Positive-Displacement Superchargers
Positive-displacement superchargers are those that spin a pair of multilobed rotors that mesh tightly to squeeze air through an outlet under high pressure. The displacement is derived from the amount of air delivered with each revolution of the supercharger. Typically, the larger the rotors, the more air the supercharger displaces.
Most enthusiasts and hot rodders were introduced to street supercharging with the “Jimmy”-style Roots superchargers that originated on large truck and bus engines but were adapted to automotive engines. Although impressive looking and sounding, these blowers are pretty inefficient, but they’re guaranteed to draw a crowd on cruise night.
Within the spectrum of positive-displacement superchargers are Roots types and Lysholm types. Following are design details and operational differences of the various supercharger types.
Roots-Type Supercharger
The Roots-type supercharger is an engine-driven air pump that contains a pair of long rotors that are twisted somewhat like pretzel sticks. As they spin around each other, incoming air is squeezed between the rotors and pushed under pressure into the engine, forcing more air into the engine than it could draw under “natural” aspiration. The rotors are driven by a pulley and belt that are connected to the engine’s accessory drive system.
With a Roots blower, a discharge hole is located at one end of the supercharger case. As the rotors mesh and squeeze air, it is forced at high pressure through the discharge hole. It is relatively efficient, particularly in the later designs refined by OEM supplier Eaton.
The Roots blower was used on a variety of high-end automobiles in the early 20th century, including Cords, Bentleys, and Mercedes, but it really made its mark on the aftermarket performance world when it was used on GMC-built transit buses of the 1930s and later. The buses used large superchargers to pump up the horsepower of their diesel engines. By the 1950s, enterprising drag racers began attaching GMC (also known as “Jimmy”) blowers to automotive gasoline engines, and the rest is history.
The 71-series GMC blowers were adapted to street cars too. Those are the iconic superchargers seen reaching through the hoods of so many vintage street machines and Pro Street hot rods.
To the generation of late-model performance enthusiasts, Roots blowers are synonymous with Eaton superchargers. That company pioneered the use of smaller-displacement, low-profile Roots blowers on everything from Jaguars to the Pontiac Grand Prix GTP. The Corvette ZR1 uses an Eaton supercharger too.
Although Blower Drive Service offers manifolds to adapt the classic, tall 71-style blower to LS engines, those considering a Roots-type supercharger system for their vehicle are selecting one with an Eaton compressor.
Refinements to Eaton superchargers’ rotor design over the years have made them quieter and more capable of greater airflow and boost; the packaging size and rotor speed is the biggest restriction to making tremendous power with them. Look around at professional and semiprofessional drag racers who rely on superchargers or turbochargers for power adders and you see virtually none use an Eaton-type blower. They just don’t generate the boost necessary to support a very large displacement or the high-RPM power needs.
That said, Eaton blowers are exceptionally durable, dependable, and on the street make reasonably good power at lower RPM, especially when compared with centrifugal superchargers and turbochargers. The OEM quality of Eaton systems makes them nearly bulletproof and delivers exceptional drivability. They’re not loud at low RPM and don’t have on/off performance characteristics; the power comes on smoothly and firmly.
And while the hardware (including a custom intake manifold) can make Eaton-based kits somewhat expensive, their installation is clean, unobtrusive, and as close to a factory-style installation as can be found in aftermarket kits. Generally, most Eaton-based bolt-on kits are offered through California-based Magnuson, which developed a number of very popular kits for many LS-powered vehicles. Indeed, many of Magnuson’s kits represent the easiest-to-install systems and have earned a reputation for excellent reliability.
Eaton’s TVS
The Twin Vortices System (TVS) represents the sixth generation of Eaton’s ubiquitous Roots supercharger design. It blends elements of a twin-screw compressor, including a four-lobe, high-helix (160-degree twist angle) rotor design. Previous Eaton superchargers featured a conventional three-lobe design.
As with the twin-screw design, the TVS supercharger was developed to expand the efficiency range of the supercharger to deliver more power at lower RPM and sustain boost at higher RPM while requiring less engine power to drive. And when compared with previous three-rotor designs, the TVS represents a night-and-day difference in overall performance. Wherever possible, the use of the TVS compressor is recommended. It is currently manufactured in 1.9-liter (MP1900) and 2.3-liter (MP2300) displacements. The design also features an internal bypass valve.
The TVS blower was designed primarily for OEM applications. In fact, it was driven by GM’s performance and efficiency requirements for the LS9/LSA engines, which represent the first production applications for this new compressor (see “GM Factory-Supercharged LS9 and LSA Engines” later in this chapter).
Since appearing under the hood of the C6 Corvette ZR1 and the Cadillac CTS-V models, the TVS supercharger has grown into the aftermarket. Eaton’s Magnuson outlet offers a number of bolt-on kits for engines that have either cathedral- or rectangular-port heads. Additionally, Australia-based Harrop Engineering offers TVS-based kits (1.9L and 2.3L versions) for the 6.0L Pontiac GTO/G8 GT, as well as the VE-series Holden Commodore; and Edelbrock offers TVS-based “E-Force” supercharger kits for most popular LS production vehicles.
The C6 Corvette ZR1 introduced supercharging from the factory, relying on an Eaton TVS-based blower to push the 6.2L engine’s output to 638 hp. It was the most powerful production engine ever from Chevrolet, and it was eclipsed only by the supercharged LT engines that came later in the C7 Corvette Z06 and ZR1 models. (Photo Courtesy General Motors)
Earlier Eaton-based supercharger systems, such as those found on the 3800 V-6 and larger V-8-size compressor, make excellent, usable power and absolutely help lower a vehicle’s elapsed time at the drag strip. However, a comparatively limited power range and a tendency for the compressor to soak up engine heat make them better suited to vehicles used primarily on the street and occasionally at the track. Also, they feature drive pulleys that are pressed onto the drive gear. Swapping them to adjust boost pressure is very difficult and almost impossible to do with the supercharger installed on the engine and in the vehicle. Only specialized pulling tools designed for the job should be used; even then, there’s no guarantee damage won’t occur to the supercharger’s nose section.
The newest edition to the Eaton lineup is the TVS 2650, a larger, 2.65L compressor that was introduced on the 6.2L LT5 engine that powered the C7 Corvette ZR1. Aftermarket versions of the blower have been introduced by companies including Harrop, as this larger supercharger promises to elevate the capability and output of LS engines. Besides offering a larger displacement with greater boost capability, the angle or “pitch” of the rotors is greater: 170 degrees versus the previous 160 degrees, contributing to greater overall airflow efficiency.
Lysholm/Twin-Screw Types
The Lysholm-type or twin-screw supercharger is similar in design and function to the Roots type, including squeezing air through a discharge hole in the case to deliver boosted