Chevrolet has recently introduced the 2006 Corvette Z06: it is the fastest, most powerful car ever offered by Chevrolet and General Motors. Optionally equipped with the all-new LS7 engine, the ’06 Z06 reintroduces the 427-cubic-inch displacement to the Corvette lineup. Unlike the previous 427 engine, which was a big-block design, the new 7.0-litre LS7 is a small-block V8 – the largest-displacement small-block ever produced by GM.
With 500 horsepower and 475 ft-lb of torque, it also is the most powerful passenger car engine ever produced by Chevrolet and GM. The LS7 is easily identified under the hood by red engine covers with black lettering.
And still With Pushrods!
With the LS7, GM engineers have achieved a feat many speculated was not possible. The new OHV (overhead valve) V8 will not only achieve 500 horsepower but will be capable of running up to 7100 rpm. The previous limit was 6600 rpm in the 6.0-litre LS2.
With this new engine GM is showing multi-valve overhead cam performance is achievable with a two-valve cam-in-block engine. For the new ZO6, higher rpm allows the driver to remain in first gear to just over 60 mph, contributes to higher top speed and improves overall vehicle performance. The LS7 is one of the first automotive OHV production engines in the industry capable of over 7000 rpm.
The new ZO6's sub four-second 0-60 mph time is achieved in part by the extended rpm range and the ability to remain in first gear past 60 mph. The over-190 mph top speed of the ZO6 is partially due to the speed capability of the engine. As aerodynamic drag becomes a factor at higher speeds, the capability to run higher rpm allows the transmission to be run in a lower gear generating more effective torque at the rear wheels. Also, having the ability to shift all gears at higher speeds improves elapsed times whether on a road course or drag strip.
"For a production engine to run at this high of an rpm blurs the lines even more between OHV and OHC (overhead cam) design," said Dave Muscaro, assistant chief engineer for small block engines. "We took a complete systems approach to achieve the high rpm. We have a tight valvetrain design along with some race-inspired materials for the reciprocating components like titanium intake valves and connecting rods."
OHV engines use pushrods to activate the valves via rocker arms, whereas with OHC engines the valves are typically actuated directly via finger followers. The extra mechanical movement and weight of the components of an OHV valvetrain present challenges to higher rpm. The LS7 design and use of lightweight, stiff components, along with GM's economy of scale, make higher rpm obtainable in a production OHV engine.
"The new LS7 cylinder head gave us the opportunity to design a new high-revving valvetrain," said Jim Hicks, LS7 valvetrain design engineer. "The new valvetrain had to be as stiff and light as possible to assist meeting the engine's aggressive performance targets without compromising idle quality and low emissions. Stiffness is increased with larger diameter pushrods and rocker arms optimized through extensive finite element analysis."
The LS7's 1.8:1 rocker arm ratio and titanium intake valves contribute to a lower effective mass (compared to the LS2 base Corvette engine) in spite of larger and stiffer valvetrain hardware. Idle quality and emissions performance is achieved with the help of a more aggressive cam that provides more lift and duration while still keeping overlap area to a minimum.
"We consulted with our Motorsports group on numerous design aspects of the cylinder head design, said Hicks. "We adopted some of the latest ideas that have been successful in the Nextel Cup and the American Le Mans Series including valve centreline positions, valve angles, valve sizes and rocker arm ratio."
Due to the relatively large bore and stroke (104.8 x 101.6mm) of the LS7, light weight pistons, piston pins, titanium rods and a steel crankshaft are also used to achieve extended rpm. The flat top pistons with race-ready valve reliefs are 482 grams each and use a shortened lightweight piston pin. Each forged titanium rod weighs only 464 grams which is almost 30 percent less than each rod in the LS2. The forged steel crankshaft is stronger, stiffer and designed to handle high-speed loads.
The LS7 shares the same basic Gen IV V8 architecture as the Corvette’s 6.0-litre LS2, but the LS7 uses a different cylinder block casting with pressed-in steel cylinder liners to accommodate the engine’s wide, 104.8-mm-wide cylinder bores; the LS2 has 101.6-mm bores. And when compared to the LS2, the LS7 also has a different front cover, oil pan, exhaust manifolds and cylinder heads.
The LS7’s specifications include:
The LS7’s CNC-ported aluminium cylinder heads are all-new and designed to meet the high airflow demands of the engine’s 7.0-litre displacement, as it ingests approximately 100 cubic feet more air per minute than the Corvette’s 6.0-litre LS2 V8 – an 18 percent increase in airflow. Consequently, a hydraulic roller camshaft with .591/.591-inch valve lift is used to allow plenty of air to circulate in and out of the engine.
To ensure optimal, uninterrupted airflow, the LS7’s heads have straight, tunnel-like intake runners. Very large by production-vehicle standards – even racing standards – they are designed to maintain fast airflow velocity, providing excellent torque at low rpm and exhilarating horsepower at high rpm. The heads feature 70cc combustion chambers which are fed by huge, 56mm-diameter titanium intake valves. The lightweight titanium valves weigh 21grams less than the stainless steel valves used in the LS2, despite the valve head having 22 percent more area.
They are complemented by 41mm sodium-filled exhaust valves, vs. 39.4mm valves in the LS2. To accommodate the large valve face diameters, the heads’ valve seats are siamesed; and, taken from experience with the engines of C5-R racecars, the LS7’s valve angles are held at 12 degrees – versus 15 degrees for the LS2 – to enhance airflow through the ports.
All LS7 engines are assembled by hand at GM Powertrain’s new Performance Build Center in Wixom , Michigan . The exacting standards to which they are built include deck-plate honing of the cylinders – a procedure normally associated with the building of racing engines and almost unheard of in a production-vehicle engine.
Dry sump oiling system
The LS7 has a dry-sump oiling system designed to keep the engine fully lubricated during the high cornering loads the Corvette Z06 is capable of producing. An engine compartment-mounted 8-quart reservoir delivers oil at a constant pressure to a conventional-style oil pump pick-up at the bottom of the engine. The pressurized oil feed keeps the oil pick-up continually immersed in oil at cornering loads exceeding 1g.
Oil circulates through the engine and down to the oil pan, where it is sent back to the reservoir via a scavenge pump. The large-capacity reservoir, combined with a high efficiency air-to-oil cooler, provides necessary engine oil cooling under the demands of the engine’s power output. With the dry-sump system, oil is added to the engine via the reservoir tank – which includes the oil level dipstick.
The LS7’s dry-sump system was developed and tested on racetracks in the United States and Europe , including Germany ’s famed Nürburgring. And while common in racing cars, the Corvette Z06 is one of just a handful of production vehicles – and the only production Corvette – to ever incorporate such a high-performance oiling system.
The Corvette Z06’s powertrain and drivetrain systems are matched to the LS7’s performance capability. The light, four-into-one headers discharge in to new, close-coupled catalytic converters and through to new “bi-modal” mufflers. The mufflers each feature a vacuum-actuated outlet valve, which controls exhaust noise during low-load operation but opens for maximum power.
At the rear of the LS7 engine, a single-mass flywheel and lightweight, high-capacity clutch channel torque to the rear transaxle. The six-speed manual transmission has been strengthened to handle the LS7’s increased torque load. The transmission includes a pump which sends transmission fluid to the front radiator for cooling. Upon its return, the fluid removes additional heat from the differential lube before returning to the transmission.
The M6 transmission connects to a limited-slip differential, with enlarged ring and pinion gears. Stronger axle half-shafts with tougher universal joints transmit power to the rear wheels.