In general terms, and assuming everything else is equal, an internal combustion engine with larger displacement flows more air than a smaller-displacement engine. The engine with the greater airflow makes more power.
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Forcing more air into an engine than it naturally draws can substantially increase the output of a smaller engine and give it the power of a larger engine. The forced or ambient air is delivered to the intake manifold at a pressure greater than the outside. It is denser, delivering more oxygen to the combustion chamber. When mixed with the appropriate ratio of additional fuel, the result is a more powerful combustion. That’s the essence of supercharging; whether through an engine-driven supercharger or exhaust-driven turbocharger.
Forced induction has been used to boost the power of engines for decades. Hot rodders made it a common practice after World War II, with engine-driven supercharging becoming popular on street and drag racing cars.
General Motors experimented with turbocharging in the early 1960s, and perfected it in the mid-1980s by combining it with electronic fuel injection. The turbocharged and intercooled V-6 engine of the 1986–1987 Buick Grand National outperformed most V-8s when new.
In the early 1990s, GM adopted supercharging for a number of V-6-powered midsize and large passenger cars, including the Pontiac Grand Prix. GM used a Roots-blown 3.8-liter engine, with the supercharger supplied by Eaton. The engines proved exceptionally robust and powerful, spawning a cult of enthusiasts who continue to modify and race the vehicles.
The technology for forced induction supercharging and turbocharging internal combustion engines has been around since the early twentieth century, with automotive manufacturers employing the power-boosting effects for more than 80 years. Both supercharging and turbocharging are currently used on dozens of regular production automobiles, and they have been staples of the high performance world since the close of World War II.
One of the most popular performance engines of today is GM’s “LS” family. As technology progresses, it continues to become an increasingly popular choice for forced induction. Since its introduction in the late 1990s, the GM Gen III/Gen IV engine family (commonly known as LS) has proven itself a capable foundation for high-performance engines. By relying on a conventional, cam-in-block configuration with the benefit of exceptionally high flowing cylinder heads, the LS engine delivers tremendous torque at low RPM and great power at the upper rev range.
GM’s relationship with Eaton superchargers reached its zenith in 2009, with the introduction of the factory-blown Corvette ZR1. With its sixth-generation supercharger atop its 6.2-liter V-8, the ZR1 is rated at 638 hp. It is the most powerful production car ever produced by General Motors. (Photo courtesy General Motors)
Along with the Corvette ZR1, GM launched another factory-supercharged car in 2009: the Cadillac CTS-V. Like the ZR1, it featured a sixth-generation Eaton supercharger on a 6.2-liter engine, but the supercharger was smaller—resulting in “only” 556 hp. (Photo courtesy General Motors)
Forced induction was attempted with early LS engines, but often with mixed results. Early adopters of supercharging and turbocharging typically encountered tuning trouble when they tried to work around the factory engine-control system and crank-triggered ignition system. That, and the greater airflow capability of the LS heads, made it difficult to match a supercharger or turbocharger to the engine. Often, the “blowers” ran out of breath.
But much has changed in the years since tuners first experimented with supercharging the LS engine. Properly sized superchargers and turbochargers, relatively easy tuning, and other elements have made supercharging or turbocharging an LS-powered vehicle a simple, yet highly effective method of generating a dramatic increase in power.
Of course, GM itself has adopted supercharging as a regular production method of building big power. The Corvette ZR1’s LS9 engine and the Cadillac CTS-V’s LSA engine use Roots-type superchargers to make 638 hp and 556 hp, respectively. The engines are designed with specific components to support forced induction.
LS Family Tree
Although all engines in the family are referred to as LS series, GM has manufactured two generations within that family. The first generation was called Gen III and includes, from an automotive standpoint, the LS1 and LS6 engines. The biggest differences between Gen III/IV engines are larger bores in the cylinder blocks of Gen IVs, and Gen IV cylinder heads in general.
There are also different camshaft position-sensor locations: front timing-cover area on Gen IV blocks and top-rear position on Gen III blocks. On most Gen IV blocks, there are cast-in provisions for GM’s Active Fuel Management cylinder-deactivation system.
Also, many Gen IV engines made from around 2006 and later are equipped with electronically controlled (wireless) throttle systems. Those engines also feature a 58X timing system that includes a different engine controller and 58-tooth reluctor wheel on the crankshaft (also known as a 60-minus-2 wheel), rather than the 24X system used with earlier LS engines (mostly those with cable-operated throttles).
