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CHAPTER 2

SUPERCHARGER TYPES AND SELECTION

Superchargers come in many different shapes and sizes, but they are related by a common attribute: they generate boost pressure via an engine-driven mechanism. Typically, superchargers are driven by a belt connected to the crankshaft.

When it comes to the commercially available superchargers for LS engines, there are two basic types: positive displacement and centrifugal. Positive displacement superchargers are “draw-through” designs, where the air charge is compressed after the throttle opening. Conversely, centrifugal superchargers, like turbochargers, are a “blow-through” design, where the air is compressed prior to entering the engine through the throttle opening.

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 air pressure to the engine.

For those who are building an LS engine for a street rod, a muscle car, or another older vehicle and appreciate the look of the old-school 71-style Jimmy blowers, Blower Drive Service offers supercharger kits for engines that use LS cathedral-port heads. This example was built by Martin Motorsports to be installed in a vintage Chevy II. Note the custom, marine-style intercooler sandwiched between the blower case and the intake manifold. It was the best solution to maintain the vintage drag racing look of the engine with its classic “bug catcher” intake.

Displacing 2.65 liters, the Eaton TVS 2650 produces 14 pounds (0.9 bar) of boost in the C7 Corvette ZR1’s LT5 engine. That’s about 4.5 psi more than the LT4 engine, but it is achieved with a slower 15,860-rpm maximum rotor speed, which helps keep down the pressurized air charge’s temperature. Compared to the rotors in the LT4’s supercharger, the LT5s are larger in diameter and have a unique, 170-degree pitch for the four-lobe design versus the previous TVS 160-degree pitch. The higher-pitch angle enhances the blower’s efficiency at high RPM, helping it sustain max boost through the top of the RPM band. (Photo Courtesy General Motors)

The Gen V Camaro ZL1’s LSA engine used a smaller, 1.7L TVS supercharger compared to the C6 ZR1’s larger 2.3L blower. The result was 580 factory-blown horsepower. (Photo Courtesy General Motors)

This photo shows the unique, four-lobe rotors of the Eaton TVS blower. The quartet of lobes combined with the high helix (rotor angle) design gives the blower a quasi-twin-screw look. That’s intentional because, similar to a twin-screw design, the TVS delivers greater performance at low and high RPM. (Photo Courtesy General Motors)

The Eaton compressor-based Magnuson bolt-on kits are very popular for C5/C6 Corvettes, Pontiac G8s, fifth-generation Camaros, and trucks/SUVs, and for good reason. They are relatively easy to install, deliver an excellent return on investment when it comes to horsepower, and have proven to be very durable. However, Corvette applications have underhood clearance problems, and an aftermarket or modified stock hood is typically required.

Based on the technology introduced on the C7 Corvette ZR1’s LT5 engine, Eaton-based 2650 (2.65L) compressors are the largest-displacement TVS-style superchargers available. The 2650’s rotors are larger in diameter than previous TVS rotors, which raises the overall height of the blower. That can present underhood clearance problems for some vehicles. The kit shown is from Harrop.

The large displacement of the 2.3L TVS compressor helps generate truly impressive performance. In the application seen here, a Harrop-supplied TVS blower was used on a 7.0L LS engine to make nearly 900 daily drivable horsepower on readily available pump gas. The engine was then stuffed into a Pontiac Solstice roadster by Thomson Automotive.

Edelbrock’s supercharger kits use Eaton’s four-rotor, 2.3L TVS compressor at their cores. Like the factory LSA and LS9 engines, the E-Force systems feature air-to-water intercooling systems with dual-brick-style heat exchangers mounted on top of the supercharger assembly. Design features of the E-Force system include a front-driven compressor and long, 12-inch intake runners that optimize low-RPM torque. For C6 Corvette owners, the advantage of the E-Force kit is that it mounts under the stock hood.

Rather than using the inter-meshing lobes of the Roots type, the Lysholm uses a pair of worm screw-type rotors that squeeze air together to generate boost. It also generates internal compression, meaning it develops pressure progressively as the air is continually squeezed by the screws on its way to the discharge hole. This can help build more low-end power and deliver more boost at lower RPM. The relative efficiency of twin-screw superchargers is greater than a conventional Roots type. They also enable generally higher boost levels than Roots or centrifugal superchargers, providing 20 pounds or more with some compressors.

