When it arrives next summer, the 2009 Chevrolet Corvette ZR1 will be themost powerful and fastest production car ever produced by GeneralMotors, with performance enabled by a new, supercharged 6.2L LS9small-block V-8 engine.
Incorporating the engineering experience already found in the CorvetteZ06's LS7 engine and the new-for-2008 6.2L LS3 of the Corvette, GMPowertain is targeting 100 horsepower per liter for the LS9, or 620horsepower (462 kW), and approximately 595 lb.-ft. of torque (807 Nm);final SAE-certified power levels will be available in early March 2008.
It is, indeed, the ultimate small-block engine for the ultimateCorvette.
"When you experience the LS9 in the Corvette ZR1, the terms performanceand refinement take on a new meaning," said Tom Stephens, group vicepresident for GM Powertrain and Quality. "The LS9 demonstrates anoutstanding performance range, with smooth driveability at low speeds,and surprisingly fierce performance when the customer wants more power."
The enabler of the LS9's performance is a new, largepositive-displacement Roots-type supercharger - with a unique four-lobedesign - developed for the engine by Eaton. It is teamed with anintegrated charge cooling system that reduces inlet air temperature formaximum performance. The LS9 represents the first of several new,supercharged small-block engines that will be introduced in GM vehiclesin the near future, each using superchargers of similar design.
"The small-block V-8 once again demonstrates it boundless horsepowerpotential, versatile design and an architecture with proven quality,durability and reliability," said Stephens. "We haven't yet realized thesmall-block's performance potential."
More than just its tremendous peak horsepower and torque numbers, thesupercharged LS9 makes big power at lower rpm and carries it in a widearc to 6,600 rpm. GM Powertrain testing shows the engine makesapproximately 300 horsepower (224 kW) at 3,000 rpm and nearly 320lb.-ft. of torque (434 Nm) at only 1,000 rpm. Torque tops 585 lb.-ft.(793 Nm) at about the 4,000-rpm mark, while horsepower peaks at 6,500rpm. The engine produces 90 percent of peak torque from 2,600 rpm to6,000 rpm.
Heavy-duty and lightweight reciprocating components enable the engine'sconfident high-rpm performance, while the large-displacement Eatonsupercharger pushes enough air to help the engine maintain power throughthe upper levels of the rpm band.
"The sixth-generation design of the supercharger expands the 'sweetzone' of the compressor's effectiveness, broadening it to help makepower lower in the rpm band," said Ron Meegan, assistant chief engineer."To put it simply, the low-end torque is amazing."
The LS9 is assembled by hand at GM's Performance Build Center, a unique,small-volume engine production facility in Wixom, Mich., that alsobuilds the Corvette Z06's LS7 engine and other high-performance GMproduction engines.
Cylinder block and reciprocating assembly details
The LS9's aluminum cylinder block features steel, six-bolt main bearingcaps, with enlarged vent windows in the second and third bulkheads forenhanced bay to bay breathing. Cast iron cylinder liners - measuring4.06 inches (103.25 mm) in bore diameter - are inserted in the aluminumblock and they are finish-bored and honed with a deck plate installed.The deck plate simulates the pressure and minute dimensional variancesapplied to the block when the cylinder heads are installed, ensuring ahigher degree of accuracy that promotes maximum cylinder head sealing,piston ring fit and overall engine performance.
Nestled inside the cylinder block is a forged steel crankshaft thatdelivers the LS9's 3.62-inch (92 mm) stroke. It features asmaller-diameter ignition-triggering reluctor wheel and a nine-boltflange - the outer face of the crankshaft on which the flywheel ismounted - that provides more clamping strength. Other non-supercharged6.2L engines, such as the base Corvette's LS3, have a six-bolt flange. Atorsional damper mounted to the front of the crankshaft features akeyway and friction washer, which also is designed to support theengine's high loads.
Attached to the crankshaft is a set of titanium connecting rods andforged aluminum pistons, which, when combined with the cylinder heads,delivers a 9.1:1 compression ratio. This combination is extremelydurable and lightweight, enabling the LS9's high-rpm capability.
The basic cylinder head design of the LS9 is similar to the L92-typehead found on the LS3 V-8, but it is cast with a premium A356T6 alloythat is better at handling the heat generated by the supercharged engine- particularly in the bridge area of the cylinder head, between theintake and exhaust valves.
In addition to the special aluminum alloy, each head is created with arotocast method. Also known as spin casting, the process involvespouring the molten alloy into a rotating mold. This makes for more evendistribution of the material and virtually eliminates porosity - airbubbles or pockets trapped in the casting - for a stronger finishedproduct.
Although the heads are based on the L92 design, they featureswirl-inducing wings that are cast into the intake ports. This improvesthe mixture motion of the pressurized air/fuel charge. The charge entersthe combustion chambers via titanium intake valves that measure 2.16inches (55 mm) in diameter. Spent gases exit the chambers through1.59-inch (40.4 mm) hollow stem sodium-filled exhaust valves. Thetitanium intake and sodium-filled exhaust valves are used for theirlightweight and high-rpm capability.
To ensure sealing of the pressurized engine, unique, four-layer steelhead gaskets are used with the LS9's heads.
