The Difference Between a Turbocharger & a Supercharger

That big positive displacement supercharger makes tons of torque but sucks fuel.

Turbos and superchargers are basically the same thing. Both are essentially compressors that shove extra air into the engine so it can burn more fuel, which makes more power. The primary difference between the two designs is how they're powered, which has huge ramifications on performance and engine personality.

Types

There are two ways to move or compress air: centrifugally and through positive displacement. Centrifugal compressors (the most common example of which would be your hair dryer) use rapidly spinning blades to shove air against a circular housing, after which it squirts out through a hole in the housing. Positive displacement (PD) compressors push air through the engine at a set amount per revolution, largely regardless of RPM. Centrifugal compressors are most efficient when spinning very fast, and PDs work best at low RPM. This is essential to understanding the difference between how superchargers and turbos work.

Drive

Regardless of type, the primary difference between turbos and superchargers are that superchargers are driven off of a pulley on the crankshaft and turbos aren't. Turbo compressor blades (impellers) are centrifugal, and are connected to another centrifugal compressor blade (called the turbine) via a shaft. The other compressor runs backward; instead of spinning to create pressure, the turbine receives pressure from the exhaust and spins in reaction. The turbine spins the impeller, which creates boost.

Low-Speed Boost

Superchargers are specifically designed to increase torque at low RPM. The first reason they do so is that the supercharger is running anytime the engine is, so it's making boost even at idle. The second reason is that many superchargers are positive displacement, which enhances low RPM boost even more. Centrifugal superchargers (which look like turbos with a belt attached) are popular in the aftermarket, but don't offer the boost response of a PD. Even so, the instant torque and boost response advantage goes to the supercharger.

Performance

Because superchargers are connected to the crankshaft, they can create a great deal of exponentially increasing parasitic drag. For instance, a Top Fuel dragster's supercharger draws about 50 horsepower at idle, but sucks an incredible 1,000 horsepower from the crankshaft at the engine's redline. By contrast, a turbo runs off of nothing but waste gases, so any power it produces goes straight into go-fast. One interesting side effect of the turbo's lack of drag is that turbo engines tend to get better gas mileage; the horsepower needed to drive that supercharger comes from gasoline, so it's always going to negatively impact fuel mileage. Advantage in horsepower and economy goes to the turbo.

Installation and Cost

Superchargers are almost always cheaper and easier to install than turbos. Centrifugal superchargers like those made by Vortech and Paxton require little more than a new bracket, a belt and some ducting. On many domestic fuel-injected V-8s, a centrifugal supercharger can add almost 100 horsepower without even reprogramming the computer. Turbos often require a completely new exhaust system as well as modifications to the oiling system and computer programming, all of which translate into more time and money.

References

• Allpar: Turbocharger FAQ
• Subaru: The Basics of Turbocharging
• AA1 Car Library: The Basics of Supercharging
• Import Tuner Magazine: Turbocharger vs. Supercharger