Though we car crafters like to think of ourselves as a more sophisticated bunch than the average car guy, the truth is we all have a warm spot in our collective hearts for excessive and gratuitous displays of horsepower. And few things scream wretched excess more than a supercharger jutting through the hood. The bigger, the better, too. An 8-71 on a small-block Chevy-why not?
Still, it is better to take a proverbial step back before plunking down your hard-earned cash for what may be a clunker of a supercharger, which is what you will have if it is improperly matched to your engine. This article will provide a basic look at positive-displacement superchargers, hopefully giving you a good base of knowledge to refer to when planning a forced-induction build.
Positive Displacement vs. Variable Displacement
What's the difference? The an-swer is simple enough. A positive-displacement pump (whether it is pumping fluid or air) is one that moves the same amount of a substance during each revolution: One liter of air goes in, one liter is pumped out the other side no matter how fast the pump is turning. In contrast, a variable-displacement pump moves less volume at low speed than it does at high speed.
Though you can think of a centrifugal supercharger as a crank-driven turbocharger, a turbo
Here's an easy analogy we all can identify with: comparing the engine in your car with the blower motor in your HVAC system (assuming you haven't thrown it away Jeff Smith style in a quest to shave every ounce of extraneous weight). A piston engine is one type of positive displacement pump. Whether it's spinning at 6,500 rpm or you're turning it over by hand, it should pump the same amount of air on every revolution. Your heater blower motor, on the other hand, does not work that way. Like all fans, it moves more air the faster it spins. This is especially noticeable when trying to cool your car after it's been sitting in the hot sun. We all know you've got to crank the fan to high to get the air moving.
Forced induction operates the same way: Though the ultimate goal is to push more air into the engine, you can either do it with a positive- or variable-displacement pump. A generic term for these devices when used on a street machine is supercharger, and they can be driven either by the engine via a mechanical connection or by exhaust gases via a turbine wheel placed in the exhaust system. The exhaust-driven type is referred to as a turbocharger, a shortened version of the term turbosupercharger-literally a turbine [driven] supercharger. All turbochargers are variable displacement-type pumps, but beltdriven superchargers can be either positive or variable displacement. Variable displacement-type superchargers are known as centrifugal superchargers, and on the positive displacement side of things, the industry has latched on to two types-Roots and twin-screw superchargers-to get the job done.
We will not be addressing centrifugal superchargers in this article, however, because we feel the trend in the industry is focusing on the positive displacement-type superchargers. Additionally, we believe that a modern, efficient positive-displacement supercharger is currently the most economical way to make more power. Note that by economical, we are considereing the amount of time involved in installing the system as some kits are as easy as swapping an intake manifold. How much is your time worth?