The broad power band enabled by the LS9's large-displacementsupercharger allows it to make strong low-end torque and great high-rpmhorsepower, which allowed engineers to specify a camshaft with arelatively low lift of 0.555-inch (14.1 mm) for both the intake andexhaust valves. This low-overlap cam has lower lift and slowervalve-closing speeds than the Z06's 505-horsepower (377 kW) LS7, helpingthe LS9 - with its approximately 620 horsepower - deliver exemplary idleand low-speed driving qualities.
The camshaft actuates a valvetrain that includes many components,including the lifters, rocker arms and valve springs, of the LS3 engine.However, LS7 retainers are used to support the engine's high-rpmperformance.
Supercharger and charge cooler details
The LS9's R2300 supercharger is a sixth-generation design from Eaton,with a case that is specific to the Corvette application. Thesupercharger features a new four-lobe rotor design that promotes quieterand more efficient performance, while its large, 2.3-liter displacementensures adequate air volume at high rpm to support the engine'shigh-horsepower aspiration. Maximum boost pressure is 10.5 psi (0.72bar).
The supercharger is an engine-driven air pump that contains a pair oflong rotors that are twisted somewhat like pretzel sticks. As they spinaround each other, incoming air is squeezed between the rotors andpushed under pressure into the engine - forcing more air into the enginethan it could draw under "natural" aspiration. The rotors are driven bya pulley and belt that are connected to the engine's accessory drivesystem.
Because the pressurized air is hotter than naturally aspirated air, theLS9 employs a liquid-to-air charge cooling system to reduce inlet airtemperature after it exits the supercharger - reducing the inlet airtemperature by up to 60 degrees C (140 F). Cooler air is denser andallows the engine to make the most of its high-pressure air charge. Thecharge cooling system includes a dedicated coolant circuit with aremote-mounted pump and reservoir.
The design of the supercharger case and its integrated charge coolingsystem was driven by the space and dimensions afforded under theCorvette's hood. To that end, the charge cooler was designed as a "dualbrick" system, with a pair of low-profile heat exchangers mountedlongitudinally on either side of the supercharger. Coupled with thesupercharger itself, this integrated design mounts to the engine inplace of a conventional intake manifold and is only slightly taller thana non-supercharged 6.2L engine. The air inlet and rotor drive pulley arepositioned at the front of the supercharger.
Additional engine details
Oiling system: The LS9 uses a dry-sump oiling system that is similar indesign to the LS7's system, but features a higher-capacity pump toensure adequate oil pressure at the higher cornering loads the ZR1 iscapable of achieving. An oil-pan mounted oil cooler is integrated, too,along with piston-cooling oil squirters located in the cylinder block.The expanded performance envelope of the Corvette ZR1 required changesto the dry sump system also used in the Z06. System capacity isincreased and scavenge performance improved to meet the demands ofChevrolet's highest-performance sportscar.
Water pump: To compensate for the heavier load generated by thesupercharger drive system, an LS9-specific water pump with increasedbearing capacity is used.
Accessory drive system: In order to package the accessory drive systemin the Corvette's engine compartment, the supercharger drive wasintegrated into the main drive system. This required a wider 11-ribaccessory drive system to be used with the LS9 in order to support theload delivered by the supercharger.
Fuel system: The LS9 uses high-capacity 48-lb./hr. fuel injectors withcenter-feed fuel lines. The center feed system ensures even fuel flowbetween the cylinders with less noise. In order to ensure fuel systemperformance during low speed operation as well as under the extremeperformance requirements of wide open throttle (WOT), a dual pressurefuel system was developed. This system operates at 250 kPa at idle andlow speed, and ratchets up to 600 kPa at higher-speed and WOTconditions.
Throttle body: An 87-mm, single-bore throttle body is used to draw airinto the engine. It is electronically controlled.
Rocker covers / ignition coils: Unique rocker covers with new,direct-mount ignition coils are used. Other Gen IV engines, such as theLS3, feature coil packs mounted to a bracket. The LS9's direct-mountedcoils offer a cleaner appearance and a shorter lead between the coil andspark plug.
Beauty cover: A special engine cover is attached to the top of the LS9.It surrounds the intercooler, which is visible through a hood window,accenting the supercharger while simultaneously reducing noise. Thecover has "LS9 SUPERCHARGED" script on the left and right sides, alongwith a debossed Corvette crossed flags emblem on the front.
New six-speed manual transmission
The Corvette ZR1's LS9 engine is backed by a Tremec TR6060 six-speedmanual transmission, with a twin-disc clutch system. It is based on theproven T56 six-speed, but upgraded to handle the LS9's torque output anddelivers improved shift quality
The twin-disc clutch system provides exceptional clamping power, whilemaintaining an easy clutch effort. The system employs a pair of 260-mmdiscs, which spreads out the engine's torque load over a wider area,enabling tremendous clamping power to channel the LS9's torque. It alsodissipates heat better and extends the clutch life (in normal driving).
Thanks to comparatively small plates, the twin-disc system enablessimilar disc inertia to the Corvette Z06, which has a 290-mm single-discsystem, contributing to low shift effort and providing exceptionallysmooth engagement and light pedal effort. In fact, the more powerful ZR1has a pedal effort similar to the Z06.
ZR1-specific gearing in the transmission provides a steep first-gearratio that helps launch the car. The car's top speed is achieved insixth gear - a change from the fifth-gear top-speed run-outs in themanual-transmission Corvette and Corvette Z06. The gears also aredesigned to balance performance with refinement. The angles of the gearteeth are optimized to provide both strength and quietness.