Chevrolet’s Corvette LT6 engine is a masterclass in natural aspiration

This is a rare moment. The Tech Talk section of the magazine is where we get to geek out on the latest and greatest advancements in the automotive world – diving deep on cutting-edge batteries, brilliant new materials, or innovative manufacturing techniques. However, unfortunately that often means the trusty internal combustion engine – motoring’s century-plus year old warhorse – goes unloved unless there’s a new electrically-assisted turbocharger, or lab-created fuel.

Not this month. Thanks to Chevrolet we are breaching new ground with good old fashioned natural aspiration. That’s right, this is the art of making more power than ever before with atmospheric air pressure, regular 98 RON fuel, and eight combustion chambers. The subject is the 5.5-litre dry-sumped flat-plane crank double overhead cam, four valve per cylinder, atmo V8 nestled amidships in the newly revealed C8 Corvette Z06.

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It is the most powerful naturally aspirated V8 ever put into production, with the headline figures being 499kW @ 8400rpm, and 623Nm @ 6300rpm. Codenamed LT6, this engine confidently usurps the previous record holder – Mercedes-AMG’s 464kW, 6.2-litre M159 from the SLS Black Series. Making the LT6’s power figures all the more impressive, is the fact that they outstrip the LT4 from the C7-generation C06. Despite being a larger capacity (6.2-litre), and using forced induction, the LT4 only managed to send 485kW @ 6400rpm to the rear wheels. To be fair to the supercharged engine, it also unleashed 881Nm on the rear tyres from just 3600rpm.

Key to the LT6’s power advancement is clever air management, with the dry-sumped unit capable of inhaling more air than its 5.5-litre swept capacity would first have you thinking. Engine power is inherently linked to how much oxygen can be combined with fuel in the combustion chambers. This is where the LT6 stands tall, huffing down O2 with a nearly unrivalled ferocity from non-forced induction engines.

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Feeding the engine is an 11-litre intake manifold, fed by two 87mm throttle bodies, with four trumpets mirrored on each cylinder bank. For context, the previous C7 Z06 had just a single 87mm throttle body. The really clever design though is the three central valves that separate the two intake plenums. Chevrolet engineers open and close the trio of valves in a variety of combinations across the rev range to take advantage of the Helmholtz resonance effect – what you would know as the heavy buffeting when lowering a single window in a car at highway speeds.

When an inlet valve in a combustion chamber closes, it causes a pressure wave to travel back up the inlet tract. The three valves between the plenums allow this fast-moving pressure wave to ram extra air into the cylinder that is about to fire on the opposite cylinder bank. In effect, Chevrolet has tuned the intake to act as a resonance supercharger with the waste air, increasing intake pressures without parasitic supercharging or turbocharging. As a result, between 2000-5800rpm the LT6 has a volumetric efficiency higher than 100 per cent.

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The hunt for extra air even involved moving the injector in each cylinder to the exhaust side of the combustion chamber, instead of the intake. This allowed for larger intake valves which measure 42mm in diameter, compared to the exhaust side’s 35mm. The injector placement also helps aid emissions by improving air and fuel mixing at highway and urban driving speeds. The intake valves are constructed of titanium for weight saving, while the exhaust valves are sodium-filled for heat dissipation, and there is no hydraulic component to the valve train at all. There are mechanical lifters, while dual springs allow for improved valve control.

Along with the extra air, the ability to rev higher than any Corvette engine before it is a central tenet of the LT6’s design. Engineers were fanatical about reducing the rotating and reciprocal mass within the engine. A lightweight forged steel crankshaft is utilised, along with forged titanium connecting rods, forged aluminium pistons, and hollow camshafts. By virtue of the flat-plane design, the crankshaft doesn’t require counterweights, further reducing mass.

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The block itself is aluminium, and while the bore spacing is Chev’s traditional 4.4-inches, the cylinders themselves are 104.3mm by 80mm. The large bore and short stroke is key to the high-revving nature of the engine. In fact, the bore:stroke ratio (1.3:1) is higher than that of the Ferrari 458, and Porsche GT3 (1.16:1 and 1:25:1 respectively).

As is traditional with every Corvette engine, each LT6 is hand built at Chevrolet’s Bowling Green plant in Kentucky. While the unit is only available in the Z06 at the moment, we wouldn’t bet against it being offered as a crate unit in the not too distant future. On an unrelated note, has anyone got an unwanted MX-5 chassis laying around that we could borrow?

Four Fast Facts

To the Moon

There is a hidden Easter egg in the LT6. Keen observers have noted that small rocket ships have been moulded into the engine block castings. This is a nod to the projects original codename, Gemini, which it shares with the US space program. Atmo V8s aren’t rocket science, but it’s a nice touch..

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BMEP for your buck

If you want to get super nerdy about comparing naturally aspirated engines, we need to use Brake Mean Effective Pressure (BMEP) – effectively a complicated measure of internal combustion chamber pressure. All you need to know is the higher the BMEP, the better. The Aston Martin Valkyrie’s Cosworth-built 6.5-litre naturally aspirated V12 revs to 11,100rpm and produces 735kW/740Nm. It has a BMEP of 14.5 bar at peak power, and 13.1 bar at peak torque. Z06? Exactly the same, just without the million-dollar price tag.

Acoustic perfection

Chevrolet’s solution to the hot side of the engine is to fit the LT6 with a pair of equal length 4-2-1 headers on each cylinder bank. Not only does the flat-plane crank design give an even alternating firing order, it provides the thrilling, shrill engine note akin to that of Ferraris of old. The Z06’s exhaust is anything but burbling and lazy.

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In the uncanny valley

Mid-mounting a relatively large capacity V8 engine resulted in a number of packaging challenges for Corvette, particularly when its customer base still demands a rear trunk that can accommodate a set of golf clubs. As a result, the LT6’s starter motor has been placed directly underneath the intake manifold, and the alternator placed within the cylinder banks. Hot-vee? Not when it comes servicing time.