Race Design Is Production Design For LS7 Piston And Rod

June 23, 2006 - GM.com

Corvette Z06 Piston Assembly Highlights Race-To-Production Engineering

PONTIAC, Mich. – The piston assembly for the 2006 Corvette Z06 LS7 7.0-liter V-8 engine is one of the latest examples of race designs carrying over to production GM engines. Almost every aspect of the short skirt, high strength pistons and titanium connecting rods used in the 505 horsepower power plant is developed through race design know-how.

Thanks to racing experience GM engineers have brought one of the most advanced design piston and rod assemblies to a production engine. “Besides light-weight, the LS7 piston assembly has an abundance of features built-in through race experience,” said Roger Duguay, chief engineer for GM small block engines. “Full floating pistons with valve reliefs and lightening pockets, plasma moly rings, tapered wall pins, high strength chromium nitrided titanium rods just begin to highlight the racing DNA of this assembly.”

The full-floating piston is made of eutectic aluminum alloy and is CNC machined for balance and tolerance consistency. For durability the top land is anodized. Between the first and second ring lands is a pressure balance groove, another racing carry-over, to control blow-by and oil consumption at high speed. Piston tops are flat with symmetrical valve reliefs providing opportunities for a stroker crank or higher lift. A polymer coating on the skirts to reduce scuffing creates an almost negative interference fit in the bore.

The ring pack uses thin rings for high speed sealing control. A 1.2 mm top compression ring is used with a plasma moly face for superior sealing and durability. The second compression ring is also 1.2 mm thick and is made of ductile or high strength iron. The three-piece oil control ring is 2.0 mm thick and is made of nitrided steel for superior wear control and spring force longevity. Nitriding also enables lower ring tension resulting in more power due to reduced friction.

Little was spared when it came to developing the connecting rods. The rod is made of a titanium aerospace alloy and uses twelve-point doweled cap screws. The rods are heat treated and have a chromium nitride coating for surface durability. The coating is applied to the thrust facing areas via plasma vapor deposition (PVD) – a process taking 12 hours to apply. Each rod has three weight pads for precise balance. Blind holes are used, to eliminate stress areas, in conjunction with a heat treated style cap screw with a neck down shank for consistent clamp and controlled stretch. The LS7 rod weighs just 464 grams which is almost 30 percent lighter than the similar forged powder metal rod in the base engine Corvette.

Racing has always been on the forefront of reducing component weight. “Racing knowledge transfer gave us the basic design elements for the LS7 piston assembly,” said Tom Halka, development engineer for GM small block engines. “Thanks to GM’s vast technical resources and racing background we now have a light, high speed assembly that balances reliability and durability with performance. What we learned with this assembly we will apply to other GM engine programs.”