What is the force that drives most "car nuts" to squeeze every last portion of horsepower from their car's engine? Since late 1950's the ultimate icon of horsepower has been the supercharger, (or "blower") and in the late 1970's the turbocharger.

Both of these means of "artificial aspiration" were developed for high altitude aircraft. The higher you go the thinner the air, the fewer horsepower your engine develops. In technical terms one atmosphere of pressure at sea level measures 14.7 pounds per square inch over a vacuum. A tire gauge at "zero" PSI is really measuring 14.7 PSI over the true zero of a vacuum at sea level. For every 1,000 feet you go up, the atmospheric pressure drops .5 (or ½) PSI. This means the atmospheric pressure in San Francisco is 14.7 PSI, and 12.2 PSI in Denver, Colorado. These numbers do vary with the weather, and are considered "baseline". This is also why gasoline octane ratings are lower at higher altitudes. Less oxygen, the less chance of pre-combustion, or "knock" in layman's terms.

Like I said, in the 1950's hot rodders found that bolting on a salvaged (or stolen) General Motors industrial supercharger made monsters out of otherwise mediocre engines. Soon, nearly every serious drag racing competitor had a "blower" on top of his engine.

At this point let us define the differences between a "supercharger" (or blower) and a "turbocharger". (Or Turbo)

Belts and pulleys mechanically drive a supercharger directly from the crankshaft. It is also usually a "displacement" type of air pump, which means is uses a "squeezing" action to compress the air. While the amount of horsepower gained is considerable, the parasite effect of the blower's need for horsepower to do its job is considerable too.

A turbocharger is a "centrifugal" (It spins) type of air pump that is driven by the waste exhaust gasses from the engine it is supplying compressed air to. Big advantage here is that you are not removing prime horsepower from the engine to do this job, but recycling waste exhaust gasses to do basically the same thing.

Now, a principle of thermodynamics. Compressing air makes it hotter. Think of inflating a bicycle tire with a hand pump. By the time you are finished, the pump is very hot to the touch. You have squeezed the molecules of air into a smaller volume, which causes the molecules to bump into each other, and that releases energy in the form of heat.

The piston in your engine does exactly the same thing. As the piston comes up on the "compression" stroke, the temperature inside the cylinder rises dramatically. To the point where it can cause the gasoline mixture to ignite sooner than the sparkplug is set to do it at, and you have a case of pre-combustion, or the "clanking" sound called knock. That is why they have different grades of octane at the gas station. The higher the octane, the greater the gasoline's resistance to pre-ignition.

Now add to those temperatures, the additional temperature of air pre-compressed by a supercharger, and you have one hot place to mix gasoline in.

To make matters even worse, a turbocharger soaks up the heat from the exhaust gasses that are driving it, which is then added to the air it is compressing, making the air temperature at the intake valve around 300 degrees Fahrenheit.