Source: GM Powertrain
2015 6.2L V-8 Supercharged (LT4)
Vehicle Applications
Chevrolet Corvette Z06
Overview
The LT4 6.2L supercharged V-8 engine is the power behind the Corvette Z06 and is the most powerful production engine ever offered in a General Motors vehicle.
The new LT4 engine builds on the design strengths of the previous LS9 supercharged engine used in the sixth-generation Corvette ZR1 and leverages the technologies introduced on the seventh-generation Corvette Stingray, including direct injection, cylinder deactivation and continuously variable valve timing, to take Corvette performance to an all-new plateau.
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, including:
Cylinder block
The LT4's Gen-V aluminum cylinder block shares two key design elements with GM's original small-block V-8: a 90-degree cylinder angle and 4.400-inch bore centers. The bore and stroke dimensions are: 4.06-inch (103.25 mm) bore x 3.62-inch (92 mm) stroke.
Compared to the previous Gen-IV small block, the Gen-V's aluminum cylinder block casting is all-new, but based on the same basic architecture. It was refined and modified to accommodate the mounting of the engine-driven fuel pump and vacuum pump. It also incorporates new engine mount attachments, new knock sensor locations, improved sealing and oil-spray piston cooling.
Rotating assembly and windage tray
Within the Gen-V block is a durable rotating assembly that includes a steel crankshaft and 6.125-inch-long, powder-metal connecting rods, as well as high-strength, forged aluminum pistons. The LT4's connecting rods are based on the design of those used in the LT1, but are specially machined for lower weight – an attribute that reduces reciprocating mass and enables the engine to rev more quickly.
The crankshaft in the Gen-V small block is located with new nodular main bearing caps – a significant upgrade over more conventional grey iron main caps. Nodular caps are stronger and can better absorb vibrations and other harmonics to help produce smoother, quieter performance.
A windage tray is also used with the Gen-V engine, which enhances performance and efficiency by improving oil flow control and bay-to-bay crankcase breathing. The cylinder block and main bearing caps maintain the optimal crankcase "windows" that were perfected on the Gen-IV engine.
The LT4 has 10:1 compression ratio, which is comparatively high for a supercharged engine, but it is enabled by the more precise fuel control of direct injection.
Forged Aluminum Pistons
The LT4 uses unique forged aluminum pistons with a structure designed for the more intense cylinder pressures that come with forced induction. They also have a unique head topography that is essential to the direct injection system. The "bowl" and shape of the top of the piston head is designed to promote thorough mixing of the air and fuel – a dished center section helps direct the fuel spray from the injector – to ensure complete combustion, which improves performance and efficiency, particularly on cold starts.
To further reduce wear, the piston skirt is coated with a polymer material, which eliminates bore scuffing, or abrasion of the cylinder wall over time from the piston's up-down motion. The polymer coating also dampens noise generated by the piston's movement. The wrist pins, which attach the piston to the connecting rod, were developed for maximum durability, with a large outer diameter and a tapered inner diameter.
The pins "float" inside the rod bushing and pin bores in the piston barrel. Compared to a conventional fixed pin assembly, in which the connecting rod is fixed to the piston's wrist pin and the pin rotates in the pin bore, the floating pins reduce stress on the pin. They allow tighter pin to pin-bore tolerances and reduce noise generated as the piston moves through the cylinder. The benefit is less engine wear, improved durability and quieter operation.
Oiling System
The oiling system is revised and features a new, dual-pressure-control and variable-displacement vane pump with increased flow capacity. As with the Gen-III/Gen-IV engines, the oil pump is driven by the crankshaft. 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.
The LT4 engine features a dry-sump oiling system with a 10.5-quart capacity. All Gen-V engines are designed to be used with GM's Dexos semi-synthetic motor oil. "Thinner" oil is used, too, which helps reduce friction to enhance efficiency. The LT4 6.2L uses 5W30.
Oil-Spray Piston Cooling
All Gen-V engines feature oil-spray piston cooling, in which 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.
PCV-Integrated Rocker Covers
One of the most distinctive features of the Gen V family is its domed rocker covers, which house a patent-pending, integrated positive crankcase ventilation (PCV) system that enhances oil economy and oil life, while reducing oil consumption and contributing to low emissions. The rocker covers also hold the direct-mount ignition coils for the coil-near-plug ignition system. Between the individual coil packs, the domed sections of the covers contain baffles that separate oil and air from the crankcase gases – about three times the oil/air separation capability of previous engines.
