Of all the production piston engines in GM’s 100-year history, only the Allison aero V-12 that powered World War II fighter planes produced more power than the new LS9 V8 that will power the 2009 Corvette ZR1.
Although GM has yet to certify the 6.2-L LS9 to SAE J1379 standards (the certification is expected next month), GM’s Small-Block V8 Chief Engineer Sam Winegarden is confident the rated power will be “at the 620-hp (462-kW) level and maybe even north of that. We like nice round numbers,” he said with a smile.
The maximum torque target is above 600 lb·ft (813 N·m). Dyno testing has shown that 90% of peak torque is produced from 2600 to 6000 rpm.
Development of the “super small block” kicked off in 2004, as part of the LS3 V8 program (see feature in the January 2008 AEI). With the ZR1 “Blue Devil” Corvette program driving the need for even more power than the 7.0-L LS7 in the current Z06 Corvette, Winegarden’s team concluded that they’d need to achieve a specific output of 100 hp/L.
So supercharging the 6.2-L engine was chosen as the big-power enabler. The key to the LS9’s prodigious output is an Eaton TVS (Twin Vortices Series) Roots-type positive displacement blower. The TVS (which Eaton calls its sixth-generation supercharger) features a pair of four-lobe impellers. Typical Roots-type blowers feature three-lobe impellers, and are deemed to be less efficient than screw-type superchargers.
According to Darren Schumacher, Eaton’s Director of Product Engineering, the benefits of the extra lobe include reduced parasitics, increased overall efficiency, and reduced noise compared with a three-lobe design.
“Input power is less than 0.3 kW at highway cruise,” he noted. “In most North American driving conditions, the unit will be in bypass mode 98% of the time.”
The TVS’s rotors are shaped in a 160º helix. They are coated with a graphite-based abradable power coating (APC) that is designed to steadily abrade away on overlap. This self-machining break-in process helps close the rotor-to-rotor tolerances and minimizes internal leakage, Schumacher said.
According to Winegarden, each full rotation moves 2.3 L of compressed air, the rotors turning slightly more than 15,000 rpm through a 2.3:1 pulley ratio. At an engine speed of 6600 rpm, the TVS produces 10.5-psi (0.72-bar) maxium boost. A clever blower and accessory drive system uses an 11-rib reinforced belt to drive the blower as well as the water and power-steering pumps. Early testing with a toothed belt proved it to be unaccepably noisy, Winegarden said.
Piggybacked on top of the blower is a Behr “twin brick” air-to-water intercooler. Each of its left- and right-side elements serves a single cylinder bank. The intercooler system contains its own coolant and helps lower inlet temperatures by up to 140ºF (78ºC) on the ZR1.
The Eaton blower feeds roto-molded (also known as spin-cast) A356-T6 aluminum cylinder heads with CNC-machined chambers that are unique to the LS9. Winegarden said the T6 alloy was required to handle the greater thermal extremes of the supercharged engine.
The heads’ intake tracts feature what Winegarden calls “swirl wing” geometry to improve the mixture. Heads on the titanium intake valves have a diameter of 2.16 in (55 mm). Exhaust valves, which have a diameter of 1.59 in (40.4 mm), are sodium-filled for improved thermal performance. The LS9’s camshaft is designed with relatively low 0.555-in (14.1-mm) lift for both intake and exhaust valves to help deliver strong low-rpm torque, high-rpm power, and improved idle stability.
The addition of the supercharger meant the LS7’s dual-layer head gasket would not be sufficient to absorb head lift-off, so the LS9 uses a thicker four-layer gasket. Forged pistons and titanium connecting rods ride on a forged-steel crankshaft, with the pistons cooled by oil jets—a first for a GM small-block V8. Compression ratio is 9.9:1, necessitating premium fuel.
The LS9’s fuel system features high-capacity 48-lb/h (22-kg/h) Bosch injectors with center-feed fuel lines that help equalize fuel flow between cylinders with lower noise. Winegarden notes that LS9 is not yet E85-capable due to the injectors’ lack of capacity. The system operates at 250 kPa (36 psi) at idle and low rpm, rising to 600 kPa (87 psi) at higher rpm.
Within the aluminum cylinder block, reinforced bulkheads boost overall stiffness by 20% over the LS3. Massive high-strength-steel bearing caps are used, rather than sintered iron.
Like the LS7, the LS9 uses dry-sump lubrication with increased oil capacity and adds an auxiliary oil tank to handle extreme operating conditions. The system will carry over to the 2009 Z06 and its LS7 engine.