The new C8 Corvette Z06 is powered by a flat-plane 670-hp monster called the LT6. Here's how the Corvette team made it.
Four basic engines in various forms have powered the Chevrolet Corvette across its nearly 70-year history. This is the fifth, and by far the most radical, the LT6. Or, as it's called internally, the Gemini.
Corvette engineers designed this entirely new engine for the C8 Z06, a 5.5-liter naturally aspirated, twin-overhead-cam V-8, notably equipped with a flat-plane crankshaft. SAE-certified power and torque figures ring in at 670 hp at 8400 rpm and 460 lb-ft of torque at 6300 rpm, making this the most powerful naturally aspirated V-8 ever to thunder across a showroom floor.
The easy route would've been to endow the Z06 with a blown small-block, but after some "discussions" with management, approval was given to do something different. In a private preview of the Z06 at GM's design center, Corvette chief engineer Tadge Juechter told Road & Track there were a few reasons he pushed for this V-8. The first is practical—a supercharged small-block wouldn't leave space for the convertible's folding metal top. The second is that he wanted to recapture the spirit of previous Z06s, especially the C6, which was defined by its 7.0-liter naturally aspirated LS7 V-8. (The C7 Z06 had a supercharged small-block, breaking tradition.)
"For a lot of our customers, they pine for the old LS7, the seven-liter that spun to 7000 rpm. They love that," Juechter said. "It was more like a bespoke engine, it had the perfect metering of power, naturally aspirated, it had instant response. And people liked the C7 Z06, it had a ton of power, but they also liked that driving experience. A lot of people said, 'Why can't you do naturally aspirated?' Well, nobody's ever done a 650-horsepower naturally aspirated V-8, it's considered impossible, but there it is."
The bore centers are the same as the small-block, as the engine has to fit within the same space as the Corvette Stingray's LT2, but that's it. This is an entirely new unit and shares no major components with other GM products. The engine breathes through intakes in the car's rear fenders that sit just above heat exchangers for the engine on the left side and transmission on the right. That air is directed to an airbox that sits above the transmission then into twin 87-mm throttle bodies ahead of an intake manifold that looks quite a bit like a Ferrari 458's.
Engineers don't shy away from the comparison. Dustin Gardner, assistant chief engineer for the small-block, told R&T that the stated goal for this new Z06 was to beat the Ferrari 458. It may seem odd to benchmark a 12-year-old car, but the 458 is arguably still the high watermark for Ferrari's mid-engine V-8 cars.
There are four intake trumpets in each plenum, one per cylinder, with valves to connect each plenum. These tuning valves open and close depending on engine speed to help accelerate the air in the intake. This gives the engine tractability at low rpm and enough air to breathe as you get closer to its 8600-rpm redline.
Beyond simply being the first four-valve Corvette since the C4 ZR-1, the valvetrain has a number of novel features. Intake valves are made from titanium and exhaust valves are sodium filled, and notably, their dual-coil springs—used to reduce vibrations—are actuated by roller finger followers rather than traditional bucket-and-shim tappets. For a more thorough explanation of finger followers, read Kevin Cameron's column over at Cycle World, but know that this system is much lighter than a traditional tappet system, much more durable, and reduces the potential for valve float. They're typical in sport bikes, but the only other road-car engines to use finger followers are Porsche's 4.0-liter 911 GT3 flat-six and the Ferrari 812 Competizione's 6.5-liter V-12. A robot custom-selects shims for each engine, and as the valvetrain is totally rigid, it's set for life. No adjustment necessary. Gardner also told us that this valvetrain is theoretically good for beyond 8600 rpm.
Juechter jokingly calls the LT6 "the big-block of flat-plane crank V-8s," as 5.5-liters is huge for a flat-plane V-8. The old Shelby GT350's Voodoo V-8 measured in at 5.2 liters—and revved to 8250 rpm—and Ferrari never went bigger than 4.5 liters with its V-8s. "You go to 104.25-mm bore, you're getting up into LS7 bore territory. That's about as big as you can go," Gardner said. "And at the engine speeds and everything we're at, 80 mm was just the sweet spot for stroke. And as we process that through balance architecture, that was giving us predictions of where we would end up that we were happy with."
"We were trying to go, frankly, as big as we could because flat-plane crank naturally aspirated engines, they like to spin up. But we didn't want it to be anemic driving around town," Gardner added. "So we were trying to get as much displacement in there as we could, still enabling the top end. And so you still had some decent responsiveness down low, where you spend 90 percent of the time on the road."