Camshaft Design
A refined camshaft helps balance the LT4's remarkable output with silky, tractable low-rpm operation. The camshaft operates the engine's valves and its design is crucial to both power and smoothness. The torque-enhancing benefits of the supercharger allowed engineers to develop a "softer," lower-lift camshaft for the LT4, compared to the typical high-rev, high-power exotic car engine. The result is smooth operation at low speed, particularly at idle.
The hydraulic roller-lifter camshaft's specifications lift include: 0.492"/0.551" intake/exhaust lift, 189/223 crank angle degrees intake/exhaust duration at 0.050 tappet lift and a 120 degree cam angle lobe separation.
Dual-Equal Cam Phasing
All Gen V engines feature dual-equal camshaft phasing (variable valve timing), which works with Active Fuel Management to enhance fuel economy, while also maximizing engine performance for given demands and conditions.
At idle, for example, the cam is at the full advanced position, allowing exceptionally smooth idling. Under other conditions, the phaser adjusts to deliver optimal valve timing for performance, driveability and fuel economy. At high rpm it may retard timing to maximize airflow through the engine and increase horsepower. At low rpm it can advance timing to increase torque. Under a light loads, it can retard timing at all engine speeds to improve fuel economy.
A vane-type phaser is installed on the front of the camshaft to change it's angular orientation relative to the sprocket, thereby adjusting the timing of valve operation on the fly. It is a dual-equal cam phasing system that adjusts camshaft timing at the same rate for both intake and exhaust valves. The system allows linear delivery of torque, with near-peak levels over a broad rpm range, and high specific output (horsepower per liter of displacement) without sacrificing overall engine response, or driveability. It also provides another effective tool for controlling exhaust emissions.
The vane phaser is actuated by hydraulic pressure and flow from engine oil, and managed by a solenoid that controls oil flow to the phaser.
Cylinder Head Design
The Gen-V small-block's cylinder head design builds on the excellent, racing-proven airflow attributes of previous small-block heads and matches it with a direct-injection combustion system. It supports tremendous airflow at higher rpm for a broad horsepower band, along with strong, low-rpm torque.
Compared to other Gen-V variants, the LT4 uses aluminum cylinder heads produced with a rotocast manufacturing process, which rotates the head mold as the molten alloy cools and essentially eliminates porosity, or microscopic pockets of air trapped in the casting. Rotocasting delivers a stronger part that helps maintain performance and structural integrity over the life of the engine.
The heads are cast in a premium A356T6 alloy, which better manages the heat generated in a supercharged engine. A356T6 also pays dividends in the thinner bridge area between the intake and exhaust valves, where effective heat dissipation is crucial to both performance and long-term durability.
Compared to the naturally aspirated LT1 head, which features 59.02cc combustion chambers, the LT4 has a slightly larger 65.47cc chamber size designed to complement the volume of the piston's dish. The chamber size and piston dish work together to produce a 10:1 compression ratio – a full 1.5 points lower than the LT1's 11.5:1 compression. Lower compression than a comparable naturally aspirated engine is required of a supercharged application to stave off knock or detonation that can occur as a result of the forced-induction engine's higher cylinder pressures.
As with other Gen-V variants, the LT4 head features large, straight and rectangular intake ports that feature a slight twist to enhance mixture motion. This is complemented by a reversal of the intake and exhaust valve positions as compared to the Gen-IV design. The exhaust port shapes are optimized for the new valve locations, with new port opening locations at the manifold face.
Large, lightweight valves are used in the LT4's heads, including 2.13-inch (54mm) titanium intake and 1.59-inch (40.4mm) hollow sodium exhaust valves. The exhaust valves are manufactured from a high-chromium steel alloy called 21-43 (SilChrome 1 is used at the tip only, the valve is made from 21-43). At normal operating temperatures, the sodium inside the valve stem melts and becomes liquid. The liquid sodium improves conductivity, promoting heat transfer away from the valve face to the cooler end of the stem, where it more readily dissipates through the valve guide. This maintains a lower, more uniform valve temperature, reducing wear on the valve seat for a consistent seal between the valve and head over the life of the engine.
