General Motors : Compact Powerhouse: Inside Corvette Z06’s LT4 Engine

DETROIT - The all-new 2015 Corvette Z06 is the most powerful production car ever from General Motors, with its all-new LT4 supercharged 6.2L V-8 engine SAE-certified at 650 horsepower (485 kW) at 6,400 rpm and 650 lb-ft of torque (881 Nm) at 3,600 rpm.

More than raw power, the LT4 delivers exceptional capability in a more efficient package, making it one of the most compact 650-horsepower automotive engines in the world. 

"The LT4 Small Block delivers the ultimate horsepower and torque for the most capable Corvette ever," said Jordan Lee, chief engineer and program manager, GM Small Block Engines. "Its tremendous low-end torque, enabled by a more efficient supercharger, maximizes responsiveness for a driving experience that is on par with the world's most exotic supercars - with impeccable manners that make it suitable for daily driving."

To achieve its world-class balance of performance and efficiency, the LT4 leverages the technologies introduced on the 2014 Corvette Stingray's LT1 Gen 5 Small Block, including direct injection, Active Fuel Management (cylinder deactivation) and continuously variable valve timing, while advancing the supercharging technologies established with the previous-generation Corvette ZR1's LS9 engine. That includes a new, more compact and higher output supercharger/intercooler assembly that helps maintain a low hood profile, while also meeting the pedestrian protection requirements in the global markets where the Z06 will take on the world's supercar comers. It's also 20 pounds lighter than the LS9's supercharger.

"The LT4 is only one inch taller than the naturally aspirated LT1 engine," said Lee. "Along with a lower profile under the hood, the new, very compact supercharger helps the engine make power more quickly - and perhaps more importantly, it helps produce more torque earlier in the rpm band."

Supercharging for low-rpm torque and high-rpm horsepower
A new R1740 TVS 1.7L supercharger spins at up to 20,150 rpm - 5,000 rpm more than the supercharger on the previous LS9 engine. Like the LS9's supercharger, the R1740 features a pair of four-lobe rotors, each with a 160-degree helix (twist) designed to broaden the effectiveness of the positive-displacement "blower" - enabling more torque production at lower rpm and sustained boost at higher rpm for greater horsepower.

Compared to the LS9's 2.3L supercharger, the R1740's rotors are shorter in length and smaller in diameter, which contributes to their higher-rpm capability - and enables them to produce power-enhancing boost earlier in the rpm band. That boost is achieved more efficiently via a more direct discharge port that creates less turbulence, reducing heat and speeding airflow into the engine.

The supercharger's charge-cooling system is also more efficient, offering 10 percent greater heat rejection through 23 percent reduced volume, compared to the LS9.

"The Small Block's cam-in-block design heritage has always enabled very high performance and responsiveness in a small, compact package - an attribute amplified by the performance of our new supercharger/intercooler design," said Lee. "With the torque the LT4 produces at low rpm, the feeling of power is immediate and thrilling."

Making boost: LT4 vs. LS9 superchargers

Indeed, compared with other supercar engines, the LT4 is a veritable fountain of low-end torque, producing 457 lb-ft (619 Nm) just off idle and 625 lb-ft (847 Nm) by only 2,800 rpm. The V-12-powered Ferrari F12 Berlinetta, for example, produces about 28 percent less torque than the Z06, despite offering about 12 percent more horsepower - and its peak torque isn't achieved until 6,000 rpm. The LT4 maintains 90 percent of its peak torque, or 592 lb-ft (802 Nm), from 2,500 to 5,400 rpm.

The new LT4 engine eclipses the Porsche 911 Turbo S engine's peak power levels by 90 horsepower (67 kW) and 134 lb-ft of torque (182 Nm).

"Torque is the pulling power of an engine and the LT4's abundance of it at every rpm in the engine's speed range helps the 2015 Corvette Z06 accelerate quicker and respond nearly instantaneously," said Lee. "It's the very definition of power on demand."

In fact, the new Z06 engine produces 40 percent more peak torque (180 lb-ft / 244 Nm) than the previous-generation's 7.0L LS7 engine - and 7.5 percent more than the supercharged 2013 Corvette ZR1's 604 lb-ft (819 Nm). At 3,200 rpm, the new LT4 surpasses the LS7 by 208 lb-ft of torque (252 Nm). On the horsepower side of the graph, the LT4's 650-hp rating is 29 percent greater than the LS7's 505 horsepower (376 kW), and 12 horses more than the ZR1's LS9 engine.

