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.
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Positive-displacement superchargers are those that spin a pair of multilobe 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.
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.
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 twentieth century, including the 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, and 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, lowprofile 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 has 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 semi-professional drag racers who rely on superchargers or turbochargers for power adders and you see virtually none use an Eatontype 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 found in aftermarket kits. Generally, most Eaton-based bolt-on kits are offered through California-based Magna Charger, which has developed a number of very popular kits for many LS-powered vehicles. Indeed, many of Magna Charger’s kits represent the easiest-to-install systems and have earned a reputation for excellent reliability.
The Twin Vortices System (TVS) represents the sixth generation of Eaton’s ubiquitous Roots supercharger design and 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 sustained 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 page 29, “GM Factory-Supercharged LS9 and LSA Engines.”
Since appearing under the hood of the Corvette ZR1 and the Cadillac CTS-V models, the TVS supercharger has grown into the aftermarket, with Eaton’s Magna Charger outlet offering 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 (1.9-liter and 2.3-liter versions) for the 6.0-liter Pontiac GTO/G8 GT, as well as the VE-series Holden Commodore.
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.
Rather than using the intermeshing lobes of the Roots type, the Lysholm uses a pair of worm screwtype rotors that squeeze air together in order 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 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).
In the performance aftermarket, Whipple Industries is just about the most recognizable name in twinscrew technology, with Lysholm-type blowers derived from industrial air compressors that 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 has used a twin-screw compressor of its own design.
In 2009, Vortech joined the twinscrew blower fray with the addition of a Lysholm-based supercharger of its own. Rather than manufacturing its own blowers, Vortech has 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 5.3-liter 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. Known primarily for its systems for Ford vehicles, the company offers bolt-on kits for 1997–2004 Corvettes equipped with the LS1, LS2, or LS6 engine. No applications for later Gen IV engines with rectangular-port heads are offered.
Although engine-driven and not exhaust-driven, a centrifugal supercharger generates boost much like a turbocharger, with 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 ATI 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 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 highboost, 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 four and six-cylinders, the V-5 is not well-suited to the airflow capabilities of LS V-8 engines.
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-20 Series: designed purely for the rigors of drag racing, these compressors, in V-24 and V-27 forms, can flow up to 2,000 cfm and produce more than 30 pounds of boost.
An excellent reference chart of Vortech’s various compressors, trims and boost/airflow capacities is available at www.vortechsuperchargers.com.
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 ATI’s ProCharger compressors are relatively loud, especially at idle, but their street-based blowers have become admirably quiet in recent years. Like Vortech, there are numerous compressors in the ProCharger portfolio, with several designed specifically for racing applications. In fact, ATI offers the largest centrifugal superchargers, with some capable of more than 40 pounds of boost and up to 4,000 cfm.
Kit and Cost Considerations
Unlike turbocharger systems, there is 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 Magna Charger 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 centrifugals typically require more extensive modification of the accessory drive system.
Positive-Displacement vs. 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 at very low in the RPM band. That means a Roots or twinscrew 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 non-linear 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 twinscrew 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.
The non-linear 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. Unlike a turbocharger that generates more boost as the engine speed increases, the boost level with a supercharger plateaus relatively quickly in the RPM band. 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 this writing, the 3.3-liter Lysholm twin-screw supercharger from Vortech is the largest positive-displacement supercharger offered in bolt-on kits, although the Eaton 2.3-liter TVS blower is offered in more kits—and even it can struggle to keep up with some large displacement combinations. Whipple offers 3.3-, 4.0-, and 5.0-liter compressors, but none had been adapted to LS engines.
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.
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.
Companies, such as Vortech, have worked hard to reduce the low-speed sound of their centrifugal units, while others, like Powerdyne, use quieter, belt-driven impellers rather than gear-driven ones. But for the most part, the sound hasn’t been eliminated. If a subtler, stealthier approach is desired, the Roots/screw blower is the way to go. For those who don’t mind rolling up to a stoplight and having all eyes focus at the hood area of their vehicle, a centrifugal does the trick.