The valves are held at 12.5 degrees intake/12 degrees exhaust angles vs. the Gen-IV's 15-degree angle. Additionally, the valves are splayed at 2.61 degrees intake/2.38 degrees exhaust to reduce shrouding and enable greater airflow.
Valvetrain components include durable valve springs and roller-pivot rocker arms with a 1.8 ratio – the amount of movement on the valve side of the rocker arm in comparison with the pushrod side. And speaking of pushrods, the Gen-V small-block features large-diameter 8.7mm (outside diameter) components that provide exceptional stiffness that enables excellent high-speed valvetrain dynamic performance.
Given the LS4's pressurized induction, special attention was paid on sealing, too. The head gaskets are extra-robust, seven-layer stainless steel, and the 12mm cylinder head bolts are hardened stainless.
Next-generation Eaton supercharger
State-of-the-art supercharging technology is the foundation of the LT4's remarkable performance. The supercharger is an air pump driven by the engine's crankshaft. It forces more air into the engine's combustion chambers than the engine could otherwise draw on its own. The increased volume of oxygen allows the engine to efficiently process more fuel, and thus generate more power.
The LT4 employs Eaton's new, twin-rotor R1740 Twin Vortices Series (TVS) supercharger, which spins up to 20,000 rpm – 5,000 rpm more than the supercharger on the previous LS9 engine. The rotors are smaller in diameter than the LS9's supercharger, 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 LT4 supercharger displaces 1.7L and generates maximum boost pressure of 9.71 pounds per square inch (0.67 bar). The TVS rotor design features four lobes on each of the supercharger's pair of rotors. The spiral-shaped rotors intermesh with each other and the four-lobe configuration provides about 20 percent more airflow than conventional three-lobe designs, as well as an improvement in thermal efficiency of up to 15 percent. Moreover, parasitic power loss – the amount of power the engine uses to operate the supercharger – is reduced 35 percent. That improves both supercharger response time and the engine's overall efficiency.
Even with its integrated supercharger/intercooler assembly mounted in the valley between the cylinder heads, the engine is only about 1 inch (25 mm) taller than the Corvette Stingray's LT1 engine.
An electronically controlled throttle is mounted to the supercharger inlet. It is a single-bore design with an 87mm bore diameter and features a "contactless" design that is more durable and enables greater control.
The design of the supercharger system incorporates advanced features for noise reduction. The rear cover of the supercharger gear case has changed from an Aluminum casting to a constained-layer damping material designed to absorb radiated noise from the supercharger drive gears. Additionally, the supercharger lid has a constrained-layer damping panel assembled over the discharge port to damp the high frequency content of the air pulsations exiting the supercharger discharge port. This reduces the traditional "whine" noise associated with superchargers and gives a refined yet powerful and pleasing sound to the engine.
Dual-Brick Air-to-Liquid Intercooler
An advanced intercooling system increases the LT4's performance and extends its supercharger's benefits. The engine's charge cooler is integrated in the supercharger case adjacent to the rotors, with two air-to-liquid cooling "bricks" that substantially lower the temperature of air used in the combustion process.
Intercoolers are familiar features on supercharged and turbocharged engines. Similar in concept to an engine's radiator, intercoolers cool the air pumped by the charging device into the cylinders. Cooler air is denser air, which means more oxygen in a given volume, resulting in optimal combustion and more power. Traditionally, intercoolers look like small radiators mounted somewhere outside the engine, with air fed into the engine through a plumbing network.
The LT4's intercooling system raises the bar in both packaging and efficiency. It uses two low-profile, aluminum clamshell heat exchangers mounted longitudinally adjacent to the rotors in the supercharger case. Air pumped by the supercharger flows directly through these bricks to the intake ports on the cylinder heads without the need to channel the air through plumbing to the front of the vehicle and back. The bricks are cooled by their own coolant circuit, with a remote pump and heat exchanger mounted in front of the Corvette's radiator. The temperature of air fed to the LT4's cylinder heads is reduced by up to 140 degrees F (60 degrees C), substantially increasing the amount of oxygen available for the combustion process.
