Positive displacement superchargers are easily recognized, as they sit directly on the motor itself. Normally this is directly on the top of the engine, as it does with the Mustang. There are some exotic cars, however, that use a positive displacement supercharger mounted more along the side of the engine. The first positive displacement supercharger on record was the Roots-type supercharger, which has origins dating back to the mid-1800’s.
The Roots-style supercharger is beginning to lose favor compared to the twin-screw (Lysholm) superchargers. This is primarily due to efficiency benefits with the twin-screw design. The twin-screw rotors use very tight tolerances to pull the air into the heart of the supercharger and compress it. Twin-screw designs also compress the air better than the rounded lobes of the Roots-style supercharger, which provides a better air charge when compared to power needed to turn the rotors. The Roots-style supercharger uses a more rounded-style lobe with tolerances that aren’t as tight as the twin-screw’s rotors.
A positive displacement supercharger still uses the basic supercharger concept of compressing air and overcoming the engine’s vacuum, but achieves this is a slightly different way than the centrifugal supercharger does. With a positive displacement supercharger, air enters it, and ends up at the rotors. From there, the rotors (whether twin-screw or the rounded Roots-style) compress the air as they spin inside the supercharger. The rotors are powered by a pulley at the front of the supercharger that is driven by the engine’s accessory belt system.
When the air reaches the rotors, they compress it, forcing it out the underside of the supercharger’s body. From there, it enters directly into the intake of the engine. As with the centrifugal supercharger, you’ll see a minimum of 5-10 psi of boost with modified and strengthened motors, often sporting much more boost.
Depending upon the exact type of positive displacement supercharger, there may be more than two rotors used. With the modern positive displacement superchargers used on Mustangs, twin rotors are normally employed. Just as with the centrifugal supercharger, the rotors spin at high speeds and lubrication is required inside the supercharger. Positive displacement nearly always use a self-contained oil system, which will requires oil changes on a regular basis.
As a result of the benefits of twin-screw rotor use, the positive displacement industry as a whole has been moving towards twin screw designs in recent years. Regardless of the design approach used, the positive displacement supercharger has one distinct advantage over the centrifugal superchargers. That advantage involves the production low-end torque. Since a positive displacement supercharger delivers an almost constant flow of air once it’s called upon, the power gained from the supercharger is nearly instantaneous, unlike the centrifugal type, which tends to apply the power more gradually as the RPM increases and reaches a certain range.