Charge It!


It's hard to believe that the automotive technology known as "supercharging" has been around for almost 70 years. When one thinks of the full-fendered, spoke-wheeled cars of the '20s and '30s, forced induction (and the amazing performance benefits that accompany it) seems rather out of place on such primitive automobiles. Although it was available, the majority of vehicle manufacturers from that era had neither the financial nor the technical capability to develop and outfit their models with a supercharger.

One notable exception came in the form of the 1932 Duesenberg Model SJ. Besides boasting twin overhead camshafts, hemispherical combustion chambers and aluminum pistons, the Model SJ sported a centrifugal-type supercharger that brought horsepower to 320 from a 6.8-liter, straight-eight engine. The SJ Phaeton, with its convertible top down, was capable of speeds approaching 130 mph in showroom stock condition...in 1932!

As impressive as supercharger technology was in 1932, it has been substantially refined over the last seven decades. Today, superchargers aren't limited to just extravagant sports cars for the super rich, as they were in the 1930s. They can currently be found on everything from the $20,000 Mini Cooper S to the $120,000 Mercedes-Benz SL55.

But what exactly does a supercharger do to boost horsepower by as much as 50 percent over a normally aspirated engine?

To answer that, we'll first take a look at how a normally aspirated engine works. If you know the four-cycle process of any internal combustion engine, you know the first phase is called the "intake" cycle. This is when the piston is moving down and the intake valve is open. During this cycle, the air/fuel mixture enters the combustion chamber because of the vacuum created by the descending piston. The pressure inside the combustion chamber is less than atmospheric pressure (about 14.7 p.s.i. at sea level), which causes the air/fuel mixture to rush in and fill the chamber. A loose, but effective, analogy is pouring water from a pitcher into a glass. Only the force of gravity is causing the water to move from one container to the other.

In contrast, a supercharged engine actually pushes the air/fuel mixture into the combustion chamber, rather than relying on simple vacuum to get the job done. This pushing force is called "boost" and refers to the amount of pressure (over-and-above atmospheric pressure) that is working to move the air/fuel mixture from the intake manifold to the combustion chamber. This boost measurement is expressed in terms of pounds-per-square inch, or p.s.i.

Going back to our water analogy, if the intake stroke of a normally aspirated engine can be compared to pouring water from a pitcher to a glass, a supercharged engine would be like holding a high-pressure car-wash nozzle over that same glass and squeezing the trigger. Obviously, in this instance, a large percentage of the water would just splash out of the glass, but the idea of filling the glass more quickly and packing the water in with a pressurized nozzle is accurate. This is exactly what a supercharged engine does when compared to a normally aspirated one. By compressing, or "stacking up," the air/fuel molecules in the combustion chamber, a supercharger creates a denser air/fuel mixture that results in a more powerful explosion. This compressing action is so pivotal to a supercharger's functionality that Mercedes-Benz uses "KOMPRESSOR" emblems on their supercharged vehicles to identify this important component.

How much of a difference does a supercharger make in terms of performance? That depends largely on how the rest of the engine is set up and on how much boost the supercharger is creating. However, the potential for a 50 percent increase in horsepower and torque is easily attainable when adding a supercharger to a normally aspirated engine. That figure is for a supercharger upgrade alone, with no other modifications to areas like exhaust system design or camshaft specifications. It's this potential for massive horsepower gains that makes supercharging a favorite method for hot rodders and aftermarket tuners to use when creating a high-performance street machine. It's also an increasingly popular method used by manufacturers to up vehicle performance and efficiency, as in the case of the new Chevrolet Cobalt SS.

A final image to remember regarding supercharger performance comes from the "Mad Max" and "Road Warrior" movies. While the concept of turning a supercharger on and off via a switch was somewhat outlandish, the change in vehicle performance between these two modes of operation was well represented in the films. In the end, it took some post-apocalyptic mutants on a nitrous oxide-equipped dune buggy to catch our hero Max in his supercharged V8 Interceptor.

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