The Importance of a Charge Cooler
Almost every supercharger and turbocharger kit for LS-powered vehicles includes some type of charge cooler or intercooler to reduce the inlet temperature of the boosted air before it enters the engine through the throttle body. It’s necessary to ward off the engine-damaging effects of detonation and/or pre-ignition— conditions LS engines are particularly susceptible to because of their high-compression ratios.
In general terms, forced-induction engines are safer with compression ratios in the neighborhood of 8.5:1 to 9.5:1, but LS engines have much greater “squeeze” from the pistons. The LS7 engine of the Corvette Z06 has 11:1 compression, the LS3 is at 10.7:1, and even the original LS1 engine had a 10.25:1 compression ratio. By comparison, the factory-supercharged Corvette ZR1’s LS9 engine has a blower-friendly 9.1:1 compression ratio.
So, routing the boosted air charge through the cooler reduces the maximum boost level, but it typically enhances horsepower because the cooler air charge is denser than a heat-soaked charge. And it’s the only option on vehicles with an otherwise stock engine assembly.
Regardless of whether it’s a Roots/screw-type or centrifugal supercharger, compressor surge occurs when the blower is making boost, but the throttle suddenly closes, such as when the driver pulls his or her foot off the gas pedal. When this occurs, the blower keeps pushing air into the closed throttle body. When the pressure inside the throttle body is higher than the pressure being generated by the supercharger, air blows back toward the compressor.
In low-boost applications, this isn’t a big problem, but with higher boost— more than 10 pounds or so—the comparatively great pressure can cause damage to the engine, supercharger, or both. Venting the excess pressure that builds when the throttle snaps closed is the cure, and that can be done with either a blow-off valve (which vents excess air back into the atmosphere) or a bypass valve (which vents the air back into the compressor).
Pulley Size and Performance
Sometimes not even the installation of an entire supercharger system suits some enthusiasts. They look to extract every pound of boost possible from the blower, and that usually leads to swapping the factory-installed drive pulley for a smaller-diameter pulley.
The smaller pulley typically generates more boost, because it forces the rotors to spin faster. You should explore all the performance ramifications of the swap before performing it, because the gain may be negligible or lead to a number of other issues that must be addressed, including the following:
- The pulleys on Eaton-based Roots superchargers, like those sold through Magnuson and Harrop, typically have pressedon pulleys that require special tools for removal. In fact, the procedure typically requires the removal of the supercharger if it’s already been installed on the engine; so if a pulley swap is considered, have it performed before the supercharger is installed on the vehicle.
- The greater boost of a smaller pulley can push against the engine’s threshold for detonation—or exceed it—requiring revised tuning and possibly the use of higher-octane fuel.
- In many cases, the smaller pulley also requires the investment in a slightly smaller drive belt. During the swap, the tensioner and/or other idler pulleys should be inspected for wear.
Another method to increase the boost from the supercharger is a larger-diameter crankshaft pulley/ balancer, which spins all of the accessories (including the supercharger) faster. The flip side to this method is that the faster rate is not necessarily healthy for the other accessories. It could affect the performance or shorten the life of the water pump, alternator, and more.
Belt Wrap and Belt Size
The drive system of a supercharger puts a tremendous amount of additional load on the beltdriven accessory system of an engine. At high RPM and under maximum boost, a supercharger belt with a production-style, cogless design can slip, robbing horsepower and possibly causing engine damage. Most blower kit manufacturers design the belt drive with a lot of belt-to-pulley contact. This higher degree of “belt wrap” helps mitigate slippage.
For bolt-on blower kits making up to about 12 pounds of boost, inserting the supercharger into the stock, six-rib accessory-drive system generally doesn’t cause a problem, especially if the belt routing ensures good belt wrap. When building for a higher boost application, however, a separate, wider belt system should be considered between the blower and crankshaft pulley.
The LS9 comes from the factory with a 10-rib belt system for the entire front drive, while many aftermarket supercharger companies offer 10-rib conversion kits for systems aimed at generating more than 12 pounds of boost. A cogged belt should be used with a racing engine to maximize belt engagement.
