It's not too often a genuinely new, genuinely exciting high-performance engine gets released for a production car, so when something like the C8 Corvette Z06's LT6 drops, it's worth going over in painstaking detail. The 5.5-liter flat-plane crank V8 gracing the engine bay of the most track-focused Corvette produces 670 horsepower without any help from forced induction. We've done our fair share of reading and research to find out how, but we also spoke with Josh Holder, a Corvette chief engineer who explained the ins and outs of this mechanical masterpiece.
To start, it's worth giving some context. This new naturally aspirated V8 makes more horsepower than the previous Z06's supercharged lump, which also had more displacement at 6.2 liters. How? It's all about airflow, and airflow is determined by an engine's cylinder head, along with everything attached to it. The blown LT4 used in the old Z06 was a classic pushrod design with a cross-plane crankshaft. Flow in a pushrod design is limited by the number of valves in the head (essentially always two in modern gasoline applications) and the pushrods themselves protruding into the path of the intake and exhaust ports. Pushrod head design has come a hell of a long way since the 21st century began—there's all sorts of trickery you can do to get pushrods out of the way of the ports—but a dual-overhead-cam layout is always going to have the capability to flow more air.
That's why the LT6 sports a DOHC layout. In that respect, as well as plenty of others, it followed the lead of the C8.R race car's engine. According to Holder, this road car power plant shares a ton of parts with what you'll find in the Le Mans competitor. "The cylinder heads are, with the exception of a couple of pieces of hardware, basically common," he said, in conversation with The Drive. "Flip the head upside down and look at... the combustion chamber and the valves and the ports, they're all CNC machined. They look like they're right off of a race car."
The heads' specs are impressive on their own. Flow numbers for the ports were not provided nor were the port volumes; however, the valves themselves are very large thanks to a 4.104-inch bore. For reference, the largest production small-block Chevy, the 7.0-liter LS7, has a 4.125-inch bore. The head's 58.8cc combustion chamber allows for a 12.5:1 compression ratio as well—premium fuel will be necessary here.
The intake valves' diameter is 1.654 inches (42mm), and the exhaust valves' diameter measures 1.378 inches (35mm). Valve lift numbers could not be confirmed, although they're definitely high considering the engine makes peak power at 8,400 rpm and peak torque at 6,300 rpm. For reference, the E46 BMW M3's high-revving, 3.2-liter straight-six, the S54, makes peak torque at 5,000 rpm and peak power at 7,900 rpm. This LT6 is likely cammed even hotter than that on account of the higher peak torque rpm, and it's got greater volumetric efficiency as well; 121.8 horsepower per liter as opposed to the M3's 104.1.
The LT6 does not have a variable valve lift system, although its hollow camshafts are equipped with variable valve timing. The cams are phased by an electro-hydraulic system that allows for around 55 degrees of authority on the intake cam and about 25 degrees on the exhaust cam, according to Holder. This amount of cam phasing ability should help the engine be more drivable lower in the rev range.
The cams themselves press down on finger followers, which in turn act on the titanium intake valves and sodium-filled exhaust valves. The valves are controlled at high rpm by twin valve springs, a smaller spring being nestled inside a larger outer. The LT6 has solid lifters—the lifter being the part between the cam follower and the valve itself—which typically require adjustment over time. Chevy says you won't have to adjust the lifters on the LT6, though. That's because it's created automated tooling that ensures a very precise fit between the finger follower and the lifter itself. On top of that, the automaker has positioned an oil jet that flows through the finger follower and sprays directly at the area where the cam surface and the finger follower meet. This, Chevy says, will help maintain extremely consistent tolerances over the life of the engine. Only time will tell whether no adjustment is actually necessary, but Chevy, having likely tested a very similar setup in the C8.R, seems confident.
"The engines are hand-built in Bowling Green and as part of that [precise lifter fit] process... the cylinder head and cam caps are fully assembled, measured for running clearance, disassembled, and that running clearance measurement goes into a calculation that a robot pre-selects shims to put back in the finger follower for the net fit." Holder said, detailing the affair. "So we're taking all build variation out, reassemble the head measure it again, make sure it's good, passes that station. And then when the head's bolted on the engine, we measure it one more time."
Without going too deep into the dry-sump lubrication system, which I know a ton of people definitely want to hear about, it's rather interesting. The LT6 uses 5W50, which is pumped through the engine extremely efficiently. There are four points in the crankcase and two points located in the heads where oil is scavenged back to its remote sump. Speaking to Road & Track, the automaker says that, most of the time, as much as 80 percent of the oil remains in the sump. The oil gets pumped through the engine, does its lubricating work, and then gets sent back into the sump very quickly. If you want to make more power, you can't have oil getting in the way, and the LT6's lubrication system keeps the oil out of the path of fast-moving components like the crankshaft.
