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 |
| L99 6.2L (2010+ Camaro SS with Active Fuel Management) | 10.4:1 |
| LS4 (front-drive application) | 10.1:1 |
| LS7 7.0L | 11.0:1 |
| LS9 6.2L | 9.1:1 |
| LSA 6.2L | 9.0:1 |
| L92/L94/L9H 6.2L | 10.5:1 |
| Vortec 6.0L (various truck applications) | 9.4, 9.6, and 10.8:1 |
Crankcase Ventilation
LS engines have a tendency toward blowby, where combustion gases and engine oil slip past the piston rings. The condition is exacerbated with forced induction, which can push a considerable amount of oil out the engine in a relatively short period; and the factory positive crankcase ventilation (PCV) system may not accommodate the additional pressure introduced by a turbocharger or a supercharger.
Some turbo and supercharger kits include replacement valve cover breathers, but they may not be sufficient in some cases. Installing larger breathers and possibly a catch can for oil may be required. In racing applications, the engine may benefit from a vacuum pump. However, the bottom line for builders is: be prepared for blowby.
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 a turbo system makes forced-induction engines extremely susceptible to detonation, particularly under high load and higher boost levels.
A high compression ratio can also contribute to detonation, making it important that an otherwise-stock engine (especially an LS engine with its comparatively high
