One of the things that make drag racing such a wonderful thing is the lack of rules governing ones imagination. This provides great parity and many different variations of engine and driveline combinations. Turbo, roots supercharger, centrifugal superchargers, and nitrous oxide injection combinations are all competitive and have certain advantages over one another. I feel that the verdict is still out on what is the most competitive combination although nitrous oxide is certainly the most commonly used.
When talking about power most people consider the horsepower the key indicator of performance; this is not necessarily accurate. Torque, the moment or rotational force, determines the acceleration curve that the engine is able to apply through the driveline. For example, nitrous oxide injected engines typically do not produce as much horsepower as a supercharged engine, but typically perform better than turbocharged engines with 1-300 horsepower more. It is all a function of Useful Power.
Torque is a function of the force exerted on the crankshaft by the piston/connection rod. Understanding that the combustion of the fuel-air mixture produces great pressures inside the cylinder while the piston is near top dead center, it is easy to identify and roughly calculate an engines ability to produce torque. If a pressure is exerted on a certain area there is a calculated force produced. In an engine, the combustion forces press against the top of the piston. This pressure acting on the piston area translates to a force exerted on the connecting rod, which pushes against the crankshaft. The offset journals of a crankshaft (stroke length) then determine the rotational forces or moment forces measured in units of torque exerted by the crankshaft. Horsepower is a measurement of the torque of an engine at engine speed. Thus the horsepower can increase as engine speeds increase even with falling torque measurements.
There is another important factor however when examining engine torque, rod angle in reference to the offset throw of the crankshaft. As the angle of the rod approaches 90 degrees or perpendicular to the crankshaft, force loads must be translated from a different axis of movement causing mechanical loads to increase as well as a loss of available force. Many engine builders refer to the rod/stroke ratio as critical to the longevity and torque production of an engine, they are manipulating the angle of the offset journal to that of the connecting rod, However this is a double edged sword because as the rod/stroke ratio increases, the piston velocity grows greater at certain crankshaft degrees pulling or pushing harder on a longer narrower connecting rod beam.
Now that there is a basic understanding of torque and horsepower it is easy to see how torque initially gets a car moving from the starting line (or causes tire spin). If the power and torque levels of the three most popular engine combinations are examined it is very apparent that nitrous oxide injected engines simply do not produce as much power as super or turbo charged engines. However, the power it produces is immediately available due to the fact that all of the increase in combustion pressures is available immediately. In a turbo charged or supercharged application, the engine speed is directly related to the increase in combustion pressure. In reverse though, as engine speed increases, the nitrous injected engine will not produce more pressure on the piston area in comparison with the charged engines. But notice the 60 ft elapsed times of the nitrous oxide injected engines in comparison to the charged engines. Nearly always will this combination be quicker. The ability to get the car moving as fast as possible is very important especially in 1/8th mile drag racing. It is very hard to overcome that ability in a relative short distance. In ¼ mile racing though, the other combinations can really show their increased high rpm torque and power.
How do the charged engines compete with the nitrous oxide injected combinations? It is not an easy task. Fine-tuning the engine combination is critical. With a supercharged engine, be it centrifugal or roots-type, it is not quiet as hard as with a turbo charged engine. By changing the supercharger drive ratio and varying the leave rpm the engine can develop close to the same power as a nitrous oxide injected combination. Increased drive ratios also shorten the life of an engine due to the high boost conditions at high rpm. There has to be a balance of starting line power and engine longevity/durability.
With a turbo charged engine things even become more complicated and more delicate to balance. Exhaust gasses drive a turbo charger and thus determine the amount of intake air and fuel going into the engine. The turbo chargers are centrifugal however meaning they depend on an acceleration of gasses by the impeller and turbine blades to provide tertiary air to the engine. This slinging of the gasses means at some point the backpressure of the gasses will equalize with the turbine outlet pressure limiting the quantity of tertiary air and turbine speed. The turbine speed (ability to produce tertiary air) is adjustable by changing turbine housings. But once again, another downfall, when the turbine housing shrinks the turbine max speed before backpressure meets the outlet pressure decreases so does maximum power.
To evaluate the combinations I have included engine operating simulations for a 565 CID nitrous injected engine, a 526 CID super charged engine, and a 400 CID turbocharged engine.
Chuck Poindexter Jr.
Paint & Performance Svc.Ctr.
1205 Masonic Dr.
(972) 227-7910 or 7911