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The Zen of Purging

"Man, nitrous sure does make the engine start good!"

The young man had been droning on about his new found love of nitrous. I had been concentrating on the job I was trying to finish and the one sided conversation was going in one ear and out the other but he suddenly had my undivided attention.

Huh? "Uh, just a moment." I walked over and opened the shop door. The hood was still on his car.

Fearing the worst, I asked him to tell me exactly what he meant by "start good." He explained that he had been hitting the nitrous button before hitting the starter key! The only thing I could picture was the classic mushroom cloud.

We went on to discuss how nitrous works and the fact that at 1500 rpm you could probably put enough nitrous in a cylinder to create 300 or 400 hp. This is a whole group of corresponding cylinder pressure that no engine can handle, particularly a street engine.

I will try to explain this by slowing down the time line. Einstein would be proud. I don't want to hear any complaints from math purists. This example is just to clarify the situation.

Let’s say that, at idle, it takes a minute for the piston to go from bottom to top. In a minute, depending on jetting, you can empty a ten pound bottle. Now let’s say that at 4000 rpm it takes one second for the piston to go from BDC to TDC. At this point you can only inject 1 oz of nitrous and the corresponding fuel.

How can this be? One ounce versus 10 lbs? Nitrous is being injected at constant (hopefully) 900 lbs of pressure average no matter what the rpm is while the speed of the engine at high rpm is allowing less and less of an open window (valves & cam) for the nitrous to get in.

The point of this is that at idle it's like trying to put 20 lbs of potatoes in a 10 pound bag. Something is going to give! Could be the manifold and hood soaring to new heights or the rods drilling for more OPEC revenues — or both.

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, email:

So don't do it! Get a purge kit. They are cheap. Next time I will get into the various uses for a purge valve.

This guy was lucky. If the cold engine had sneezed he would have been picking pieces up for a week. If I had sold him his nitrous kit to begin with I would have insisted on the purge kit. I wrongly assumed everyone understood this "purge thing." We both learned something that day.

The young man grew up and went on to be an automotive engineer for one of the Big Three and to this day we still laugh about averting a world wide holocaust or at least a neighborhood one.


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