Supercharging Your Engine

What hot rodders learned from the big bad wolf: If you huff and you puff, you'll produce more horsepower. Well, maybe not quite, but you get the idea. Think of your engine's pistons and cylinders as basically a pump. To run, it inhales big lungs-full of air spiked with some flammable gasoline vapor (the ideal ratio is 14.7:1). The more air that can be sucked in, the more fuel that can be burned, and the more power this fast-revving pump under your hood can crank out.

The more air that can be sucked in, the more fuel that can be burned, and the more power is cranked out.

That's all well and good, but the volume of air (and hence power-producing fuel) is inherently limited by atmospheric pressure. (That's why your engine seems less gutsy when it's climbing in the thin air of Pikes Peak.) How do we increase air pressure, then? Aside from heading back down the mountain, we can do it mechanically with a forced-induction system, either a turbocharger or a supercharger. From the standpoint of forcing more air into an engine, these systems are essentially two sides of the same coin. The basic difference is that a turbo uses exhaust flow to drive a fan, which in turn packs in more air. In contrast, a supercharger is driven by the engine's crankshaft, usually by a cogged belt and pulley system.

Although spinning a supercharger actually takes power away from the engine (as much as 20 percent of the stock output), it makes up for this parasitic loss by producing far more horses than what's required to drive it. All three types of blowers—Roots, centrifugal, and screw—can be adjusted to varying levels of boost (how much pressure they create), and a 50 percent increase in power is pretty typical. It's no wonder they're so popular—whether for towing, hauling or just pure performance, superchargers really pack a punch.

Roots

The Roots-type supercharger sits on top of the engine's intake manifold and is identified by its large rounded case. The name comes from the Roots brothers, who designed it in the 1880s as an air conveyor for mine shafts. General Motors later adopted the design in the 1930s as a supercharger for its GMC diesel engines, using it mainly to pump out exhaust gases. Eventually these "blowers" were adapted to increase the performance of gasoline engines by feeding in more air.

In general, Roots blowers have a two- or three-lobe rotor design, depending on the size of the case. They can move mass quantities of air, which stacks up inside the intake manifold to create positive pressure, or boost. Even though they look cool sticking through the hood, the downside is that they have higher discharge temperatures than other designs.

Screw-Type

The screw-type supercharger is also mounted on the manifold, but it usually is smaller and less conspicuous than a Roots blower. Bigger isn't necessarily better, though, because the screw-type actually compresses the air inside its housing between two screws. The screw supercharger offers potential advantages over non-compressor designs in that the outlet temperatures are normally lower, producing a denser air charge.

Centrifugal

The centrifugal type looks like it sounds: It's circular in shape, somewhat like a big snail shell. It's actually a turbo system that's switched sides. Instead of relying on exhaust flow to spin its air pump, the centrifugal blower relies on (you guessed it) crankshaft rotation.

Pros & Cons

As with most performance parts, the application should dictate the supercharger style. Both the Roots and screw-type have the advantage of providing immediate boost, making massive amounts of torque instantaneously. That's great for getting off the line—assuming you can maintain tire traction. The Roots doesn't have any other distinct advantage, other than it really looks awesome sticking out of the hood of a hot rod.

If space restriction is an issue, either the screw-type or the compact centrifugal-type can be mounted out of sight to create a stealthy hot rod. Due to the centrifugal blower's radial configuration and smaller size, the power increase is not as immediate or intense (similar to the lag of its half-brother the turbo). Even at a cruising speed of 1,800 to 2,500 rpm, it's not producing any significant amount of boost, and thus consumes less fuel than its supercharger cousins. When it spools up to higher revs, though, hang on.

Before you run out and buy a blower (figure on at least $3,000 plus labor), a few additional expenses should be considered. Exhaust-system upgrades are a must since the additional air a supercharger pumps into the engine must exit quickly and efficiently. Consider smog-legal headers, a low-restriction catalytic converter and a name-brand cat-back exhaust system.

To reduce the chance of detonation (pinging), install a low-temperature thermostat (160 degrees) and consider a boost/retard ignition device. This will retard ignition timing as the boost pressure increases in order to prevent detonation. Also, once supercharged, plan on running the good gas: 92 octane or better to prevent detonation. Other considerations that will affect supercharger installation include the camshaft profile and compression ratio of the engine.

Speaking of compression, if your engine has a fair number of miles on it, know that a supercharger does add internal stress, so you may have to do a piston-ring job to avoid blow-by of exhaust gases. On a more positive note, late-model cars equipped with electronic fuel injection (EFI) can handle superchargers rather easily because the computer control module precisely adjusts fuel delivery to match increased boost. Whichever type of blower you choose, you can be sure of one thing: It'll turn your vehicle into one big bad wolf.