Dale Amy
October 1, 2003

Horse Sense:
Keep camshaft duration short with supercharging, otherwise boost will just blow the mixture right out the exhaust port.

Show me a Mustanger who doesn't place a supercharger high on his wish list, and I'll show you a statistical oddity. Our lust for blowers is seemingly universal-whether our ride's a V-6, a GT, or a Cobra, a pushrod, or a modular-and with good reason, since the perceived effect of boost is not unlike a hefty increase in cubic inches, no matter what we're beginning with. Today's properly engineered blower kits can also be emissions-legal and have little or no effect on everyday driveability, especially in comparison to a nervously cammed, high-strung, naturally aspirated screamer capable of the same power levels. The bottom line is, superchargers are efficient power adders for race or street use, but they're also expensive, so it's important to consider your needs and understand some blower basics before making the sizeable financial commitment for one kit or another.

Different Approach,Same Result
There are two markedly different categories of supercharger vying for your Visa card's attention-centrifugal and positive displacement-and within the latter category are two distinct types.But before examining the differences, let's look at the similarities. The job of any supercharger is no more complicated than to force air into an engine's combustion chambers at higher-than-atmospheric pressure, ramming in lots of additional oxygen molecules for combustion. In simplest terms, superchargers are belt-driven air pumps (as opposed to turbochargers, which are driven by exhaust heat energy).

Since an engine's ability to produce power is directly proportional to the mass of its air/fuel mixture, our pre-vious suggestion that the effect of supercharging was comparable to an increase in displacement was not mere jour-nalistic hype. An engine's displacement can be thought of as a measurement of the total volume of air it takes to fill its cylinders, so that the eight cylinders of the current modulars, for instance, theoretically displace a total of 4.6 liters, or about 281 ci of air. Fairly basic stuff, but we say "theoretically" because the volumetric efficiency of a stock, naturally aspirated engine is typically only about 75 percent, meaning a cylinder is never really filled to capacity during the intake stroke (volumetric efficiency being a comparison of the amount of air/fuel charge that an engine could draw in, to the amount that it actually draws in). Because a supercharger forces air into the cylinders, it can raise an engine's volumetric efficiency to considerably more than 100 percent, quite literally making it act as though it has more displacement.

So the goal of a higher density of oxygen molecules in the cylinder is the same, but centrifugal and positive-displacement blowers go about achieving it in different ways, with resulting variations in boost characteristics. The design of positive-displacement superchargers, including both Roots-type and twin-screw type, is such that they displace the same volume of air with each revolution, so that their boost increases quickly in a linear fashion with rpm. With centrifugal blowers, on the other hand, the air output per revolution is not fixed, but rather increases as the square of rotational speed. Centrifugals produce comparatively little boost at low rpm and really come on as revs climb. Only you can decide if you'd prefer generous boost throughout the rev range, or a big heady wallop at the top of the tach.

Blower designs also exhibit differing levels of mechanical and thermal efficiency. Think of mechanical efficiency in terms of how many horsepower it takes to operate the supercharger at a given boost level, and thermal efficiency as how much heat is added to the air charge for every pound of boost generated. More on this as we go along. Let's look a bit closer at the different players.