Of course, getting relatively large pistons to spin to such high speeds meant that much attention had to be paid to keeping weight down. Gardner describes the forged aluminum pistons as "a work of art," and naturally, there are forged titanium rods, too.
The big problem with any flat-plane V-8—and the reason that the more traditional cross-plane V-8 was created—is vibration. And that posed a big challenge to the Corvette team. "The shaking forces on this engine are crazy," Juechter said. "It's the largest displacement flat-plane crank engine that's ever been done. And the bigger the engine, the more the shaking forces, then integrating that into the rest of the vehicle, it's quite an engineering challenge. I think everybody who's done a flat-plane crank talks about it. The engine shakes everything around it apart."
Juechter even said that early prototypes were so violent, oil filters were spinning off during testing. "It was like there were ghosts in the machine," he recalled. Thankfully, the team figured everything out and took a number of measures, including strengthening the gearbox casing, to deal with vibrations.
The Corvette team is also quite proud of the oiling system in the LT6. As with the C8 Stingray's LT2, the LT6 is dry-sumped, but with a slightly larger 8-quart tank and six-stage scavenge pump. Four of those stages pull from the crankcase, which helps reduce parasitic losses from having too much oil. "Think about running in water versus running on land," Juechter said. "Which way are you faster?" The other two scavenge stages pull from the heads and the front of the engine, and Gardner says the system is so effective that at any given time, 80 percent of the LT6's oil is in the sump.
As a flat-plane crank is, essentially, two four-cylinders sharing a common crank, the exhaust sound is quite unique. Juechter said that the team spent around a year developing the aural signature of the LT6. "We'd originally planned to carry over or just retune the Stingray exhaust," he said. "But with the flat-plane crank engine, the results were okay, but they weren't as good as we were hoping." Part of the sound comes from a four-into-two-into-one exhaust manifold, but much attention was paid to the exhaust tips themselves. The Corvette's signature central exhaust returns, but for the first time, the chrome tips you see at the rear of the car aren't directly attached to the exhaust pipes. Juechter is at pains to point out they're not "fake," though.
They're shaped like trumpets, with the bells facing the front of the car. "What that surface does is it actually reflects some of the exhaust sound forward," Juechter said. "So the sound comes out of the pipe, expands, bounces off that little parabolic surface, and reflects forward so the driver can hear it. Because these tips are so far rearward, and we've got a lot of insulation between the engine and the driver to get the bad noise out, we wanted to hear more of that music, and so that was actually a technical solution to do that." From what we've heard thus far, the results speak for themselves, recalling the glorious sounds of Ferraris like the F355 and 458.
Automakers love to talk about the link between motorsports and road cars, but the link is often tenuous. With the LT6, it's not, as the Corvette C8.R has run a version of this engine for two years now, giving the road-car team valuable data. Since the C8.R is restricted by balance of performance, which limits horsepower to around 500, the racing LT6.R uses a slightly smaller crankshaft and has a redline set around 7000. The oiling system is very similar to the road car's, though, and so is the finger follower valvetrain. Really, the biggest difference between the LT6 and the LT6.R is the block. In the race car, the engine is mounted lower, but the top ends of the two engines remain very similar.
Juechter said that the C8.R almost ended up with the 5.5-liter small-block that powered the C6.R and C7.R to many victories, but for the sake of road-car development, it was worth going for the LT6. Even though it let the world know that the Corvette would get a flat-plane V-8 two years before the Z06 debuted. But this should mean that the LT6 has the reliability and durability we've come to expect from Corvette engines, despite being so radically different from a conventional small-block.
The LT6 comes at an interesting time for the internal-combustion engine, as the industry shifts toward electrification. GM is no exception, with plans to invest $35 billion between 2020 and 2025 into EV development and production. We asked Juechter if the driving force behind the LT6 was the fact that this was the last chance to develop something like this.
"It's more in hindsight," he said. "I mean, the world has changed so much since we started on this. We didn't think it would be the end of the line, but now the way that everything is going to charged engines, EVs, and everything, I suspect now, looking at where we are right now, it probably will be."
So what we're looking at here is, possibly, the most powerful naturally aspirated V-8 to ever exist. If the LT6 is the end, it’s one hell of a way to go.
text by Chris Perkins
photos by Andi Hedrick
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