There is great interchangeability between LS engines, including the Gen III and Gen IV versions. Cylinder heads, crankshafts, intake manifolds, and more can be mixed and matched, but there is not absolute compatibility. Not every head matches every intake manifold and not every crankshaft works with every engine combination. (See Chapters 8 and 9 for more information on parts interchangeability and building a forced-induction LS engine.)
What follows here is a quick primer on production LS engines and their notable features.
Gen III Automotive Engines
The Gen III LS engine is defined by its 3.90-ci (99 mm) cylinder bore. Like later LS engines, it carries the basic design attributes and good flowing heads, but the bore dimension ultimately limits the type of cylider head that can be used on it. Later, larger-bore engines can use almost all LS-style heads, while the Gen III is limited to the heads from LS1, LS6, and Gen IV LS2 engines.
LS1
1 LS1 5.7-liter (346 ci) engines were installed between the 1997 and 2004 model years in North America (Corvette, Camaro, and Firebird, as well as the GTO in the United States only) and encompassing some 2005 models in other markets (primarily Australia).
LS1 5.7-liter Gen III. (Photo courtesy General Motors)
LS6
The LS6 was introduced in 2001 in the Corvette Z06 and was manufactured through 2005, when it was also installed in the first-generation Cadillac CTS-V. The LS6 shared the LS1’s 5.7-liter displacement, but used a unique block casting with enhanced strength, greater bay-to-bay breathing capability, and other minor differences. The heads, intake manifolds, and camshaft are unique LS6 parts, but fit the LS1 block.
Gen IV Automotive Engines
In automotive applications, the Gen IV introduced a larger, 4.00-ci (101.6 mm) bore dimension that enables heads of larger-displacement Gen IV engines to be used with it. The Gen IV engines also introduced electronic throttle control, while the Gen III LS1 and LS6 engines only used cable-operated throttles. It is possible to adapt either throttle style, although the proper controller must be used.
LS2
In 2005, the LS2 6.0-liter (364 ci) engine and the Gen IV design changes made their debut. In GM performance vehicles, it was offered in the Corvette, GTO, and even the heritage-styled SSR roadster. It is the standard engine in the Pontiac G8 GT. Its larger displacement brought greater power. The LS2 is one of the most adaptable engines, as LS1, LS6, LS3, and L92 cylinder heads work well on it.
LS3/L99
Introduced on the 2008 Corvette, the LS3 brought LS base performance to an unprecedented level: 430 hp from 6.2-liter (376 ci), making it the most powerful base Corvette engine in history.
The LS3 block not only has larger bores than the LS2, but a strengthened casting to support more-powerful 6.2-liter engines, including the LS9 supercharged engine of the Corvette ZR1. The LS3 is offered in the Pontiac G8 GXP and is the standard V-8 engine in the 2010 Camaro SS.
The L99 is essentially the same as the LS3, but it is equipped with GM’s fuel-saving Active Fuel Management cylinder-deactivation system. It is standard on 2010 Camaro SS models equipped with an automatic transmission.
LS3 6.2-liter Gen IV. (Photo courtesy General Motors)
LS4
Perhaps the most unique application of the LS engine in a car, the LS4 is a 5.3-liter version used in the front-wheel-drive Chevrolet Impala SS and Pontiac Grand Prix GXP. The LS4 has an aluminum block and unique, low-profile front-end accessory system, including a “flattened” water pump, to accommodate the transverse-mounting position in the Impala and Grand Prix. It is rated at 303 hp and 323 ft-lbs of torque.
LS7
The LS7 is the standard engine in the Corvette Z06 and its 7.0-liter displacement (427 ci) makes it the largest LS engine offered in a production car. Unlike the LS1/LS6, LS2, and LS3 engines, the LS7 uses a Siamese-bore cylinder-block design— required for its big, 4.125-inch bores. Competition-proven heads and lightweight components, such as titanium rods and intake valves, make the LS7 a street tuned racing engine, with 505 hp.
LS9
The most powerful production engine ever from GM, the LS9 is the 6.2-liter supercharged and charge cooled engine of the Corvette ZR1. It is rated at an astonishing 638 hp. The LS9 uses the strengthened 6.2-liter block with stronger, roto-cast cylinder heads and a sixth-generation 2.3-liter Roots-type supercharger. Like the LS7, it uses a dry-sump oiling system. It is the ultimate production LS engine.