Sweden-based Lysholm Technologies AB (a company that has undergone several corporate changes in recent years) is the name behind the technology, and it manufactures many sizes of twin-screw compressors, ranging from 1.2 to 3.3 liters in displacement. Rather than offering retail systems, the company licenses its product to other manufacturers, including OEM companies such as Ford, which used a Lysholm supercharger on the 2003–2006 GT sports car (through a licensing agreement with Eaton that essentially made them Eaton superchargers).

A Vortech supercharger system is installed on a fifth-generation Camaro SS.

In the performance aftermarket, Whipple Industries is just about the most recognizable name in twin-screw technology. Its Lysholm-type blowers derived from industrial air compressors were adapted to automotive use. For years, Whipple relied on the twin-screw compressors from the company currently known as Lysholm Technologies AB, but since 2005, it has used a twin-screw compressor of its own design. Whipple offers kits for Gen V Camaros, C6 Corvettes, LS-powered trucks, and the Chevrolet SS sedan.

In 2009, Vortech joined the twin-screw blower fray with the addition of a Lysholm-based supercharger of its own. Rather than manufacturing its own blowers, Vortech licensed the compressors from Lysholm Technologies AB and developed its own installation kits. Vortech offers 2.3- and 3.3-liter superchargers. Currently, the only dedicated twin-screw kit for LS engines from Vortech is for 5.3L truck engines. The company also offers “tuner” kits that can be adapted to a variety of LS engines, as long as a suitable manifold is available to match the ports on the cylinder heads.

Another player in the twin-screw market is Kenne Bell. At the time of publication, the company offered several twin-screw supercharger kits for the Gen V Camaro with compressors displacing 2.8, 3.6, 4.2, and 4.7 liters.

This Lysholm twin-screw supercharger system is installed on a fifth-generation Camaro SS.

The principle of the twin-screw design is a pair of screw-shaped rotors that intermesh much like the rotors of a Roots blower, but the rotors’ shapes create internal compression that helps boost low-end power. Whipple designed its own compressor in 2005. It features self-contained lubrication, a large bypass valve, and the capability of up to 30 pounds of boost. It is manufactured in a variety of sizes with many of them larger than the largest-displacement Eaton TVS compressor.

Vortech’s Lysholm-supplied twin-screw supercharger system for 5.3L LS-powered GM trucks includes an integral bypass valve within the supercharger housing. The kit also includes a charge-cooler/intake manifold assembly (with fuel rail mounts), higher-rate fuel injectors, a pump system for the intercooler, and a cold-air-style air intake system. Tuning calibration is provided via a DiabloSport Predator programmer. Similar systems are expected for a variety of other LS-powered vehicles.

Centrifugal Superchargers

Although engine-driven and not exhaust-driven, a centrifugal supercharger generates boost much like a turbocharger. It uses an impeller (similar to a turbine) that spins upward of 40,000 rpm to draw air into the compressor and blow pressurized air into the engine.

The impeller is the engine-driven part of the supercharger, as it is linked via a pulley and belt to the crankshaft. After the impeller draws air into the compressor head unit, it is squeezed and forced into the supercharger’s scroll (a chamber within the head unit that funnels the compressed-air charge out of a discharge tube and toward the engine’s throttle body). The scroll has a progressive shape that gets larger the farther it is from the center of the head unit. That design feature reduces airflow while simultaneously increasing the air charge’s pressure.

Air is compressed in the head unit when it leaves the impeller and is forced into the scroll. A venturi-like outlet, through which the air is forced, creates boost pressure, so the greater the impeller speed and the faster the air moves through the venturi, the higher the boost pressure.

A centrifugal supercharger is comparatively efficient, requiring relatively little engine power to drive, but its downside is the need for very high impeller speed to make horsepower-building boost. That’s why centrifugal blowers are known mostly as mid- and higher-range power adders; the impeller speed at lower RPM doesn’t make sufficient boost, and once the maximum impeller speed is achieved (usually around the peak horsepower mark), boost levels trail off at higher RPM.

A change to a smaller-diameter drive pulley can add a few extra pounds of boost, but matching a properly sized compressor head unit with the displacement and airflow capabilities of the engine is the key to sustaining power throughout the middle and upper ranges of the RPM band.

The two main players in the centrifugal supercharger business are Vortech Superchargers and ProCharger. Another centrifugal blower manufacturer is Rotrex, but currently, there were no direct applications for LS engines. The following is a closer look at the offerings from Vortech and ProCharger.