GM Factory-Supercharged LS9 and LSA Engines
Launched in 2008, the Corvette ZR1 and Cadillac CTS-V were the first LS production engines from GM to use forced induction. Each features an Eaton-developed Roots-type supercharger blowing—via a charge cooler—into a 6.2-liter engine.
The ZR1’s LS9 engine is rated at 638 hp and 604 ft-lbs of torque, while the CTS-V’s LSA engine is rated at 556 hp and 551 ft-lbs of torque. Despite both engines using a similar supercharger design and 6.2-liter displacement, the LSA is not simply a de-tuned version of the LS9. The engines are built with a number of different components.
Among the most notable differences between the engines are the superchargers. The LS9 uses a 2.3-liter compressor, while the LSA uses a smaller, 1.9-liter blower. Both superchargers are based on Eaton’s sixth-generation Roots design that features four-lobe rotors for greater efficiency. The comparatively large displacement of superchargers (particularly on the LS9) helps the engines overcome two of supercharging’s biggest hurdles: low-end torque and high-end horsepower.
The LS9, for example, makes big power at lower RPM and carries it in a wide arc to 6,600 rpm. GM testing has shown the engine makes approximately 300 hp at 3,000 rpm and nearly 320 ft-lbs of torque at only 1,000 rpm. Torque tops 585 ft-lbs at about the 4,000-rpm mark, while horsepower peaks at 6,500 rpm. The engine produces 90 percent of peak torque from 2,600 to 6,000 rpm.
Both engines are offered through GM Performance Parts as complete crate engine assemblies, offering enthusiasts a ready-to-go alternative to building a supercharged engine from scratch. The PN for the LS9 engine is 19201990; the PN for the LSA is 19211708.
Below is a look at the components and processes that comprise these unique powerplants.
Cylinder Block and Rotating Assembly
Both engines feature an aluminum cylinder block with the LS-standard six-bolt main bearing caps. The LS9 uses steel main caps and the LSA uses nodular-iron caps. The block also features enlarged vent windows in the second and third bulkheads for enhanced bay-to-bay breathing. Cast-iron cylinder liners, measuring 4.06 inches in bore diameter, are inserted in the aluminum block, and they are finish-bored and honed with a deck plate installed. The deck plate simulates the pressure and slight dimensional variances applied to the block when the cylinder heads are installed, ensuring a higher degree of accuracy that promotes maximum cylinder head sealing, piston-ring fit, and overall engine performance.
A forged-steel crankshaft delivers the engines’ 3.62-inch stroke. On the LS9, it features a nine-bolt flange (the outer face of the crankshaft on which the flywheel is mounted) that provides more clamping strength. The LSA uses an eight-bolt flange. Other, non-supercharged LS engines, have a six-bolt flange. A torsional damper mounted to the front of the crankshaft features a keyway and friction washer, which is designed to support the engine’s high loads.
With the LS9, a set of titanium connecting rods and forged-aluminum pistons are used. The LSA uses powdered-metal rods and hypereutectic (cast) aluminum pistons. Both engines have a 9.1:1 compression ratio.
The basic cylinder head design of the LS9 and LSA is similar to the L92- type head found on GM’s LS3 V-8, but it is cast with a premium A356T6 alloy that is better at handling the heat generated by the supercharged engine—particularly in the bridge area of the cylinder head, between the intake and exhaust valves.
In addition to the special aluminum alloy, each head is created with a rotocast method. Also known as spin casting, the process involves pouring the molten alloy into a rotating mold. This makes for more even distribution of the material and virtually eliminates porosity (air bubbles or pockets trapped in the casting) for a stronger finished product.
Although the heads are based on the L92 design, they feature swirlinducing wings that are cast into the intake ports. This improves the mixture motion of the pressurized air/fuel charge. Both engines feature 2.16-inch-diameter intake valves and 1.59-inch-diameter exhaust valves, but the LS9 uses more exotic titanium intake and hollow-stem sodium-filled exhaust valves.
Camshaft and Valvetrain
The broad power band enabled by the LS9’s large, 2.3-liter supercharger allowed GM engineers to specify a camshaft with a relatively low lift of .555 inch for both the intake and exhaust valves. It is a lowoverlap cam with lower lift and slower valve-closing speeds than the Z06’s 505-hp LS7, giving the LS9 very smooth idle and drivability qualities.