LSA
A detuned version of the LS9, this supercharged 6.2-liter engine is standard in the 2009 Cadillac CTS-V. It is built with several differences compared to the LS9, including hypereutectic pistons versus the LS9’s forged pistons; and a smaller, 1.9-liter supercharger. The LSA also has a different charge-cooler design on top of the supercharger. Horsepower is rated at 556.
LSA 6.2-liter supercharged Gen IV. (Photo courtesy General Motors)
L94 Vortec 6.2-liter Gen IV. (Photo courtesy General Motors)
Gen III/IV Vortec Truck Engines
Although performance car engines have typically carried “LS” designations, truck engines built on this platform have been dubbed Vortec. They are generally distinguished by iron cylinder blocks and smaller displacements than car engines. Interestingly, a 5.7L Vortec LS engine has never been offered. Here’s a quick rundown of the previous and current production LS truck engines.
4.8-liter: The smallest displacement LS engine (293 ci); it uses an iron block with 3.78-inch bores and aluminum heads.
5.3-liter: The most common LS truck engine, it uses the same iron block with 3.78-inch bore as the 4.8-liter, but with a larger, 3.62-inch stroke (327 ci). Later versions came equipped for Active Fuel Management. They were manufactured with iron and aluminum cylinder blocks.
6.0-liter: Used primarily in 3/4 and 1- ton trucks, the 6.0-liter (364 ci) uses an iron block (LY6) or aluminum block (L76) and aluminum heads, with provisions for Active Fuel Management; some are equipped with variable valve timing.
6.2-liter: Commonly referred to by its L92 or L94 engine codes, the Vortec 6.2-liter (376 ci) engine uses an aluminum block and aluminum heads, and incorporates advanced technology including variable valve timing. The 6.2-liter is used primarily as a high-performance engine for the Cadillac Escalade and GMC Yukon Denali, but also in some Silverado and Sierra pickup models. In 2010, some 6.2-liter engines took the L9H name to reflect changes that included E85 fuel compatibility.
Supercharging vs. Turbocharging
At their most basic, turbochargers and superchargers are air pumps, but with different pumping characteristics. The turbocharger is an exhaust-driven pump that saps no engine power when not making boost. A supercharger is an engine driven pump that is essentially another component on the accessory drive system that requires a modicum of power to drive, even when it’s not producing much boost.
The thermal efficiency (the amount of combustion energy that is converted to power) is greater with a turbocharger system than a supercharger, because it recycles a significant amount of exhaust energy to spin the compressor. That exhaust energy is lost to the exhaust system in normally aspirated and supercharged engines. That said, centrifugal and Lysholm (screw-type) superchargers are up to 85 percent efficient.
Excellent airflow characteristics of the basic LS cylinder head design greatly exploit the benefits of forced induction, as air is easily and quickly moved through the engine. Because of this, a higher-capacity supercharger or larger turbo is often used, when compared to older, previous generation Chevy small-block designs, to fulfill the airflow capability of the free-flowing LS engine.
Superchargers (particularly Roots and screw-type blowers) are excellent at delivering low-RPM power, as they are always making at least a minimal amount of boost when the engine is running. That’s because the supercharger is directly linked to the crankshaft via the drive belt. That connection also requires a small amount of horsepower to simply turn the supercharger.
Turbochargers require no engine power to drive, and therefore are considerably more efficient than an engine-driven supercharger. However, boost only occurs when the engine RPM rises. At low speeds, particularly off idle, the turbocharger provides no power increase.
The advantage of turbocharging in a racing application is clearly illustrated in this partially constructed fourth generation Firebird, as two very large turbochargers have been adapted to an LS engine. Except for the older, “71”-series superchargers used in Top Fuel, Top Alcohol, and some Pro Mod-type drag racing classes, there aren’t Roots and screw-type superchargers capable of delivering the airflow of a pair of extra-large turbos. Even large centrifugal blowers are typically limited to only one per engine. With a pair of turbos, each driven by half of the cylinders, the only real limit is keeping the engine itself together under maximum boost.
In general terms, superchargers deliver greater power and torque at low- and mid-range RPM levels, with nearly full boost available immediately at wide-open throttle (WOT). A supercharger’s effectiveness tends to trail off at higher RPM, while turbochargers typically deliver their greatest power contribution at midto high-RPM levels, with boost building progressively in line with an increase of engine speed. Turbochargers are also very good at building mid-range torque, and when properly sized, can deliver excellent low-end power, too.