Vortech Superchargers

Vortech centrifugal superchargers have been mainstays of both the street and racing worlds. Typically, Vortech blowers are known for their relatively quiet performance and engine-oil-fed lubrication system (except for the V-3 compressor). Vortech has also been at the forefront of developing bolt-on kits, which are available for most popular LS-powered vehicles, including the fifth-generation Camaro. Several aftermarket companies, such as A&A Corvette, use Vortech compressors as the basis for tailored supercharger systems (see chapter 5 for details on installation).

Vortech offers a number of different compressors designed for a wide variety of performance requirements. They’re also subdivided among “trim” types: X trim, F trim, SCi trim, etc. Here’s a quick rundown on them.

V-1 Series: A high-performance compressor with high-speed ball bearings that makes it compatible for high-boost, cog-belt racing applications. Depending on the trim, a V-1 is capable of up to 26 pounds of boost and 1,200 cfm of airflow.

V-2 Series: Lower maximum boost (17 to 22 pounds, depending on the trim) and slightly lower maximum airflow than the V-1, but designed as a direct replacement. V-2 SQ trim is known for exceptionally quiet operation.

V-3 Series: The only internally lubricated compressor in Vortech’s portfolio. A V-3 compressor fills the mounting brackets for V-1, V-2, V-4, V-5, and V-7 compressors. Maximum boost and airflow is similar to V-2 compressor trims.

V-4 Series: A racing-intended compressor that Vortech claims is twice as efficient as a Roots blower at 12 pounds of boost. Depending on the trim, a V-4 can produce up to 32 pounds of boost and flow 2,000 cfm.

V-5 Series: Designed for smaller-displacement engines, typically 4- and 6-cylinders, the V-5 is not well-suited to the airflow capabilities of LS V-8 engines.

Vortech centrifugal superchargers typically make 6 to 8 pounds of boost in most bolt-on kits, but a range of higher-performing, racing-oriented compressors can supply more than 30 pounds of boost. Most of Vortech’s compressors are interchangeable with the company’s brackets, allowing you to swap compressors to better suit your engine combination. (See chapter 5 for installation details on a Vortech-based bolt-on system.)

V-7 Series: A high-flow, racing-intended compressor designed for modified engines built to accommodate high boost levels. Depending on the trim, a V-7 can flow more than 1,400 cfm and generate 30 pounds of boost.

V-9 Series: This more compact compressor is designed for engine compartments with little room, such as the fourth-generation F-Bodies. They’re also designed for smaller-displacement V-8s (smaller than 400 ci). Maximum boost is about 14 pounds and maximum airflow is 750 cfm.

V-30 Series: Replacing the V-20 series, the V-30 is designed for racing applications and can push enough air to support 1,000 hp with about 35 pounds of boost.

An excellent reference chart of Vortech’s various compressors, trims, and boost/airflow capacities is available at vortechsuperchargers.com.

ProCharger

Unlike Vortech blowers, most ProCharger compressors have a self-contained lubrication system, meaning there’s no need to tap the oil pan for the oil feed source. Some of the ProCharger compressors are relatively loud, especially at idle, but their street-based blowers have become admirably quiet in recent years. The company offers bolt-on kits for most LS-powered production models, including C5 and C6 Corvettes, fourth-generation F-Bodies, Gen V Camaros, Pontiac GTOs and G8s, and more.

Like Vortech, there are numerous compressors in the ProCharger portfolio with several designed specifically for racing applications. In fact, ProCharger offers the largest centrifugal superchargers, with some capable (including the colossal F-3X-143 compressor) of producing up to 60 pounds of boost and flowing 4,500 cfm. And with less-complex tubing routing than high-boost turbo systems, ProCharger offers a viable alternative for the street and drag strip.

ProCharger Compressor Comparison Chart
Compressor	Maximum Airflow (cfm)	Maximum Boost
P600B	1,200	24
P-1SC	1,200	30
P-1SC-1	1,200	32
P-1SC-2	1,200	30
D-1	1,400	32
D-1SC	1,400	32
F-1	1,525	38
F-1A	1,650	38
F-1C	1,850	38
D-1R	2,000	32
F-1R	2,000	38
F-2M	2,250	40
F-2	2,700	38
F-2R	2,750	38
F-3A-117	2,800	40
F-3A-123	3,100	40
F-3R-131	3,600	45
F-3R-139	4,000	45

Here’s a typical bolt-on ProCharger system on a fifth-generation Camaro SS. As with Vortech superchargers, ProCharger’s larger compressors are mostly interchangeable with the bracketry, allowing custom combinations. Proper tuning is paramount when using a high-boost, large-displacement compressor, as is the durability of factory engine parts.

