To the Moon and Track

In the age of turbo- and electric-boosted four-bangers, General Motors chose to go bold when it came time to power the 2023 Corvette Z06. So big, in fact, that development of the Z06’s 5.5-liter LT6 engine was nicknamed Project Gemini, a nod (in part) to NASA’s early manned spaceflight program in the 1960s. The result? The world’s most powerful naturally aspirated production V8, producing an eye-popping 670 horsepower at 8400 rpm. Plus, a set of massive twin-throttle bodies, a variable tuned intake manifold, and a glorious quad exhaust.

This rocket launch is the endgame of a seven-year passion project for a Corvette development team that includes Corvette executive chief engineer Tadge Juechter, global powertrain/engine chief engineer Jordan Lee, and Corvette vehicle performance manager Alexander MacDonald. We sat down with all three as they discussed the challenges that came with building their dream Corvette.

Jordan Lee: We built our first prototypes in September 2015. We wanted an engine that was true to the Corvette DNA and yet unlike any conventional Small Block engine that we’ve done in the past. So: Double overhead cams, four valves per cylinder, unique cylinder block, and a sophisticated dry sump lube system. Small Block history is “near and dear” to Corvette, so we kept our historic 4.4-inch bore center dimension.

We were looking for a name that would stick with us. We really liked that astronauts in the late ’60s and ’70s drove Corvettes; they got a pretty lucrative deal I think to lease the car for a dollar a year. We came up with Project Gemini. Gemini is Latin for twins, and the engine carries that theme with two intake plenums, two throttle bodies, two camshafts per cylinder head, etc. The fact that this was our moonshot — essentially, the most complicated and sophisticated engine we’ve ever come up with — made Gemini the perfect name for us.

Tadge Juechter: Historically, the whole engine side of the business kind of operates at arm’s length from the rest of the company. Requirements that are confidently achievable are written down, and the engineers set off to work on them. At Corvette, we are much closer than that. We’re friends outside of work, we go to Corvette events together — it’s almost like an extended family. Our mantra is to make decisions together that are the best decisions for the customer and stretch ourselves to push the limits of what is possible.

Alexander MacDonald: I first joined GM around the time the [505-horsepower, 7.0-liter] LS7 was part of Z06, back when we launched the C6. I think everybody recognizes the LS7-powered Z06 as a very successful naturally aspirated raw track car. Then the C7 moved toward something more powerful with the addition of the supercharged LT4. But for the C8 Z06, I think all of us immediately recognized this as another opportunity to build an amazing car around this super-special naturally aspirated engine.

Juechter: However, we were also taking on a massive risk. Historically, new Corvettes outperform old ones, and we typically see a horsepower bump as part of that. We knew when we started down this path that we might not exceed the horsepower of the C7 Z06, which was 650 horsepower. We all had to swallow hard and admit laws of physics might not let us exceed that with a naturally aspirated V8. The C7 Z06 was a great car, but the front-engine layout limited the amount of power we could get to the ground. You could do all-wheel drive, except that doesn’t work well with our front-engine layout; there’s no place for driveshafts. It would’ve been a horribly heavy car. We had to do something.   

Jordan Lee, General Motors’ global powertrain/engine chief engineer, has driven plenty of performance vehicles. The Z06 hit different. “When I finally drove a prototype on our test track for the first time, I was floored with how incredible it was.”

That “something” was the long-rumored move to a mid-engine configuration when the eighth-generation Corvette, the C8, arrived in 2020 featuring the 495-horsepower, 6.2-liter Small Block LT2 V8. Moving the engine behind the cabin put more weight on the drive wheels and helped unlock the Z06’s future potential.

Lee: The mid-engine layout gave us more engine compartment room, so this allowed us to use a DOHC architecture and not be too limited with the size of the engine. So DOHCs and a large intake manifold were now possible. We also went with a flat-plane crankshaft design to maximize volumetric efficiency so we could make the most power possible. The only downside of a flat-plane crankshaft is the shaking forces can be quite high, not unlike a paint shaker. We often affectionately referred to the LT6 as a paint shaker.