Direct Injection
Direct injection is featured on all Gen-V engines. This technology moves the point where fuel feeds into an engine closer to the point where it ignites, enabling greater combustion efficiency. It fosters a more complete burn of the fuel in the air-fuel mixture, and it operates at a lower temperature than conventional port injection. That allows the mixture to be leaner (less fuel and more air), so less fuel is required to produce the equivalent horsepower of a conventional, port injection fuel system. Direct injection also delivers reduced emissions, particularly cold-start emissions.
The pistons play an integral role in the direct injection system, as they feature dished heads designed to direct the fuel spray for a more complete combustion. Design of this advanced combustion system was optimized after thousands of hours of computational analysis, representing one of the most comprehensively engineered combustion systems ever developed by General Motors.
To support the requirements of an engine producing approximately 40 percent more power than the naturally aspirated LT1, the supercharged LT4 uses higher-capacity, 141 lb./hr fuel injectors. The LT1 injectors are rated at 123 lb./hr.
High-Pressure Fuel Pump
The LT4 direct injection system features a new higher fuel pressure pump, capable of pressures up to 20Mpa (200bar). The LT1 high pressure pump is capable of 15Mpa (150bar). Direct Injection requires the high-pressure, engine-driven fuel pump in addition to a conventional, fuel-tank-mounted pump. On all Gen-V engines, the pump is mounted in the "valley" between cylinder heads – beneath the intake manifold. It is driven by the camshaft at the rear of the engine.
A "soft stop" control strategy for the pump's internal solenoid significantly reduces the characteristic "ticking" sound of direct injection systems. Mounting the pump in valley, where it is covered by an acoustically treated intake manifold, also helps reduce noise, while also maintaining the tight, compact packaging for which all small-blocks have been known.
Active Fuel Management
Active Fuel Management temporarily deactivates four of the cylinders and seamlessly reactivates them when the driver demands full power. When cylinders are deactivated, the engine's pumping work is reduced, which translates into real-world fuel economy improvements. The transition takes less than 20 milliseconds and is virtually imperceptible.
The key to AFM's efficiency and seamless operation is a set of two-stage hydraulic valve lifters, which allows the lifters of deactivated cylinders to operate without actuating the valves. In engineering terms, this allows the working cylinders to achieve better thermal, volumetric and mechanical efficiency and lowering cyclical combustion variation from cylinder to cylinder. As a result, AFM delivers better fuel economy and lower operating costs. The only mechanical components required are special valve lifters for cylinders that are deactivated, and their control system. Active Fuel Management relies on three primary components: Collapsible or "de-ac" (deactivation) valve lifters, a Lifter Oil Manifold Assembly (LOMA) and the engine controller, which determines when to deactivate cylinders.
Exhaust Manifolds
The LT4 exhaust manifolds are constructed of cast Austenitic Stainless Steel. The smooth flow passages and equal length runner geometry were carefully developed using CFD analysis to maximize the volumetric efficiency tuning of the exhaust gas flow.
LT4 exhaust manifold flow performance is equivalent to the LS7/LS9 tube and jacket design at lower overall cost, optimized exhaust sound characteristics, and more consistent exhaust flow.
The manifolds are fitted with a pair of close-coupled catalytic converters that heat quickly, achieving light-off temperature and closed-loop operations in seconds.
58X Ignition System
The Gen-V family uses an advanced 58X crankshaft position encoder to ensure that ignition timing is accurate throughout its operating range. The 58X crankshaft ring and sensor provide more immediate, accurate information on the crankshaft's position during rotation. This allows the engine control module to adjust ignition timing with greater precision, which optimizes performance and economy. Engine starting is also more consistent in all operating conditions.
In conjunction with 58X crankshaft timing, the Gen-V applies the latest digital cam-timing technology. The cam sensor is located in the front engine cover, and it reads a 4X sensor target on the on the cam phaser's rotor which is attached to front end of the cam. The target ring has four equally spaced segments that communicate the camshaft's position more quickly and accurately than previous systems with a single segment.
The dual 58X/4X measurement ensures extremely accurate timing for the life of the engine. Moreover, it provides an effective backup system in the event one sensor fails.
Additional Features
E92 Engine Controller
Operation and performance of the Gen-V family is overseen by this next-generation engine controller.
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