ProCharger’s large compressors and cog-style belt-drive systems are designed for racing applications, and several compressors are capable of 40 pounds of boost or more. It is these big blowers that are giving some turbo systems a run for their money in drag racing. This is ProCharger’s F3R compressor.

It is worth reviewing the complete compressor lineups from Vortech and ProCharger before investing in a bolt-on kit or a stand-alone setup for a custom engine project. Take the time to compare their respective performance traits and match them to the goals of your project.

Kit and Cost Considerations

Unlike turbocharger systems, there are a great number of bolt-on blower kits designed to work on stock LS engines. The number of kits changes constantly as new vehicle models are introduced and supercharger manufacturers and other aftermarket companies develop kits for them. For Roots-type systems, Magnuson’s kits cover most popular LS-powered vehicles. When it comes to twin-screw systems, there are few choices for vehicles with rectangular-port heads; most are designed for earlier, cathedral-port engines (LS1, LS2, and LS6). Vortech’s new twin-screw blower is offered in kit form for the rectangular-port LS3 engine of the Camaro and G8 GXP with more applications expected.

To ensure pump-gas compatibility and to lower the risk of detonation, bolt-on kits typically make less than 10 pounds of boost and deliver around 80 to 125 additional horsepower with preprogrammed tuning. Greater performance is attainable with custom tuning, smaller-diameter pulleys, and the like, but such changes increase the risk of detonation on stock engines with high compression ratios and cast rotating parts.

Of course, cost is an important factor for any enthusiast selecting a supercharger kit. One of the important factors in the centrifugal supercharger’s favor is generally a lower purchase cost in kit form when compared with Roots/screw kits. That’s because the ability to mount the compressor head unit almost anywhere allows manufacturers to bundle most of the kits with universal components. Typically, only relatively inexpensive mounting brackets and other related components separate, say, a 2006 GTO kit from a 2002 Camaro Z28 system.

The Roots/screw-type systems generally require a dedicated intake manifold that must be matched to the heads, and casting an entire intake manifold is a lot more expensive than laser cutting a steel mounting bracket for a centrifugal blower.

Where Roots/screw blowers can narrow the price gap with centrifugal kits is in the installation labor charge. Typically, it takes less time to install a Roots/screw-type system on most vehicles, as centrifugal blowers typically require more extensive modification of the accessory drive system.

The relative ease of installation and tuning, as well as the limited impact on other factory vehicle systems, makes a bolt-on supercharger an increasingly cost-effective alternative to a custom-built engine. That’s exactly what Berger Chevrolet did with the latest versions of its limited-production Berger Camaros. On its earlier fourth-generation models, the dealership sourced custom, 500-hp, naturally aspirated engines that cost much more to build and install than the Magnuson kits used on its fifth-generation cars.

Here’s the 550-hp engine of the Berger Camaro. It uses nothing more than a Magnuson kit (non-TVS compressor) and the kit’s supplied tuning upgrade. It is a simple upgrade that delivers a huge increase in performance.

Positive-Displacement Versus Centrifugal Blowers

When it comes to supercharged horsepower, positive-displacement superchargers and centrifugal blowers produce it differently. In simple terms, a centrifugal supercharger’s boost increases exponentially with engine speed, while a positive-displacement supercharger’s airflow is linear with maximum boost occurring very low in the RPM band. That means a Roots or twin-screw blower that delivers, for example, 500 cfm of air at 2,500 rpm pushes 1,000 cfm at 5,000 rpm.

With a centrifugal supercharger, boost builds in a nonlinear way, much like a turbocharger. As RPM increases, the airflow from the compressor increases at a faster rate. Because of that, maximum boost is not achieved until the engine’s redline, or maximum RPM level.