Juechter: From the beginning, we designed the C8 to minimize vibration from the engine. We knew the LT6 would have a lot of shaking forces, and we wanted to make sure the vibration was not noticeable to the driver and the passenger. This required meticulous engineering of the mount system and body structure — from the way the powertrain mounts attach to the structure and that connection all the way to the seat track and steering wheel.

Lee: As we began our engine development effort and started running power tests, the results were spectacular. We were hoping we’d hit 625 horsepower, and it was clear early on we were going to exceed that by a wide margin. Everything was going full bore until Covid hit. Luckily, we were able to keep working relentlessly from home, and we were all quite amazed at how well our engineering teams collaborated.

Juechter: Covid also gave us time to re-architect the exhaust.

Tadge Juechter, Corvette executive chief engineer, says that new Corvettes typically outperform their predecessor in terms of horsepower. After admitting the laws of physics might not make that possible this time, they went and made it possible.

Lee: We wanted the Z06 to tantalize all of your senses — what you see, hear, and feel. The LT6 makes really glorious sounds, and we wanted to be sure the people in the car could hear the symphony.

Juechter: Full credit to Alex’s team for developing an amazing exhaust system that mounts these quad-reverse megaphones under the lower fascia. It not only created the noise that we thought was best from the outside, but it sent it forward into the car.

MacDonald: The difficulty of making a great sound in a V8 or muscle car is in the way you do an X-pipe over the two banks of cylinders. The center exit actually has that happen outside the car. So that crossover between the cylinders is happening in the air, after the exhaust system, but it’s super important to what you hear. You couldn’t fit a traditional X-pipe with hardware in that space because it’s so small. So the novelty of it wasn’t the appearance of a center-exit exhaust, but the sound quality we get from the mixing happening out there, in the turbulent air behind the car — which is pretty wild for me to think about.

Midway through development, the Gemini team rolled their unfinished Z06 to the track to put it through its paces. The engine performance exceeded expectations, but the rest of the car seemed off. Ultimately, the team concluded that the car’s other systems — like the transmission mapping and throttle response — needed to catch up.

Juechter: Once we set that mission, the rest of the car came together.

Lee: You never really know how great the engine and car are until you drive it (laughs). When I finally drove a prototype on our test track for the first time, I was floored with how incredible it was. It was super-fast, super-exciting. A perfect Z06. But Alex is the expert — he has racing experience and is an expert driver. I asked him what he thought. When Alex says a car is great, I believe him. He confirmed we had something special.

Corvette vehicle performance manager Alexander MacDonald and his team took on the challenge of bringing the raucous sound of the Z06’s engine, now positioned behind the driver, into the cockpit.

MacDonald: My team of development engineers and I are pretty lap-time focused. We sort of had our benchmark of where we thought the C8 Z06 was going to land in the C7 lineup, from the ZR1 on down, and we were pretty certain our new C8 Z06 would be faster than the C7 ZR1. It was simply that good. At our first development track event, the C8 Z06 was running lap times faster than the 2019 ZR1, and we were shocked. That ZR1 has its places where 750 horsepower is hard to argue with. Generally, the new Z06 punches above its weight class.

Juechter: Most of the performance car industry is migrating to turbocharging. We went to supercharging for the C7, and part of that was for the rapid torque response with a supercharged engine. But people love the responsiveness and sound of a high-performance naturally aspirated engine. It’s an emotional experience. The scream of the engine, the linear response to your foot pressing the gas pedal — it’s a joyous part of driving a high-performance sports car, a track car. It’s what the Z06 was meant to be.

As GM and other manufacturers navigate toward an all-electric future, it is a boon for customers to have amazing choices in both types of propulsion systems. In the meantime, the Z06 is a glorious celebration of a charismatic high-performance internal combustion engine. The Z06 is a milestone for Corvette and General Motors. Looking back 100 years from now, if people buy one of these and keep it, they’ll be really, really happy.

STORY: ANDREW LAWRENCE / PHOTOGRAPHY: JENNY RISHER