Tech Talk #80 – Getting to Know Nitrous

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“Something that looks simple from the outside is actually incredibly complex.”

There was a time in my life when I thought that Pro Stock was the center of the universe. I’d spend every minute of the day – and many sleepless nights – thinking about how to extract more horsepower from a 500-cubic-inch Pro Stock engine. Unfortunately, focusing so intensely on one combination can lead to a severe case of tunnel vision. I didn’t appreciate the much wider world of racing engines until I retired from Pro Stock.

Walk though the Reher-Morrison Racing Engines shop today and you’ll find powerplants for professional and sportsman drag racing classes, alongside engines built for tractor pulls, land speed record cars, sport compact drag racing, vintage road racing, seriously fast street cars, and other high-performance pursuits. Building, testing, and developing such a wide variety of internal combustion engines has opened my eyes to the possibilities of transplanting technology from one type of racing to another.

During my Pro Stock days, I had some preconceived notions about nitrous oxide injection. Along with the predictable comments about hidden bottles (some serious and some in jest), the consensus among Pro Stock racers seemed to be that going fast with nitrous oxide was simply a matter of using a bigger nitrous jet. For the last few years, I’ve been heavily involved in developing nitrous-injected Pro Mod engines. Once again, I’ve learned that something that looks simple from the outside is actually incredibly complex. Pro Mod development is as difficult as any engine program. Working with knowledgeable Pro Mod racers like Shannon Jenkins, I’ve discovered that it’s just as rewarding when you get it right.

The nitrous-injected engines that compete in the NHRA Get Screened America Pro Mod Drag Racing Series, the ADRL, and several regional circuits are impressive by any standard. A state-of-the-art RMRE Pro Mod engine displaces 854 cubic inches, thanks to an aftermarket billet aluminum block, 4.975-inch diameter cylinders, and a 5.500-inch stroke crankshaft. When all four stages of nitrous hit, peak power jumps from 1680 to approximately 2800 horsepower. The current Pro Mod performance marks are 3.81/198 mph in the 1/8th mile and 5.88/242 mph in the quarter-mile. I expect that these will be eclipsed in 2011.

The sheer size of these mammoth motors creates immense loads and makes extreme demands on components. Average piston speed is one measure of the stress on internal parts. In a typical NASCAR Cup engine running at 8500 rpm, the piston speed is approximately 5100 feet per minute. The pistons in a 500ci Pro Stock engine turning 10,500 rpm are traveling 6300 feet per minute. In contrast, when a Pro Mod motor crosses the finish line at 8400 rpm, the piston speed is a staggering 7700 feet per minute – far exceeding the generally accepted limits for piston velocity.

The long crankshaft stroke and high engine speed also subject the connecting rods and pins to enormous loads. During the overlap cycle when both valves are open, there is no cushion of compressed gases to lessen the loads – the 800-gram piston and 200-gram pin effectively weigh thousands of pounds as they reverse direction. Working with manufacturers to develop connecting rods, caps, and bolts that are capable of withstanding these forces has been a satisfying experience.

A common misconception is that the nitrous oxide is a powerful fuel additive. In fact, nitrous oxide won’t burn and contains no BTU’s. What it does have is an oxygen molecule that becomes available at elevated temperatures to burn more fuel. Earth’s atmosphere contains about 21 percent oxygen by volume, and that’s all the oxygen that is in the combustion chamber of a naturally aspirated engine to oxidize the fuel. When nitrous oxide is added, the percentage of oxygen increases to 26-28 percent. This extra oxygen allows more fuel to be burned, increasing output dramatically.

Years ago we ran combustion pressure tests on a Pro Stock engine with GM Racing. I expected that the pressure trace would show a steady rise after ignition, leveling off at its peak, and then gradually declining as the piston moved down the cylinder. In fact, what we saw was a pressure spike that lasted for only a few degrees of crankshaft rotation and then dropped rapidly. The GM engineer explained that this was typical in a naturally aspirated racing engine, while the cylinder pressure in a forced induction engine has a more gradual rise and fall that pushes on the piston for a longer time.

While I haven’t seen data on cylinder pressure in a Pro Mod engine, the evidence suggests that nitrous oxide injection produces an extremely short and exceptionally strong pressure spike, further increasing the loads on the components. Our Pro Mod engines require 20–25 degrees less spark advance than a very efficient 500ci Pro Stock engine, in the range of 5-7 degrees BTDC. This shows just how fast the burn rate is with high levels of nitrous.

Even with nitrous oxide injection, a Pro Mod engine is still a “barometric” engine. By this I mean that the amount of air and fuel entering the combustion chamber depends on the pressure differential between the induction system and the cylinder. When these pressures reach equilibrium, the flow stops. Putting more fuel and more nitrous into the system at this point won’t make more power, it just creates an overly rich condition. It’s like adjusting an acetylene torch – increasing the flow of gas beyond the capacity of the oxygen to burn it only creates a cloud of black smoke, not more heat.

The path to increased power in a nitrous-injected engine is the same as in any other naturally aspirated engine: the induction and exhaust systems must maximize flow. If the system is too restrictive, the engine can’t reach its potential. If the cross-sections are too large, the airspeed drops and the efficiency falls. In this respect, refining a nitrous-equipped engine is exactly the same as developing any normally aspirated racing engine.

As a lifelong gearhead and professional engine builder, I’ve been fortunate to work on many different engines for many types of racing. Although every application has specific requirements, there are opportunities to apply knowledge gained in one area to a wide spectrum of racing. Many of the lessons I learned 40 years ago racing a small-block Chevy in Modified eliminator still apply to today’s behemoth Pro Mod motors.