The differences in airflow delivery create very different performance curves and driving experiences. In general terms, a positive-displacement supercharger has a flatter power curve with more low-RPM power. The centrifugal delivers a greater feeling of increasing power as the revs climb. On the street, and all other things being as equal as possible, a positive-displacement blower feels stronger on the low end, especially directly off idle. A Roots or twin-screw blower makes a small amount of boost whenever the engine is running. The centrifugal, on the other hand, “rolls” into its boost and is generally easier to launch, with a stronger feel through the mid- and upper-range RPM levels.

Unlike a Roots or twin-screw blower, which delivers maximum boost at relatively low RPM, a centrifugal (such as this ProCharger D1SC) increases its airflow with the RPM, much like a turbo. Maximum boost comes at the engine’s maximum RPM.

The nonlinear airflow delivery also makes the centrifugal supercharger better suited for drag racing, because the graduated boost application enables an easier launch, with greater power coming on as the RPM increases. Of course, with peak boost not occurring until redline, the blower’s effectiveness is not fully realized at lower RPM.

In general terms, a street vehicle with a positive-displacement blower feels the effects of the blower immediately and at all low-RPM levels, while a centrifugally blown car feels more like stock until around the 3,000-rpm level. There is also a more pronounced application of the power with a centrifugal blower, but not the “on/off” feeling of a turbocharger.

How Much “Blower” Do You Need?

Unless you are adapting a GMC 71-series-style Roots blower, which is offered in tremendous size increments for drag racing, there is a limit to the effectiveness of many bolt-on, underhood-type superchargers. If the supercharger (be it a positive displacement or centrifugal) can’t flow enough air to support the engine’s high-RPM requirements, horsepower falls off and the effectiveness of the supercharger is greatly diminished. Increasing the boost pressure increases the effectiveness to a certain degree, but in the end a supercharger with a larger compressor is the best way to optimize the blower’s performance across the RPM band.

The great airflow capability of LS engines and the larger displacements offered in production and aftermarket versions of the engine make sizing a supercharger particularly important, as the smaller-displacement superchargers that were common on a street car only a few years ago simply don’t flow enough to support later and larger-displacement LS combinations.

At the time of publication, the 4.7L Lysholm twin-screw supercharger from Kenne Bell was the largest positive-displacement supercharger offered in bolt-on kits, although the Eaton TVS 2650 blower topped the Roots-type offerings. Whipple offers 3.3-, 4.0-, and 5.0-liter compressors, but none had been adapted to LS engines in bolt-on kits.

When it comes to centrifugal superchargers, both Vortech and ProCharger offer a number of large compressors to suit high-powered street engines and dedicated racing combinations.

Getting the most from a supercharger, regardless of the compressor design, is dependent on flowing enough air to satisfy the airflow capability of the engine. A blower’s maximum boost will not be realized on a large-displacement engine that isn’t matched with a commensurately sized compressor.

Positive-Displacement Blowers: Calculating Boost and Increased Boost with a Pulley Change

Changing the supercharger drive and/or the crankshaft pulley/damper is the common method for increasing the boost output of the blower. Generally speaking, reducing the size (diameter) of the supercharger drive pulley will spin the rotors faster to produce more boost. Increased temperature in the boosted air charge is an inevitable byproduct as well. Depending on the engine combination, there can be a point of diminishing return with such a change, but it’s the most effective way to increase the output of the supercharger.

Determining the approximate amount of boost a positive-displacement blower such as an Eaton TVS or Whipple will produce, as well as how much more it will produce with a pulley change, is determined with a few simple calculations. First, start with the theoretical max boost of the combination. It’s determined with this formula:

PR x 14.7 x SV / EV (½) − 14.7 = max boost (psi)

Air flowing through the heat exchangers of an intercooling system will reduce the maximum pressure of the boosted air charge before it enters the engine. This cutaway shows the integrated charge-cooling brick on an Edelbrock E-Force supercharger with Eaton TVS 2650 rotors.

PR is the pulley ratio, which is the size of the crankshaft pulley divided by the supercharger pulley. For example: An 8.2-inch-diameter (208-mm) crankshaft pulley and a 3.1-inch supercharger pulley (78 mm) delivers a pulley ratio of 2.64 (8.2 / 3.1 = 2.64).

14.7 is the normal air pressure: 14.7 psi (1 bar).

SV is the supercharger volume in liters. For an Eaton 2300 TVS-type blower, that would be 2.3 liters.

EV is half of the engine volume. The total engine volume is divided in two because one rotation on a four-cycle engine is only half of a complete cycle. For an LS3 6.2L engine, the engine volume number for the equation would be 3.1L.

