While General Motors often comes under fire for comically rigid decision making, confusing corporate silos that often don’t talk to one another, their silly logo, or the constant re-invention of the Cadillac brand, there is one area where absolutely no one else can hold a candle to the General: engine building. And if you’ve been paying attention for the past few weeks you’ll know what we’re on about. Sure, the 10.4L 1,004 horsepower pushrod big-block the company showed off during SEMA 2021 is pure cake. It’s the 5.5L DOHC LT6 V8 equipped with a flat-plane crank designed and built for the 2023 C8 Corvette Z06 that really shows off how talented GM’s powertrain department really is.
Everyone want’s to go on and on and on about those horizontal oil eaters from Stuttgart, especially in 4.0L form, or the spezial M159 V8s from Mercedes-AMG, or any of the high-pitched Italian screamers from Ferrari. Never mind any of those, because it’s the Corvette Z06’s 670 hp LT6 that wears the crown as the most powerful naturally aspirated (V8) motor ever built by an OEM for road car use.
That doesn’t happen by accident. Luckily the crew from Hot Rod was able to illuminate some of the technical revolutions that went into building the LT6.
It all starts with airflow. The thing about a flat-plane crank V8 is that in reality, they’re just two inline fours sharing the same rotating assembly, meaning in order to optimize the engine design two separate intake plenums and throttle bodies should be employed. Unlike Ford’s 5.2L Voodoo engine built for the GT350 and GT350R, the C8 Z06’s 5.5L LT6 V8 employs two huge 87mm TBs which fill individual plenums.
In total, the plenums can ingest 11 liters of air, which is then fed to each cylinder by individual runners angled in different directions housed within the plenums. In order to facilitate the intake’s magic volume, the fuel injectors were actually relocated to the side of the cylinder head, beneath the exhaust valves. It’s a trick learned from Chevy’s IndyCar engine program.
All that air is then put to work creating a resonant supercharging effect within the intake manifold. There’s a series of valves connecting the two intake plenums that open in different combinations to vary the degree to which pressure waves from the intake valves closing communicate within or between the LT6’s plenums. This helps cram extra air down into each cylinder and allows the naturally aspirated LT6 to offer a volumetric efficiency greater than 100 percent, which is actually unheard of for a road-legal production engine.
A major factor in the LT6’s impressive horsepower figure is the high-revving nature of the rotating assembly. The engine is an oversquare design with a 104 mm bore and an 80 mm stroke, which actually reduces peak piston speed at high-rpm operation. Peak piston speed is the source of those nasty secondary vibrations flat-plane crank engines suffer from.
The piston heads themselves are forged aluminum, while the connecting rods are forged titanium. The intake valves are made from titanium and the exhaust valves are sodium-filled. The lifters are selected by high-precision robotic measurements and will never, ever, require a lash adjustment. The valves are snapped shut by dual-coil springs.
But the absolute best part about the Corvette Z06’s LT6 engine is that it’s expected to have a significant shelf life. The engine was developed to meet every current and forecasted emissions regulation. Pretty impressive considering the LT6 started life as a pure-bred racing engine in the C8.R before being adopted for road work.