The Zen of Purge, Part 2

If you look up purge in the dictionary you find all kinds of examples, most of which sound disgusting. Some refer to bloodily removing a head of state from power or perhaps the transference of that strange pizza to the porcelain round file. A more accurate description pertaining to nitrous is to evacuate. We want to evacuate or get rid of some unwanted air and gaseous nitrous and control the pressure at the same time.

Unwanted? Sounds counterproductive to our goals of shoving as much as we can cram down the engine’s throat without choking it. However, purging does serve a couple of useful purposes.

Consistency is important in tuning a nitrous car. If you do not launch at the same bottle pressure each time you will not be able to tell what, if any, changes you need to make the next time. Nitrous pressure is something you do have control over, at least at launch, so it is in your best interest to dial it in.

GET THE AIR OUT
When you first turn the bottle on there will be an air lock between the liquid nitrous and the solenoid. That is the primary function of purging. You want to get rid of that airlock and have the right pressure in relation to the fuel you are adding so the car will launch the same each time. Without purging, the car will leave soft because the fuel goes in first and the liquid nitrous has to play catchup. Of course there may be a time or two that you may want it to happen this way due to traction problems, but the bottom line in drag racing is to put as much horsepower as possible to the ground as soon as you can.

BOTTLE PRESSURE DEPENDS ON TEMPERATURE
The second reason for primary purging is bottle pressure. Bottle pressure is relative to temperature. When the nitrous is at about 85 degrees it will be in the desired liquid state at a pressure of about 900 lbs. To give you an idea of how temperature is important to your tuning techniques the following chart should show you the possibilities.

“Trick” launch pressures abound but we will use the most commonly used number of 900 lbs for our discussion. 900 lbs matches up to most of the manufacturers’ baseline settings. More important is the fact that drag racing is done in (hopefully) nice weather and 85 degrees and 900 lbs just happens to match up nicely. Most of the time anyway. Granted, you could run lower pressures and tune the fuel jet accordingly, but it’s a bigger pain than trying to keep the bottle pressure consistent. The best advice is to pick a number and stick with it. Higher pressures are a problem however. Many of the solenoids will not open above 1100 lbs so there is not much point in taking that risk.

VOLUME KEEPS PRESSURE UP
The idea is to have the bottle pressure slightly higher than launch pressure so you have a little room to purge down to your desired launch pressure. Some bracket racers have this down to a minor science. They try to keep the bottle at 925 to 950 and use only a short purge to hit the 900 mark. Their thinking is not to waste any nitrous but the real benefit of this approach is to keep as much volume in the bottle as possible. This volume helps keep the pressure up longer as the car goes down the track.

The biggest problem with bottle pressure is how to keep it at the pressure point you desire. If it’s too high, a wet towel will bring it down fairly quickly. I am assuming that it’s about 1000 lbs when I say that. If you just topped off the bottle and have extreme 1800 lbs of pressure, well, you have a problem. It’s going to take a while, especially on a hot race day. Many racers keep an ice chest handy just for this occasion.

WARMING THE BOTTLE
Warming up the bottle is a whole different can of worms. There are commercial heaters available that attach to the bottle. These are thermostat controlled. These are a love / hate deal. They work up to a point but suck up amps. What some racers use is a 110 bolt heater and just use it in the pits. They have their program down to the minute so they have a good idea where the temp will be in the staging lanes and so on. There is a pressure controlled heater available. It measures the pressure and turns the heater on and off accordingly.

The downside as I see it is most bottles would have to be turned open since the sensor is in the main line. This is not a good idea since a leaking solenoid seal could lead to a hood launching condition. If you choose this type of heater I recommend that you change your bottle valve to one on the Super Hi Flow valves that have a port available on the bottle side of the valve. This way the pressure can be controlled without having the main line and solenoids exposed.

Another warming technique being used is what I call the bottle jacuzzi. The bottles are submersed in a water container that uses a hot water heating element and preferably a thermostat. The idea, and it works, is to saturate both the bottle and the contents to a desired temperature. My motorcycle customers are limited to a little 2 lb bottle and they have found that if they start the “jacuzzi” process a day in advance they have minimal pressure drop at the end of the quarter mile. Food for thought.

DON’T DO THIS
Warming techniques that you should never attempt? A propane torch. Get real. I have seen highly intelligent individuals with backgrounds in metal working doing this and I have to tell you I can’t go the other way fast enough. Racing can sure make dolts out of Mensa candidates. Torch warming is outlawed most places but somebody is going to get killed by an exploding bottle due to this stupidity and that will be the end of nitrous racing. Think about it. It doesn’t take much to weaken aluminum and you are standing there with an open flame? All it takes is an old valve O-ring, a flaw in the bottle or soft point from the torch to create a bomb.