14.7, again, is atmospheric pressure.

Putting it all together for an LS3 with a 2300 supercharger and a 2.64 pulley ratio lands at 14.09 pounds of max boost: 2.64 (PR) x 14.7 x 2.3 (SV) / 3.1 (EV ½) − 14.7 = 14.09 pounds of boost (0.97 bar).

Changing the pulley sizes changes the pulley ratio, thereby affecting the maximum boost capability of the compressor. With the example above, changing only the blower drive pulley from 3.1 inches to 2.8 inches (approximately a 10-percent reduction) changes the pulley ratio to 2.93. When plugged into the boost calculation formula, the maximum boost increases to 17.25 psi (1.2 bar).

The comparatively minor change in pulley size makes a significant change in the speed of the supercharger and its output. The caveat here is that a significant increase in boost comes with a significant increase in heat that can lead to detonation.

It is also important to note that the boost calculation formula does not take into account a couple of important factors that will reduce the maximum boost pressure that actually enters the engine. The first is the overlap factor. All engines, even those with blower-friendly camshafts, have a measure of valve overlap, where the intake valve opens before the exhaust valve closes. The amount of overlap determines how much boost is siphoned off; there is approximately 5-percent loss for every 10 degrees of overlap. On a combination with 8 pounds of calculated boost (0.55 bar) and 10 degrees of overlap, the loss is 0.4 psi (0.27 bar), for a total of 7.6 pounds of boost (0.52 bar).

The other factor is the intercooling system and other airflow restrictions. The boosted air charge will lose some of its maximum pressure as it travels through the intercooling circuit’s heat exchangers. The bottom line is the maximum theoretical boost will not be the pressure of the air that enters the engine. The formulas given, however, provide guidelines for determining the output of a blower and the expected results of pulley changes.

Centrifugal Superchargers: Calculating Maximum Boost

The airflow output of a centrifugal supercharger increases with the square of its impeller speed. That generally means it makes very low boost at low engine speeds and increases with engine speed.

Calculating max boost for a centrifugal blower, at a given RPM level, starts with determining the engine airflow requirement:

D x RPM / 3,456 x 0.9

D is the engine displacement in cubic inches.

RPM is the engine speed.

3,456 is a calculation factor.

0.9 is the estimated volumetric efficiency of the engine without boost.

Let’s assume the calculation is for an LS3 engine, which has a displacement of 376 ci, and we’re calculating for the engine’s performance at 6,000 rpm. The formula works out like this: 376 (D) x 6,000 (RPM) / 3,456 x 0.9 = 587.5. That means the airflow requirement for the engine is 587.5 cfm at 6,000 rpm.

Maximum supercharger boost depends on the airflow capability of the compressor and the displacement of the engine. Because it takes more air to fill the cylinders, the same compressor will produce less boost on a larger-displacement engine.

Next, the max airflow of the supercharger (let’s say 1,000 cfm) is divided by the engine’s airflow requirement, multiplied by 14.7 (atmospheric pressure) and, finally, one “atmosphere” (14.7) is subtracted from the total to arrive at the theoretical max boost for the given engine speed. It looks like this: 1,000 / 587.5 x 14.7 − 14.7 = 10.32 pounds of max boost (0.71 bar) at 6,000 rpm.

Increasing the supercharger’s airflow, moving up from the 1,000-cfm ProCharger C-2 compressor to the 1,500 P-1SC, for example, increases boost at 6,000 rpm to 22.83 pounds of boost (1.57 bar), a 220-percent increase in boost for a 50-percent increase in supercharger airflow.

As with positive-displacement superchargers, the max boost of a centrifugal system is affected by valve overlap and the restriction of the charge-cooling system, as well as other factors, such as ambient air temperature. Similarly, more boost brings more heat, which can lead to detonation without proper tuning considerations.

Music to the Ears?

For many contemplating a supercharger, the sound, or lack thereof, is an important consideration. Whether it’s the whir of a centrifugal’s impeller or the meshing of a set of rotors, superchargers generate sound during operation. Some think it’s noise, while others think it’s music to their ears.

Generally speaking, centrifugal superchargers are noisier. At least, they make more sound than Roots and screw-type blowers at idle and low RPM. The Roots/screw-type compressors are, for the most part, silent at idle.