Below are a number of factors to consider before purchasing a bolt-on system. The performance requirements and engine demands for custom combinations and racing applications are different, but for the enthusiast seeking to add a forced induction system to his or her vehicle, the following points are the most relevant.
Horsepower and Power Adjustability
Although supercharger and turbocharger kits deliver approximately the performance their manufacturers advertise, turbocharging generally delivers more power for the equivalent dollar spent on a supercharger kit. Turbo systems also offer almost unlimited upgrade potential.
Apart from the capacity to change the drive pulley on some superchargers, the output of a blower is pretty much determined by the size of the compressor. With a turbo system, a number of elements are easily manipulated to increase power. In fact, the almost infinite adjustability of turbo systems is one of their primary appeals.
Modern Roots and screw superchargers make excellent choices for street driven performance vehicles, as they deliver instant power at low-engine speeds. They’re also quieter and offer greater drivability than ever before. And when compared to custom or bolt-on turbo kits, they are very cost effective.
Performance Range
As noted earlier, superchargers— particularly Roots/screw types—generally deliver gobs of low-end power and become less efficient at higher RPM. The opposite is generally true for turbochargers; they tend to deliver their greatest performance as maximum boost is delivered with higher engine speed.
Drivability
Because an engine-driven supercharger is “on,” it tends to give a street-driven vehicle an abundance of off-the-line/low-speed pull—to the point where it is difficult to manage part-throttle driving in some instances, as tire spin becomes an issue. The higher-RPM power application of turbo systems typically makes them more tractable at low speeds. The enthusiast wishing for supremacy off the line at stoplights with the instant application of full boost will probably enjoy a supercharger; while the enthusiast seeking a wider performance range will likely find a turbo system more rewarding.
Noise
Generally speaking, the compressors of most supercharger and turbocharger systems are very quiet these days. Turbos are essentially silent until they start spinning at high RPM, and the same is true for most Roots/screw-type blowers. Centrifugal superchargers are much quieter than they used to be, but at idle, they’re not as quiet as turbos or Roots/screw-type superchargers.
Tuning
There’s no real advantage between tuning a supercharged or turbocharged engine, as the need to maintain an adequate air/fuel ratio and optimal spark to avoid detonation is paramount with both methods. Both types of systems have unique needs for delivering safe, optimal performance, but the basic approach to tuning is similar. There’s no clear advantage to either system.
Maintenance and Reliability
When installed and used properly, supercharger and turbocharger kits are very reliable, with the compressors for both lubricated with engine oil—although some Roots/ screw-type blowers feature self contained lubrication systems. Over time, the drive belt for a supercharger must be inspected just like the engine’s standard accessory belt, and after a few years, the compressor may require an inspection to ensure the tolerances and clearances are within specification limits for the rotors. Turbochargers are very susceptible to heat and even with adequate lubrication, the internal seals and turbine can wear and allow oil blow-by. This requires the turbo to be rebuilt.
System Cost
Because of the myriad of extra equipment—from the wastegate to the exhaust manifolds—turbocharger bolt-on kits generally cost two to three times more more than supercharger kits. Additionally, turbocharger systems generally take longer to install than supercharger kits. This adds up when outsourcing the project to a professional shop.
Installation Impact on the Vehicle
Assuming all turbocharger and supercharger systems employ an intercooler, the Roots/screw-type supercharger systems generally require the fewest compromises and/or fabrication modifications during installation. Because they install in place of the intake manifold, few changes are required at the front of the engine or in the engine compartment.
Consequently, they offer the most integrated, “factory-looking” appearance under the hood. Centrifugal superchargers require a mounting bracket on the front of the engine that can require moderate modification, removal, or relocation of factory components. With bolt-on turbocharger systems, the installation of the exhaust manifolds, turbochargers, and associated plumbing typically require considerably more fabrication, modification, and relocation of stock parts than supercharger systems.
A couple of the biggest advantages of a supercharger for a primarily streetdriven vehicle are comparatively easy installation and a lower labor investment. Bolt-on kits (particularly Roots/screw-type systems that essentially swap out the original intake manifold) can be installed relatively quickly, with little impact on the rest of the vehicle’s components or systems. The quicker the installation, the lower the labor cost at a professional shop.
Installation Cost
Again, because of the extra equipment associated with them, turbocharger kits are generally more time consuming to install, and therefore, more labor costs So, while a turbo kit offers greater performance potential, the cost involved with this investment may steer some toward a supercharger. In fact, there are other factors to consider before ordering a system for your car.