Similarly, the LSA’s camshaft delivers a relatively low .480-inch lift on the intake and exhaust sides.
The valvetrains of both engines feature many parts-bin components from the production LS3 engine, including the lifters, rocker arms, and valvesprings. However, the LS9 uses the valve-spring retainers from the LS7 engine.
Supercharger and Charge Cooler
Both engines use a sixth-generation supercharger from Eaton. Its primary improvement is a four-lobe rotor design that promotes quieter and more efficient performance. The LS9’s R2300 supercharger features a case that is specific to the Corvette ZR1. Its maximum boost pressure is 1.5 psi. The LSA uses the 1.9-liter R1900 compressor and delivers up to 9 psi of boost.
Both engines employ a liquid-toair charge-cooling system to reduce inlet-air temperature after it exits the supercharger, reducing the inlet-air temperature by up to 140 degrees F. Each charge-cooling system includes a dedicated coolant circuit with a remote-mounted pump and reservoir.
The charge-cooler design differs for each engine, because of the production packaging within the cars. The LS9 uses a “dual brick” system, with a pair of low-profile heat exchangers mounted longitudinally on either side of the supercharger. The LSA’s charge cooler is mounted atop the supercharger, owing to the Cadillac’s greater hood clearance. Coupled with the supercharger itself, this integrated design mounts to the intake.
The LS9 uses a dry-sump oiling system that is similar in design to the LS7’s system, but features a highercapacity pump to ensure adequate oil pressure at the higher cornering loads the ZR1 is capable of achieving. An oil-pan-mounted cooler is also integrated, along with pistoncooling oil squirters located in the cylinder block.
The LSA engine uses a conventional, wet-sump oiling system, but also includes oil squirters in the cylinder block.
To compensate for the heavier load generated by the supercharger drive system, an LS9-specific water pump with increased bearing capacity is used.
Accessory Drive System
In order to package the accessory drive system in the Corvette’s engine compartment, the supercharger drive was integrated into the main drive system. This required a wider 11-rib accessory drive system to be used with the LS9 to support the load delivered by the supercharger.
Both engines use fuel injectors with center-feed fuel lines. The center-feed system ensures even fuel flow between the cylinders with less noise. To ensure fuel system performance during low-speed operation and under the extreme performance requirements of WOT, a dual-pressure fuel system was developed. This system operates at 36 psi (250 kPa) at idle and low speed, and ratchets up to 87 psi (600 kPa) at higher-speed and WOT conditions. The LS9 uses 48-lb/hr injectors.
An 87-mm, single-bore throttle body is used to draw air into the LSA; an 89-mm throttle body is on the LS9. Both are electronically controlled.
Real-World Project: Pratt & Miller’s LS9.RS— The Ultimate Supercharged LS Street Engine
It’s a logical question: What happens when you marry the 6.2-liter LS9 engine’s forced-induction system with the larger, 7.0-liter displacement of the LS7? As it turns out, it’s the recipe for huge horsepower. Pratt & Miller (P&M) discovered this when it designed such a combination for its premium Corvette C6RS sports car. P&M is the racing shop that builds, maintains, and supports the factory-backed Corvette C6R racing team (and, previously, the all-conquering C5R team). Its C6RS features wider, carbon-fiber body panels, an air-adjustable suspension and, until recently, a normally aspirated 8.2-liter engine rated at 600 hp. That was before the ZR1 trumped its output at the factory.
So, for a premium product, P&M wanted an elevated performance level. The company also wanted to keep its engine project as close to OEM as possible, making the hybrid assembly of an LS7 rotating assembly and LS9 induction parts all the more logical. P&M consulted LS engine guru Thomson Automotive for some advice on the rotating assembly and airflow requirements of the engine. (Thomson Automotive also built the 2,000-hp twin-turbo engine discussed in Chapter 9.) The result is what Pratt & Miller calls the LS9.RS.