Fuel, combustion and rotating assembly details
The new LT4 engine is based on the same Gen 5 small block foundation as the Corvette Stingray's LT1 6.2L naturally aspirated engine, incorporating several unique features designed to support its higher output and the greater cylinder pressures created by forced induction.

Fuel system -The direct injection system features a specific fuel pump and fuel rail assembly consisting of:

• A higher-flow, 2,900-psi (20 MPa) fuel pump. The LT1 pump is rated at 2,175 psi (15 MPa)
• Higher-flow, 25cc/second fuel injectors (LT1 is 20cc/second), with a unique spray cone for optimized combustion under boost
• Higher capacity rail designed for the flow capacity of the new pump and injectors.

Cylinder heads / valvetrain - The LT4 features the same combustion system design as the LT1, but uses Rotocast A356-T6 aluminum cylinder heads, because they're stronger and handle heat better than conventionally cast aluminum heads. The manufacturing process involves rotating the mold during casting to eliminate porosity and ensure a denser, more accurate component. Additional unique features include:

• Larger, 65.47cc combustion chambers, compared to the LT1's 59.02cc chambers
• Lightweight 54mm solid titanium intake valves designed for great strength at high rpm and exception heat resistance. The LT1 uses 54mm hollow intake valves
• The 40.4mm sodium-filled exhaust valves are the same as the LT1
• High 10.0:1 compression ratio - for a forced-induction engine - enhances performance and efficiency and is enabled by direct injection
• A camshaft with longer exhaust duration - 189/223 crank angle degrees (intake/exhaust) at 0.050-in. tappet lift vs. 200/207 degrees for the LT1 - holds the valves open longer to process the greater volume of airflow enabled by the supercharger's boosted air charge
• Stainless steel exhaust manifolds that offer greater structure at higher temperatures.

Rotating assembly - Each component of the rotating assembly is unique to the LT4 to support the cylinder pressures the boosted engine is capable of generating, as well as the unique requirements for a lower compression ratio. Elements include:

• A 1528MV forged steel crankshaft with tungsten balancing inserts, ground pin collars and intermediate pin drills for rods 1-6
• A lightweight, forged 6061 aluminum damper with a T6 anodized hub and an iron inertia ring (the LT1 uses an iron damper)
• Lightweight, high-strength powder-metal steel connecting rod design for reduced reciprocating mass and quicker rev capability. It has the same balance mass as the LT1 rod, but a higher load capability, with features including machined lightening slots and additional lightening features, as well as a "stepped"/wide pin end with a premium bushing
• Forged aluminum pistons with flat top crown that supports the 10.1 compression ratio and internal ribs for strength (similar to the LS9 piston design). A premium ring pack includes a PVD-coated top ring, chrome-coated second compression ring and a nitrided oil control ring - all for exceptional wear resistance and durability under high cylinder pressure. The bottom ring land also features eight oil drains
• Floating piston wrist pins with a durable diamond-like coating for durability and low friction.

Oiling system - Similar to the LT1, the new LT4 uses an engine lubrication system featuring a dual-pressure-control and variable-displacement vane pump. Variable displacement enables the pump to efficiently deliver oil pump flow as demanded. Dual-pressure-control enables operation at a very efficient oil pressure at lower rpm coordinated with the Active Fuel Management and operation at a higher pressure at higher engine speeds, providing a more robust lube system with aggressive engine operation. 

Dry-sump oiling is standard and the LT4's crankshaft-driven oil pump uses higher-strength scavenge pump gears than the LT1. A large, 10.5-quart oil tank is used with a larger-capacity oil cooler. The engine uses Dexos 5W30 oil.

Additionally - and like the LT1 - oil-spray piston cooling is standard. Eight oil-spraying jets in the engine block drench the underside of each piston and the surrounding cylinder wall with an extra layer of cooling, friction-reducing oil. The oil spray reduces piston temperature, promoting extreme output and long-term durability. The extra layer of oil on the cylinder walls and wristpin also dampens noise emanating from the pistons.

LT4 6.2L supercharged engine specifications

Manufacturing
The LT4 is assembled at the new Performance Build Center at GM's Bowling Green Assembly Plant and at GM's Tonawanda engine plant in New York.

The 2015 Corvette Z06 goes on sale in the fourth quarter of 2014.

Founded in 1911 in Detroit, Chevrolet is now one of the world's largest car brands, doing business in more than 140 countries and selling more than 4.9 million cars and trucks a year. Chevrolet provides customers with fuel-efficient vehicles that feature spirited performance, expressive design, and high quality.www.chevrolet.com.