Each crankpin also gets its own compartmentalized region of the crankcase, which makes scavenging the oil that gets thrown off the crankpins—the journal where the forged aluminum piston is attached—more efficient by reducing windage. These regions are where the scavenge points are located. Each piston also gets its own oil squirter underneath to help cool the combustion chamber.
A vital part of keeping this oil system working is filtration, of course, and the LT6 has a bolt-on cartridge-style filter. That's not because the engineering team simply preferred it that way, but because of the vibrations associated with a flat-plane crank. "When we first were building our very earliest engines on a dyno, we saw an engine spin a cartridge filter right off [due to vibration]," Holder said. "We now have this cartridge filter, [which] doesn't spin on. It's bolted on."
That pesky vibration was an obstacle for the team, not unlike the issues faced by the Ford Mustang Shelby GT350's 5.2-liter Voodoo V8. Big flat-plane V8s shake like crazy, and Chevy addressed this issue not only with an oil filter that won't screw itself off but also with other smart features like, most importantly, a crank-mounted fluid damper. There aren't any balance shafts or anything going on here, to be clear. The damper works alongside a series of lighter rotating assembly components to attempt to cut back on resonance. Smaller components like electrical connectors also had to be reconfigured to make sure they didn't work themselves off through the course of the engine's life, too. All of these features, Holder says, were proven on the racetrack.
The crankshaft itself is forged steel and gets bolted down to the block with four bolts per main cap. It defines the engine's 3.150-inch (80mm) stroke, which is rather short in comparison to the engine's aforementioned 4.104-inch bore. This is typical for high-revving engines, and indeed the LT6 has one of the shortest bore/stroke ratios of any production power unit. Engineering Explained thankfully did the research and laid out a few other car's bore/stroke ratios for comparison so we don't have to. The LT6's bore/stroke ratio is 1.3:1; compare that to the current Porsche 911 GT3's 1.25:1, the Ferrari 458's 1.16:1, and the regular C8's 1.12:1. For an extreme reference, an F1 car's bore/stroke ratio is 1.51:1. The higher the ratio is, the higher the theoretical "revability" is. This revability is made sustainable by forged titanium connecting rods. The LT6's entire bottom-end is forged including the pistons and crank, too.
Other intake and exhaust tricks like three throttle valves between the two primary intake plenums help boost torque, power, and drivability as well. These valves affect the intake resonance and intake plenum volume. They're programmed to open and close throughout the rpm range in order to allow maximum torque to stick around longer. How this actually makes an engine create more or less output is a rabbit hole, but it's briefly explained in the embedded E/E video.
The exhaust system's tricks include reverse-trumpet shapes, which were detailed in the car's reveal, helping boost exhaust noise in the cabin. The exhaust system itself is a 4-2-1 setup, with the four exhaust pipes combining into two, and then those pipes merging into one. This setup, working with the engine's flat-plane layout, allows for great exhaust scavenging, which is vital at high rpm. Exhaust scavenging, if you're unsure of how it works, is the process by which an exhaust pulse traveling down the exhaust pipe helps draw other pulses down the tube with it. When you have a high-rpm engine, increasing exhaust flow is vital. Efficient exhaust scavenging enables increased exhaust flow.
All of these features add up to the LT6's incredible output. What's even more fascinating is the fact that the car this engine lives in will likely be priced below $100,000, and it shares real parts with the C8.R racer. "It's not just a marketing slogan. It is the truth," Holder told us. "[We] literally share parts with the race team. We may be short on parts to update some of our development vehicles, we'll call them. They'll bring them over and vice versa. We go to meetings together."
That joint development has allowed for the most powerful naturally aspirated V8 ever fitted to a production car to be, in relative terms, a bargain. Impressive engines in the likes of multi-million-dollar hypercars are expected, but they'll be enjoyed by a limited number of people. It's not often that enthusiasts can dream of owning something so special without also having to scheme their way to becoming a millionaire. And speaking of normal enthusiasts, Holder thinks there's still plenty of them who want machines like the one he helped create.
"[The LT6 is] a pinnacle of many, many years, like standing on the shoulders of giants," he told us. "That's built into this engine and that's kind of a way to celebrate, not just the internal combustion engine, but the pure joy of driving the sounds it makes, the way it makes you feel connected to the machine."
"There's still a lot of people that pine for that experience," he concluded.
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