With the weight breaks offered to supercharged cars in many classes, the boost capability of the latest blowers puts racers on par with turbocharged competitors. Racer Tom Kempf, who has driven a turbocharged 10.5 Outlaw Firebird for more than a decade, is ready for the change.
“I’ve had a lot of success with turbochargers, but that has come with a number of compromises,” Kempf says. “First and foremost is these big, powerful turbo engines are very hard on transmissions, when it comes to staging and building boost at the starting line. That’s not an issue with a supercharger.”
Supercharger systems are much less complex than turbo systems with far less plumbing. That reduces fabrication time during the vehicle’s build and makes it easier to do between-round maintenance. The bottom line is turbochargers are still the power adder of choice for most racers, but the tide is turning.
“Turbo cars may still be running the quickest times,” says Kempf. “But it seems that, more and more, the blower cars are winning the races. If we can get the boost we need from a blower, I’m ready for the change.”
A new generation of centrifugal superchargers is challenging the boost capability of turbochargers that have long ruled Outlaw-type drag racing, offering comparable boost capability with less plumbing complexity and reduced stress on the transmission, particularly when staging.
LS Performance Potential
Simply put, the performance potential of a boosted LS engine is almost unlimited. Whether 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-Body 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.
Cast Rotating Parts: Pushing the Factory Parts’ Envelope
Production LS engines (except the C6 ZR1’s LS9, the Gen V Camaro ZL1, and the Cadillac 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 a turbocharger takes its toll on the engine’s internal components.
The only LS engine from the factory to come with forged pistons was 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 as well, 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 to 11.0:1.
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-have items to ensure engine strength and durability.
A high compression ratio supports greater power output but 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 about 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 to 9.5:1.
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.
| Production Engine Compression Ratios | |
| GEN III Engines | |
| LS1 5.7L | 10.1:1 |
| LS6 5.7L | 10.5:1 |
| Vortec 5.3L (early trucks, including SSR) | 9.5:1 |
| Vortec 5.3L (later trucks) | 9.9:1 |
| Vortec 4.8L (truck applications) | 9.1:1 |
| GEN IV Engines | |
| LS2 6.0L | 10.9:1 |
| LS3 6.2L | 10.7:1 |
|
|