For one, the tight confines of the engine compartments in Corvettes, Camaros/Firebirds, and GTOs/Monaros make packaging and installing a turbo kit very difficult. This not only makes the installation a painstaking and difficult procedure, but can make future servicing all but impossible without an extensive teardown of the vehicle’s front end.
There is more room in the engine compartments of full-size trucks SUVs, and TrailBlazer SSs; but stuffing a turbo system—excluding the STS-type rear-mount system—can be a challenge in a regular street car.
My opinion is that turbocharging is great for vehicles destined to spend most of their time on the drag strip; but for typical, street-driven vehicles, a supercharger system is the easier and more economical method to build power. Many tuners and manufacturers who fall on the turbo side of the argument will undoubtedly disagree; but when it comes to bolt-on, forced-induction kits, superchargers are easier and cheaper to implement, with less maintenance.
Understanding Boost Including PSI vs. Bar
Whether it is a supercharger or turbocharger system, the measure of pressurized air fed into the engine is referred to as “boost.” It is the difference between the ambient air pressure and the increased air pressure the boost-producing device generates at the intake manifold. Boost is the opposite of vacuum, which is what a non-boosted engine makes during normal operation.
When an engine isn’t running, it generates no vacuum or boost (negative pressure), meaning the pressure in the intake manifold is the same as the ambient air pressure—about 14.7 pounds per square inch (psi). At idle and low-throttle conditions, an engine generates vacuum, indicating the pressure in the intake manifold is lower than the ambient pressure.
In a supercharged or turbocharged engine, boost is created as more throttle is applied and the boost-generating device forces air into the intake manifold at a higher pressure than ambient (positive) pressure. The air pressure at the intake manifold swings from negative to positive—that’s why high-performance boost gauges indicate both vacuum and boost measurements.
In North America, boost is generally measured in psi, while bar is more common in other countries. When measuring in psi, the ambient air pressure is regarded as the base, or 0 pounds of boost. The positive pressure builds on that base, with 1 pound of boost indicating 1 psi greater than ambient pressure.
With bar measurements, bar is roughly the equivalent of ambient air pressure. Technically, 1 bar is equivalent to 14.7 psi, not 14.5 psi, but many enthusiasts equate it to the normal atmospheric pressure, so a .5-bar pressure reading is roughly 7.25 pounds of boost. A full, 1-bar reading would indicate 14.5 pounds of boost.
Boost is generated when the supercharger or turbocharger creates air pressure greater than ambient when it is introduced to the engine (at the throttle body). Supercharged engines generate a small amount of boost whenever the engine is running—even at idle. Turbocharged engines require higher RPM to generate boost.
The adjustability and boost capability of turbocharging has made it the power adder of choice for most professional and semi-professional drag racers in the Fastest Street Car series. One of the quickest competitors has been Tom Kempf, whose red, single-turbo Camaro was capable of mid-7-second ETs.
Drag Racing
An inspection of the competitors at any outlaw-type drag racing event shows more turbochargers than superchargers and there’s a reason for it: Turbos make more power. More specifically, there aren’t superchargers that have the capability to match the performance of very large turbochargers. That is changing with the advent of larger centrifugal supercharger compressors from ProCharger, but at the time this book was published, even ProCharger’s largest compressors didn’t quite match the maximum boost from the largest turbo systems.
That’s not to say that turbocharging is the optimal power adder for a drag car. Dan Millen, a longtime championship drag racer and proprietor of the renowned tuning shop Livernois Motorsports in Dearborn Heights, Michigan, has been driving single-turbo race cars since 2001.
“Right now, turbos make more power than superchargers—with twin-turbo combinations making more than single turbos—and that’s why I use them,” Millen says. “I like the simplicity of blowers and the relative ease of tuning with them, but superchargers just don’t match turbos yet on the drag strip. Also, there is comparatively little parasitic loss with a turbo, whereas a blower requires a lot of power just to turn the compressor.”
LS Performance Potential
In a word, the performance potential of a boosted LS engine is almost unlimited. Whether you’re simply adding a bolt-on kit to an otherwise unmodified engine, or building an engine from the ground up to support a larger horsepower goal, the parts are available to do it all—including dedicated performance cylinder blocks designed to withstand nearly 30 psi of turbocharged boost and more than 2,000 hp.
