Muscle Mustangs & Fast Fords
Turbos & Superchargers - Forced Introduction
A breakdown of turbos and superchargers that everyone can understand.
The crankshaft drives the supercharger either by the front engine accessory drive (FEAD) belt or a secondary belt designated solely to drive the supercharger. These belts are often either 6-, 8-, or even 10-rib design. Higher boost pressures often require more than 180 degrees of belt wrap on the supercharger pulley to prevent belt slip. Blower size is determined by the amount of air pumped in one revolution. For instance, a 4.0L Kenne Bell can pump four liters of air per revolution (of the supercharger). This is why boost is almost instant, and max boost is reached sooner than a centrifugal supercharger or turbo.
Throttle bodies on cars or trucks equipped with positive-displacement superchargers are mounted either on or near (via an inlet elbow) the inlet port of the supercharger. This reduces air pulsation caused by the supercharger. A bypass valve is also usually integrated into the blower to allow excess inlet air to bypass the supercharger when the boost is not needed (deceleration and closed throttle). This also prevents pulsating air at the MAF, which would result in false readings.
Though the term “Roots” is commonly misused to describe all positive-displacement superchargers, Roots-style superchargers refer to only those using opposing rotors with a matching lobe-and-cavity design. The rotors draw air through the inlet and outward along the perimeter of the housing toward the outlet.
TVS, or Twin Vortices Series, superchargers are Roots-style blowers manufactured by Eaton, but are unlike normal Roots-style superchargers in that the four-lobe rotors feature 160-degree twists instead of the 60-degree twists found on normal Roots-style blowers. This helps with efficiency, reduces noise, and reduces heat production.
In more recent years, the twin-screw supercharger has gained popularity, which uses one male-lobed rotor and one female-cavity rotor (see photos) to compress the air charge. “The twin-screw compresses the air between the two rotors, instead of between the rotors and the case,” says Jim Bell of Kenne Bell. This is what makes the twin-screw different, and it also reduces heat produced by the supercharger by reducing air velocity inside the unit. Both types are very effective and will work well in most applications.
CentrifugalLike a positive displacement supercharger, a centrifugal supercharger is a mechanical air compressor that is belt driven by the engine’s crankshaft, but the similarities between the two end there. Instead of lobed rotors, a centrifugal supercharger uses an impeller to draw air into a snail-shell-shaped housing. They feature an increasing-diameter housing. The impeller forces the air from the center (inlet) toward the outside of the housing, which causes an increase in air velocity. This forces the air through the outlet to the engine’s intake system.
“A centrifugal supercharger compresses the air aerodynamically. The impeller, which spins as fast as 60,000 rpm, pulls air in and accelerates it into the surrounding compressor housing and then discharges it via the supercharger outlet. The faster the impeller spins, the more air it pulls in, compresses, and then flows to the intercooler and ultimately the engine,” says Jeff Lacina of ProCharger. Naturally, there are many impeller and housing designs to meet all desirable horsepower levels.
Because most centrifugal superchargers are mounted to the front of the engine, it is necessary to have sturdy mounting hardware. This will prevent deflection of the blower under strain of the belt. Manufacturers of centrifugal supercharger systems have taken this into consideration and provide heavy-duty mounting hardware. This mounting point also makes these systems relatively easy to install, even for someone with basic hand tools in their driveway.
Unlike positive-displacement superchargers, centrifugal superchargers don’t have a fixed output that increases directly with rpm. In other words, as rpm increases, output potential of the supercharger increases exponentially. This is why centrifugal superchargers are more efficient at higher rpm but are known for being less efficient down low.
Centrifugal superchargers are also known for being noisy, which some consider cool. Since they must be spun at a higher rpm to be efficient, most are driven by an internal transmission or step-up gears (to multiply impeller shaft speed). This gear set can be noisy, though some manufacturers have opted for helical-cut gears, which reduces noise for street applications.