The Bottom End
The official word from GM engineers on using the smaller-bore, 6.2-liter platform for the LS9 is safety. The thicker cylinder walls provide a greater margin of safety for an engine designed to withstand a 100,000-mile durability standard that takes into account all manners of use, load, and abuse. That doesn’t mean that using the 7.0-liter LS7 block is a recipe for a meltdown. In fact, the LS7 is commendably strong.
So, while the LS7’s cylinder block wasn’t a worry, its stock rods and pistons were. The featherweight titanium connecting rods from the LS7 simply weren’t designed for the load, stress, and horsepower range expected of the supercharged engine. At Thomson Automotive’s suggestion, they were swapped with a set of aftermarket (by Oliver) forged I-beam rods.
The hypereutectic (cast) material of the LS7 pistons may have caused concern in a supercharged application, too, but the question was rendered moot by the fact they would deliver too much squeeze in the cylinders. The LS7 has a high, 11.0:1 compression ratio—way too much for a street-driven blower engine. In their place, a set of aftermarket Diamond forged-aluminum pistons was used, each with a sizable dish to help lower the compression to a more manageable 9.0:1.
Anchoring the rods and pistons was a factory LS7 crankshaft. As a forged-steel component and one that has proved exceptionally strong and resilient, it was more than suitable for the job. For the record, the bore and stroke of this engine is 4.125 x 4.000 inches—the same as a stock LS7.
Curiously, one of the most significant elements of the big-inch LS7 engine that wasn’t carried over was the cylinder heads. Although known for their cavernous ports and straight pathways to the combustion chambers, they simply didn’t match the intake ports of the LS9 supercharger manifold. P&M had little choice but to use the factory LS9 heads; and while their intake flow is somewhat less than the LS7 heads, it isn’t a detriment when the boost is up and air is being rammed through the engine with terrific velocity.
LS9 heads are based on the respected L92 design, but feature swirl-inducing wings cast into the intake ports. The valvetrain consists of 2.16-inch titanium intake valves and 1.59-inch Inconel valves (in place of the hollow-stem stock valves). Standard springs, retainers, and push rods were used. Even the stamped-steel arms were retained, owing to the OEM parts list.
Atop the heads, of course, was mounted the LS9’s supercharger/ intercooler assembly. Apart from a different drive pulley (more on that below), P&M used the whole assembly without modification, along with the factory 10-rib belt system and LS9 front accessories. P&M omitted the plastic, decorative engine cover, milled off the “LS9 SUPERCHARGED” embossed letters on top of the intercooler lid, and painted the entire intercooler cover to match the car.
On the Dyno
Testing was conducted at Thomson Automotive, with the engine using an E67 controller and blowing gases through stock LS7 exhaust manifolds, which would deliver more representative results of the horsepower and torque of the engine in the vehicle. It was immediately clear the extra volume of the 7.0-liter engine was a factor to contend with. With the stock supercharger drive pulley, the blower could only deliver about 5 pounds of boost on the larger cylinders. It wasn’t acceptable, but even with that, the engine spit out about the same power as a stock LS9—at only half the boost.
A new, smaller pulley was quickly milled and the testing resumed. The results were more impressive: 760 hp and 830 ft-lbs of torque. That’s about 20 percent (120 hp) more power than the LS9 from an engine that is only about 13.5-percent larger in displacement. It was certainly a successful experiment, but there was more power on the table.
During the numerous test pulls on the dyno, it was noted that the airpressure drop after the throttle body was about 1.7 psi (12 kPa). That told the tuners the engine wasn’t drawing enough air for maximum power. It wore an LS7 90-mm electronic throttle body (the LS9’s is 89 mm in diameter), but it was determined at least a 100-mm unit was needed.
An aftermarket 100-mm throttle body (a cable-actuated unit that required “tricking” the controller to think it was electronically operated) was ordered, but its inlet turned out to be smaller than the factory 90- mm unit. P&M tuners figure this unique engine combination is good for 800 to 820 hp with the properly sized throttle body.
Regardless of the throttle body, the LS9.RS confirms everyone’s thoughts: Adding the LS9 blower to the LS7 bottom end is a great idea.
Written by Barry Kluczyk and Posted with Permission of CarTechBooks
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