Case in point. I was tig welding on my dragster one evening about a foot from the bottle. The shop was quiet and when I was done I heard this little hiss. The heat from the welding had progressed through the chassis and metal brackets to warm the bottle. Now I have to tell you I was real surprised by that hiss. It turned out to be a flaw in the bottom of the bottle. When it cooled to room temperature it was fine. Subsequent testing showed that if the pressure exceeded 1000 lbs it would open up a leak. Moral of story. What would have happened if an open flame had been used? No shop, no Dave, and a hopefully interesting obituary!! Nuff said? I made a lamp out of the bottle, by the way.

A BETTER WAY, BUT…
Hot air is probably a better way to hand warm the bottle. Maybe. I have seen the exhaust from the tow truck used to warm the bottle. BUT (you knew there was a “but” didn’t you?) you have to stay with the bottle and monitor the pressure. Exhaust heat brings the bottle up FAST.

A BAD EXAMPLE
This story was related to me by a customer. I don’t remember the team involved and it does not matter but it should serve as an example. The crew chief put the bottle inside the generator compartment in his trailer and forgot about it. A while later there was a big whooosh as the bottle pop-off blew. No big deal, except that the nitrous being released in the compartment sent the generator rpm sky high. Kaboom goes the generator as it kicked a rod out. Bad deal, yes, but it’s not over yet. The kicked rod started a fire!! A big expensive mess all around. So, just give some thought to what you are doing when dealing with a pressurized container.

INSULATION
If you can find a way to insulate the bottle from its surroundings it will be much easier to control the bottle pressure. Full bodied race cars and street machines have the advantage of trunk space. Insulation can be placed on the floor and the inside of the trunk lid to help control bottle temperature over time. I did one street truck with the bottle mounted in a gutted tool box in the bed. The original idea was to hide the bottle, but I insulated the box to keep the pressure under control. I went one step further and installed a purge valve inside the box to cool it down on hot days if needed. It was needed. A gauge in the cab was connected to the bottle to monitor the pressure. A neat deal but way too much work. It worked very well.

PURGE AFTER A RUN
That little purge valve is handy for shutting down the system also. When you are finished with the run, shut your bottle off and purge off the nitrous left in the line. I know, I know, it’s wasteful-but is it? Let’s say that it is 95 degrees out and it’s 3 hours until your next pass. If you check the pressure you might find that it’s really high. The reason is that in addition to the outside temperature you also have heat coming off the pavement, off the headers and engine, etc. Now what if the solenoid seeps a little bit and gets nitrous in the engine? The next time you fire up the engine could be exciting and expensive.

I think street machines need to pay attention to this even more. The car sits out in the asphalt parking lot for 8 hours while you work. Think hatch back microwave. You jump in the car and IF you don’t have a leak (you did shut the bottle off last night?) you may have a really lean condition due to the high pressure. Get a purge valve and a gauge!! If the pressure is too extreme it will blow the solenoid plunger and shut off the nitrous altogether. A lesser of two evils, but still annoying.

If you do all the above and the pressure is extreme, do not turn the bottle on. Take a wrench and crack the main feed line open a little at the solenoid. Then open the bottle valve slowly till the pressure drops to reasonable limits. Take care when doing this if your solenoid and bottle are in the same vicinity because leaking nitrous will burn you like RIGHT NOW. Been there.

DYNAMIC PURGING
Sometimes called the “bump” or the “burp,” this is when you bring the engine rpm up to about 3500 and hit the nitrous activation switch momentarily. The idea is to have both the fuel system and the nitrous system purged at the same time. If you have a well designed return style fuel system this is not necessary and static purging the nitrous will take care of itself.

Dave Koehler has driven Junior fuel dragsters in the 60s, Super-charged Top dragsters in the 70s, and Nitrous powered cars in the 80s, in 1991 after two decades of running a high performance engine / machine shop, he sold it in order to spend more time doing R&D and manufacturing of specialty Nitrous Oxide and Alcohol fuel injection systems and engine balancing. He was also the onsite NOS tech rep at IHRA and NMCA events from 1995-1997.  Contact Dave Koehler at: Koehler Engineering 1888 CR 1400 N Urbana, IL 61802. phone (217) 469-7663 fax (217) 469-7910 www.koehlerinjection.com

Now I admit it, I have done this. Most nitrous racers have at one time or another. I do not, however, believe it to be a good thing. Any time you free rev an engine, with or without nitrous, you shorten its life. The reason is that the components move so fast that the oil system can’t keep up, so for a brief nano-second in time there is the potential for the rod bearing to touch the crank without the oil cushion. This “touch” increases the bearing clearances and it just gets bigger and bigger as time goes on. No load means that the pistons and rods are swinging out and jerking back in the opposite direction. The load keeps a cushion effect that keeps things under control. In the bad old days of stick shifts and no rev limiters it was a given that if you missed a gear you must immediately change the bearings. If you did not it was a sure bet you would be picking the rods off the ground within the next 5-6 laps. Its up to you.

To be an effective nitrous tuner you must get as many things under control as possible in order to be fast and consistent. Purging the bottle is just one of those tools. Take control. The good news is that its not an expensive project compared to all the stuff you already purchased.