Realistically, most enthusiasts and builders are aiming for something more modest in a street-driven or street/strip car. But the already high power levels of stock LS-powered vehicles—from the 305 hp of the 1998–2002 LS1-powered F-cars to the 505 hp of the LS7-powered Corvette Z06—means the return on a supercharger or turbocharger investment will be impressive.
In most cases, a standard street based bolt-on supercharger or turbocharger kit adds approximately 100 to 125 hp. Bolt-on twin-turbo systems can approach or exceed 200-hp gains, but extreme care must be taken with tuning on an engine with a stock rotating assembly, as factory installed cast pistons and rods don’t stand up long if detonation occurs, or even if there is excessive heat from a slightly lean air/fuel mixture.
In fact, when a forced-induction system is planned to exceed the stock engine’s output by more than about 150 hp, the builder should consider fortifying the engine with forged rotating parts and lower compression pistons.
Perhaps the ultimate demonstration of forced-induction LS power is the twin turbocharged 1996 Impala SS built by GM Performance Parts. Its 400-ci LSX iron-block engine produces more than 2,000 hp, with help from a pair of 88-mm turbos. (See Chapter 9 for a complete look at the engine, including the components and dyno testing.)
When building a forced-induction combination that’s planned to exceed the performance level of a bolt-on kit with relatively mild boost, the investment in stronger rotating parts must be made. Most LS production engines don’t come with a forged crankshaft, rods, or pistons. They’re must-haves for ensuring engine strength and durability.
Cast Rotating Parts: Pushing the Factory Parts’ Envelope
To put it simply, production LS engines—except the ZR1’s LS9 and CTS-V’s LSA—weren’t designed for supercharging. And while the basic engine design has proven to be remarkably durable, the cylinder pressure generated by a supercharger or turbocharger takes its toll on the engine’s internal components.
The only LS engine from the factory to come with forged pistons is the LS9. All of the rest (the LS7 and LSA included) use hypereutectic (cast) aluminum pistons. Powdered metal rods and a mix of cast and forged crankshafts are used, too, but the bottom line is the basic rotating assembly was not designed for the rigors of forced induction.
That’s not to say the factory parts don’t withstand forced induction. In fact, typical bolt-on blower and turbo kits survive very well with otherwise-stock engines. Generally speaking, however, bolt-on kits deliver less than 15 pounds of boost and vehicles that are primarily street driven don’t see extended use at wide-open throttle. When tuned properly, stock engines survive admirably. It’s when the boost level is turned up and the vehicle’s use sees increased racing duty that the longevity of the factory internal components is reduced. (See Chapters 8 and 9 for engine-building guidelines, including the use of forged rotating components.)
Compression Ratio and Recommended Boost Limits
Another performance limitation when using forced induction on an LS engine with stock internal components is the high compression ratio. The engines in most popular LS-powered performance vehicles, from the LS1-powered F-bodies to the LS7-powered Corvette Z06 have comparatively high compression ratios that range from 9.0:1 to 11.0:1.
A high compression ratio increases the tendency for the engine damaging conditions of detonation and preignition. Those conditions can be especially hard on the factory-installed cast pistons. As a result, the boost pressure on otherwise-stock engines should be limited to prevent damage and ensure performance longevity.
Most intercooled/charge-cooled, street-intended bolt-on supercharger and turbo kits deliver between 5 and 8 pounds of boost—and that’s sufficient for stock-engine vehicles. Some kits push toward 10 pounds, with turbo kits easily tuned to deliver much more, but anything more than about 12 pounds is pushing the boundary of engine safety. Enthusiasts and builders seeking more than 12 pounds of boost from an LS engine should consider rebuilding it with forged rotating parts and a lower compression ratio of approximately 9.0:1 to 9.5:1.
Importance of Tuning and Avoiding Detonation
The previous sections that described boost levels, compression ratios, and forged engine components are all tied together by the importance of proper tuning of a forced-induction engine. Without it, even the strongest engine parts don’t last long under pressure if the air/fuel ratio is too lean or the engine is prone to detonation.
Detonation is the uncontrolled combustion that is typically caused by excessive heat in the cylinders, whether through a too-lean air/fuel mixture or other factors. The added heat generated by a blower or turbo system makes forced-induction engines extremely susceptible to detonation, particularly under high load and higher boost levels.
ad and higher boost levels. A high compression ratio can also contribute to detonation, making it important that an other wisestock engine—especially an LS engine with its comparatively high compression ratio—is tuned properly to prevent detonation at all costs. Many builders are adept at installing the hardware of a turbocharger or supercharger system, but don’t have the knowledge to upload the proper software when it comes to the engine controller. Anyone who isn’t proficient at tuning should leave it to someone who is (see Chapter 7 for more tuning details).
