By David Reher, Reher-Morrison Racing Engines
“The only source of power in any internal combustion engine is the energy in the fuel.”
The hottest topic in engine building these days is power adders. It seems like every magazine, website, and online forum is talking up nitrous oxide injection, superchargers, and turbochargers. What’s the big deal?
It seems to me that increasing power has always been the goal of engine builders.
Back in the days of steam power, engineers figured out how to improve performance with firebox fans, higher operating pressures, and multiple stages to extract energy from superheated steam. The history of internal combustion engines has followed a similar development curve. We’ve progressed from flatheads to overhead valves and multi-valve overhead cam designs. Hot rodders and racers have always been relentless in their pursuit of more power. High-lift camshafts, ported cylinder heads, and high-flow carburetors are all “power adders” in the pure sense of the term.
Drag racers have employed power adders for decades. Fuel-burning dragsters and Funny Cars rely on multiple power adders to achieve their stunning performances – superchargers that dramatically increase manifold pressure and oxygen-bearing nitromethane fuel.
So it seems strange to me when I hear some people attach a stigma to power adders like nitrous oxide and superchargers in sportsman eliminators that allow such devices. Isn’t the whole idea of racing to make more power and go faster? Apparently many racers agree, because the popularity of fast sportsman racing is exploding. At the peak of racing season, four or five NHRA divisions are running Top Sportsman and Top Dragster eliminators simultaneously, often with 40 or more entries vying for a 32-car show with a 6.70- or 6.60-second bump spot.
Nitrous oxide injection has become the power adder of choice in these fast sportsman eliminators. While there are some supercharged cars competing, the relative simplicity and economy of a nitrous-injected engine combination is hard to beat – a bottle, injectors, and plumbing is as close to “bolt-on” power as you can find.
But there is a downside to the tremendous power gains available with nitrous oxide. The harsh reality is that the engine must be built to withstand the higher power level. An engine intended to produce 1,000 horsepower will be overstressed and short-lived when it’s boosted to 1,800 horsepower.
Most racers understand intuitively that you can’t bolt a blower onto a naturally aspirated engine and expect it to live. They know that the internal components must be beefed up to withstand the higher pressures and loads produced by a supercharger. Nitrous oxide effectively does the same thing, but since there is not a mechanical air pump on top of the engine, some don’t make the connection. Pistons, wrist pins, connecting rods, and other components must be stronger (and inevitably heavier) to survive in this high-pressure environment. Unfortunately, nitrous gets an unwarranted bad reputation when an engine fails because it wasn’t built to survive at the power level it can achieve with nitrous oxide.
The only source of power in any internal combustion engine is the energy in the fuel. Blowers, turbos, and nitrous don’t add power by themselves – they simply increase the oxygen supply in the cylinder and thereby allow more fuel to be burned and more heat to be released. Superchargers achieve this by raising the pressure in the induction system above atmospheric pressure. Nitrous oxide (N2O) works by breaking down at elevated temperature, releasing oxygen. Increasing the oxygen supply, either mechanically with a supercharger or chemically with nitrous oxide, allows the engine to burn more fuel. The energy in this additional fuel is what adds the power.
With 36 percent oxygen content, nitrous oxide is a tremendous oxidizer. Think of blowing on the charcoal in your barbecue, or a blacksmith pumping air into his forge with a bellows – with more oxygen available, the fuel burns faster and hotter. Since the fuel burns faster in an oxygen-enriched cylinder, the combustion process requires less spark advance – single digit spark timing can produce best power with nitrous oxide injection.
Nitrous oxide leaves the injector nozzles at 59 degrees below zero. This blast of super-cold gas chills the intake charge and increases its density, further enhancing the oxygen content. This charge-cooling effect is a major benefit of nitrous injection.
The intake valve notch typically has a thin cross-section in a racing piston, and that is where trouble often begins in an oxygen-enriched engine. The piston material must be able to conduct enough heat to prevent localized hot spots that can produce pre-ignition and damaging detonation. It’s like electrical wiring – if you have a 14 gauge wire where an 8 or 10 gauge wire is needed to carry the current, what happens? The wire heats up, the insulation melts, and the system shorts out.
There is a misconception that power adders can transform a mediocre engine into a winner. The truth is that the best engine usually wins the horsepower race. A nitrous-injected motor responds like any naturally aspirated engine – it depends on the pressure differential between the atmosphere and the cylinders. An engine with the correct port size and mixture velocity, the right camshaft profile, and an optimized intake and exhaust system will outperform an engine with restrictive or lazy ports, an unstable valvetrain, and mismatched parts. A supercharged engine obeys the same laws: If it has too much resistance to airflow, turning the blower faster just heats up the intake charge without producing more power.
A more efficient engine can move more air and burn more fuel, whether it is naturally aspirated, nitrous injected, or supercharged. Regardless of the power adders used, the importance of the basic engine never changes.