completely closed storage vessel. When the temperature is raised to standard room temperature, the nitrous oxide tries to convert back to a gas. In other words, it starts to "boil". However, as liquid nitrous oxide boils, the pressure of the gas trapped between the surface of the liquid and the inside of the container rises. As the temperature rises, the liquid nitrous oxide continues to convert to a gas until the gas pressure above the liquid nitrous is high enough to prevent further boiling. At this point the contents have reached a stable condition, and the vapor pressure holds the rest of the nitrous oxide in a liquid form.

As long as the temperature inside the tank rises, the vapor pressure rises until conditions reach a "critical point". At temperature above the critical point the vapor pressure can no longer hold the nitrous oxide in liquid form. The critical point for nitrous oxide is 97.6 degrees Fahrenheit, which results in a bottle pressure of 1069 PSI. At and above 97.6 (F)/ 1069 PSI the entire contents of the bottle becomes gaseous. Pressure takes increasingly large jumps with each temperature increase and nearly doubles on the last step as the gas molecules are trying to jump out of the liquid.

As the nitrous temperature increases the containment pressure increases, but conversely, the density decreases. Racers heat the nitrous bottle and go strictly by the bottle pressure, but it can leave more questions than answers because it neglects density. Here is a chart that explains what happens to the nitrous density at different temperatures, It shows the density of the liquid in the bottle and you can see from the numbers that weight of a cubic meter (DENSITY) decreases as the temperature goes up. The jet understands volume flow, but density has less effect (a square root function). When you activate nitrous flow while the bottle temperature is higher, you are going to get LESS NITROUS! Also, as you come close to the edge of heating up the nitrous, near the critical point 97.6 degrees (F) or 1069 PSI the density and weight delivered through a given jet changes more drastically.

THERMODYNAMIC PROPERTIES OF SATURATED NITROUS OXIDE

TEMP (F) PSI DENSITY kg/M3

32 460 913.1

40 520 876.2

50 590 837.8

60 675 788.1

70 760 744.9

80 865 640.7

97 1069 424.5

A good compromise is achieved when the bottle temperature is about 83 to 90 degrees (F) these temperatures will produce a bottle pressure of 900 to 1000-PSI. It is important to achieve as little pressure drop as possible from the bottle to the point of injection. Smart tuner's prefer to make a single pass on a bottle and then refill it. This will reduce pressure drop during a run. The bottle begins to drop pressure from the very beginning of the run. FULL BOTTLE = LESS PRESSURE DROP. The shorter the supply line from the bottle to the solenoid the more it will flow due to less pressure drop from one end to the other. Some racing organizations require the bottle to be mounted in the rear of the vehicle. This will require a long supply line to carry the nitrous to the engine. It is not uncommon for a supply line to lose 125-200 PSI of pressure between the bottle and metering orifices, this is a surprisingly high-pressure drop. This is normally due to restrictions, friction or viscous losses or a combination of these factors. All components of the supply system must be carefully selected and coordinated to eliminate or reduce the losses. The moral here is the larger #6AN line is better. If rules require you to use a "Long" supply line or a -4 AN supply line or both, you should install a pressure gauge at the solenoid inlet and one at the bottle and observe pressure drop after a run. You don’t want the pressure to drop below 850 PSI after a pass. If it does adjust your starting line bottle pressure to compensate. Dropping below 850 PSI during a pass will cause a rich condition and you will be down on power and down on MPH on the top-end of the track.

Consistency is the key to reliability and tuning with nitrous. Do yourself a favor and get a ACCURATE Nitrous Pressure Gauge and do some testing to determine what bottle pressure works best with your particular combination and do it the same way every time. If you have trouble hooking on a small tire try starting with 850 PSI and it will still make good power (Although MPH will be down on the top end), but it will not hit the tires as hard. Excessively high bottle pressures are not the key towards more power at this time.

How and where you heat the nitrous bottle does make a difference! You can build up pressure fast by heating the top of the bottle and expanding the gas, but the pressure will flash off and your car will slow down because there was no heat in the liquid nitrous. Heating the bottle evenly will cause the liquid nitrous to put out more vapor pressure and maintain the pressure during the run. The vapor pressure in the bottle is what sends the liquid nitrous through the system to be metered.

A full bottle is faster than one that already has a couple of runs on it. There is a time lag in pressure recovery during a shot of nitrous. The lower the liquid nitrous level, the larger the time lag. When you trigger the nitrous system the gas in the top of the nitrous bottle goes to work pushing the liquid out of the bottle. When the gas does work it cools off and builds less vapor pressure. When the liquid is leaving; the nitrous bottle has more area to boil off in and pressurize. Therefore the bottle pressure drops quicker on a bottle with less nitrous in it. If the bottle was heated properly and heat soaked the pressure recovers quicker and does not lose much pressure.

The most important point is EVEN heating of the nitrous, you can get the pressure up to 1000 PSI fairly quick with heaters, but it will quickly fall when you remove the heating element because the entire volume is not yet at an even temperature. To do this you need to heat soak the bottle for an extended period of time. (Especially in the winter months) When heating the bottle with a 12-volt bottle heater to 950-PSI/85 degree’s (F) I found that the bottom of the bottle was 13-14 degree’s cooler than the top half of the bottle. This indicates that the bottle isn’t fully heat soaked. After the bottle is heated to the desired pressure. Pick the bottle up and shake the bottle and rock it back and forth. (Turn the bottle upside down and then back upright several times) Then observe the pressure gauge. If the bottle is not heated evenly the bottle pressure will drop after this procedure. Which indicates that you need to heat the bottle back up again to the desired pressure. Repeat until the bottle pressure does not drop. Around 925-1000 Starting Line Bottle pressure is optimum,which is a bottle temperature of 83-90 degree’s. It is wise to heat the bottle up to 1025-PSI and then do the procedure described above. That is why you see the Pro’s using the multi-bottle heaters in the pits. They get their bottles fully up to racing temperature and can always cool them down quickly if necessary. Then purge the bottle down to the 925-1000 Bottle pressure range on the starting line. It is far easier to cool a bottle than to heat it up! In cool weather when you have to sit in the staging lanes for an extended time you can also use a 12-volt heater to keep the bottle at pressure. Getting the bottle up to pressure is the hard part, keeping it there is far easier for the heater to do. Just soak a towel in your cooler and lay it on the bottle and it will bring the temperature down quickly.

Thanks for reading!

Robert @ FASTLANE NITROUS INJECTION

Any questions for Robert Lane please go to his forum @ http://excoboard.com/exco/index.php?boardid=18653

FASTLANE NITROUS TECHNOLOGY is a nitrous development company. We work with each person as an individual to develop or modify their nitrous system for their specific application. We even repair/modify most competitor’s designs. Most update/repair programs are affordable. These programs include wet-flow testing to verify proper operation, nitrous flow analysis to calibrate the system, which includes a wet-flow portfolio for easier tuning. There's a reason we have these world records and championships, we spend time with the customer to build what they need to win and enjoy their day at the races. We specialize in higher end nitrous systems designed for competition in "Heads Up" Drag Racing. If you have the desire to win races, championships and set records then you have come to the right shop.

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