Charge Cooling/Intercooling
To put it simply, compressing air, as superchargers and turbochargers do, generates heat. In the engine, that means an increase in the inlet air temperature—the boosted air that enters the engine—of up to 200 degrees F at 8 pounds of boost.
grees F at 8 pounds of boost. Hotter inlet air significantly reduces the effectiveness of the boosted air charge, because it is less dense than cooler air. It also makes the engine more susceptible to detonation. A charge-cooling system, commonly called an intercooler, combats the effects of a hotter cooling system by forcing the air charge through a radiator-like device to reduce its temperature before it enters the engine at the throttle body. Because of the concern for detonation on LS engines with their relatively high compression ratios, almost all bolt-on supercharger and turbocharger kits include a charge cooler.
There are two basic types of charge coolers: air-to-air and liquidto-air (also known as water-to-air). With an air-to-air intercooler, the boosted air charge simply blows through a “radiator,” where air rushing over the fins provides the cooling effect. A liquid-to-air system is more like a conventional radiator and includes a dedicated circuit of coolant (typically a 50/50 mix of anti-freeze and water, just as in the engine’s radiator).
Because production LS engines have relatively high compression ratios, extreme care must be taken to avoid detonation with superchargers and turbo systems. Bolt-on kits can be tuned to minimize the risk, but lower compression pistons should be used when building an engine for greater power and higher boost levels.
Generally speaking, a liquid-toair charge-cooling system is more effective on higher powered street engine combinations and racing combinations. It requires a separate cooling circuit, a coolant reservoir and an electric-driven water pump.
Forced-Induction Terms
Throughout this book, a number of terms are used to describe or support specific characteristics, components, and performance related to forced induction. Reviewing them through the definitions below will enhance your comprehension of the following chapters.
Adiabatic Efficiency: The amount of heat generated when air is compressed by the supercharger or turbocharger in relation to the amount of the air compressed. Superchargers and turbochargers typically have adiabatic efficiency ratings of 50 to 75 percent. A 100-percent efficiency rate equals no heat generated during compression.
Air Compressor: With either a supercharger or turbocharger, it is the fan-like device that blows pressurized air into the engine’s air inlet.
Air Density Ratio: The difference between the denser air under boost and the outside air.
Air/Fuel Ratio: The mass difference between air and fuel during the combustion process. For gasoline engines, the optimal (see Stoichiometric) air/fuel ratio is 14.7:1, or 14.7 times the mass in air to fuel. A higher number indicates a leaner mix, or lower fuel content in the mix. A lower air/fuel ratio number indicates a richer mix, or one with greater fuel content. A lean mixture—one with a higher air/fuel ratio—can lead to detonation.
Blow-off Valve: A vacuum-actuated valve that releases excess boost pressure in the intake system of a supercharged or turbocharged engine when the throttle is lifted or closed. The excess air pressure is released to the atmosphere.
Boost: The pressure of compressed air at the intake manifold that is generated by the supercharger or turbocharger. It is generally measured in pounds per square inch (psi) or bar. A 1-bar measure is equal to 14.7 psi.
Boost Controller: A device used to limit the air pressure that acts upon a turbo charger’s wastegate actuator in order to control the maximum boost at the engine. It can be a mechanically or electronically controlled device.
Bypass Valve: Similar to a blow-off valve, it is a vacuum-actuated valve designed to release excess boost pressure in the intake system of a turbocharged car when the throttle is lifted or closed. The air pressure is re-circulated back into the non pressurized end of the intake (before the turbo) but after the mass airflow sensor.
Charge Cooler: A radiator-like device that is used to dissipate or reduce some of the heat generated by the compression of the boosted air charge, enabling greater power and/or helping reduce or eliminate the tendency for detonation.
Detonation: Abnormal and uncontrolled flame activity in the combustion chamber that can cause engine damage, typically due to excessive heat. In a forced-induction engine, detonation is generally caused by a lean fuel mixture, too high compression, improper tuning, or a combination of all three.
Heat Exchanger: The radiator-like part of a charge-cooling system.
Intercooler: See Charge Cooler.
Pre-ignition: Similar to detonation, pre-ignition is a potentially catastrophic condition whereby heat retained in the cylinder causes the spark plug to act like a diesel engine’s glow plug, igniting incoming fuel charge before the piston reaches the top of its stroke. A cooler air charge can reduce the chance of pre-ignition.
Stoichiometric Combustion: The ideal combustion process that completely burns the air/fuel mixture. Generally speaking, an air/fuel ratio of 14.7:1 in a gasoline engine delivers stoichiometric combustion (see Air/Fuel Ratio).
Turbine: The part of a turbocharger that is acted upon by the engine’s exhaust gases. Hot exhaust gases flow into the turbine, spinning it. In turn, the turbine spins the corresponding air compressor that blows fresh air into the engine.
Turbo Lag: The time difference between the application of the throttle and the power boost delivered by the turbocharger.
Wastegate: A boost-pressu reactivated valve that allows excessive exhaust gas to bypass the turbo-charger’s turbine. It is used to control boost pressure.
Real-World Project: Steve Gilliland’s 1,000-hp Twin-Turbo Z06
Out on the windswept plains of Oklahoma, Steve Gilliland’s Millenium Yellow Corvette Z06 must seem like a UFO, with the locals sometimes glimpsing a fast-moving, strange-sounding object that seems to defy the laws of physics. And while the car may have seemingly unearthly capability, it’s the result of decidedly earth-based technology.
Gilliland sent his Z06 to Katech Performance in Detroit. There, the stock LS7 engine was reinforced with Katech’s Air Attack 7.0-liter engine package and fitted with an APS twin turbo kit. On Katech’s engine dyno, this combination made a staggering 1,008 hp and 827 ft-lbs of torque. Of course, there’s more to the engine than a collection of parts and a pair of turbochargers. Here’s a closer look at the combination.
Rotating Assembly
Because the factory LS7 uses cast pistons, the engine’s rotating assembly was completely replaced with racing-spec parts to withstand the pressure and heat applied by the twin-turbo system. A Callies Dragonslayer forged-steel crankshaft anchors the assembly, with a set of Carrillo forged-steel H-beam connecting rods and Katech’s forged-aluminum pistons. The pistons are dished slightly to reduce the engine’s compression ratio from a high 11.0:1 to a more boost-friendly 9.0:1.
Heads and Camshaft
The engine uses the factory LS7 heads, but the exhaust valves were swapped for Inconel units that better withstand high heat. Also, higher rate valve springs were added to withstand the greatly increased cylinder pressure that can make it difficult to close the valves. Racing-spec ARP head studs and fasteners along with premium head gaskets have provided a leak-free seal of the heads against the block.
Steve Gilliland’s twin-turbo Corvette Z06 makes more than 1,000 reliable horsepower.
As for the camshafts, it’s a custom grind designed especially for this engine combination, with .615-inchintake and .613-inch-exhaust lift specs; duration of 220 degrees on the intake valve and 229 degrees on the exhaust; and a lobe separation angle of 116 degrees. (That compares with .591/ .591-inch lift, 211/230-degrees duration, and 121-degree lobe-separation angle on the stock LS7 cam.)
Turbo System
Katech fitted the reinforced LS7 with APS’ Z06 intercooled twin-turbo kit. It uses a pair of Garrett GT3582R ball-bearing turbochargers, featuring 61.4-mm inlets and 82-mm outlets on the compressor and a 68-mm turbine. The turbochargers mount very low on the engine—essentially in the transmission tunnel—because of the space restrictions and proclivity for heat buildup under the Corvette’s hood. Gilliland wasn’t concerned about pump-gas compatibility, so the system was tuned to push 12 pounds of boost on 100-octane gas and 15 pounds of boost on 105-octane racing fuel. An A’PEXi AVC-R boost controller is used to maintain boost at predetermined RPM levels as Gilliland shifts through the gears.
Mounted low and out of sight on the LS7 engine, the APS twin-turbo kit on Steve Gilliland’s Corvette works with additional components from Katech Performance to produce more than 1,000 hp; and it’s a completely streetable, easy-driving combination.
Beyond the engine compartment, the drivetrain was upgraded with a strengthened transaxle from RPM, a stronger triple-plate carbon clutch from Exedy, a Quaife differential, and stronger, 300M alloy axle shafts.
Written by Barry Kluczyk and Posted with Permission of CarTechBooks
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