Technical presentation: Corvette LT6 engine
Tit is a rare moment. The Tech Talk section of the magazine is where we can learn about the latest and greatest advancements in the automotive world – delving deep into advanced batteries, shiny new materials, or innovative manufacturing techniques. Unfortunately, however, this often means that the trusty internal combustion engine – the workhorse of the automobile over a century old – is not loved unless there is a new turbocharger with assist. electric or a fuel created in the laboratory.
Not this month. Thanks to Chevrolet, we are exploring new horizons with good old-fashioned natural aspiration. That’s right, it’s the art of producing more power than ever before with atmospheric pressure, regular 98 RON fuel and eight combustion chambers. The subject is the 5.5-liter, dry-sump, flat-sump dual overhead camshaft, four valves per cylinder, atmo V8 tucked amidships in the new Corvette C8 Z06.
It is the most powerful naturally aspirated V8 ever put into production, the main figures being 499 kW at 8,400 rpm and 623 Nm at 6,300 rpm. Codenamed LT6, this engine confidently usurps the previous record holder – the Mercedes-AMG’s 464 kW 6.2-liter M159 from the SLS Black series. The LT6’s horsepower figures are all the more impressive as it overtakes the C7-generation C06’s LT4. Despite its larger capacity (6.2 liters) and the use of forced induction, the LT4 only managed to send 485 kW at 6,400 rpm to the rear wheels. To be fair to the supercharged engine, it also triggered 881 Nm on the rear tires from just 3,600 rpm.
Key to the LT6’s power advancement is smart air management, with the dry sump unit capable of inhaling more air than its 5.5L swept capacity would initially suggest. . Engine power is intrinsically related to the amount of oxygen that can be combined with fuel in the combustion chambers. This is where the LT6 sits high, blowing O2 with a ferocity almost unmatched by unforced induction motors.
The engine is powered by an 11-liter intake manifold, powered by two 87mm throttle bodies, with four mirrored trumpets on each row of cylinders. For context, the previous C7 Z06 only had a single 87mm throttle body. The really smart design, however, lies in 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 call the violent shaking when lowering a single window in. a car at highway speed.
When an intake valve in a combustion chamber closes, a pressure wave rises in the intake duct. The three valves between the plenums allow this rapid pressure wave to send additional air into the cylinder which is about to pull on the opposite row of cylinders. This is because Chevrolet has tuned the intake to act as a resonant supercharger with the stale air, increasing intake pressures without any parasitic boost or boost. As a result, between 2000 and 5800 rpm, the LT6 has a volumetric efficiency greater than 100%.
The additional air flush even involved moving the injector from each cylinder to the exhaust side of the combustion chamber, instead of the intake. This made it possible to have larger intake valves which measure 42mm in diameter, compared to 35mm on the exhaust side. The placement of the injectors also helps reduce emissions by improving the air-fuel mixture at highway and city driving speeds. The intake valves are constructed of titanium for weight saving, while the exhaust valves are filled with sodium for heat dissipation, and there are no hydraulic components in the valve train. There are mechanical valve lifters, while dual springs provide better valve control.
With the added air, the ability to climb higher than any Corvette engine before that is a central tenet of the LT6’s design. Engineers were fond of reducing the rotational and reciprocal mass in the engine. A lightweight forged steel crankshaft is used, along with forged titanium connecting rods, forged aluminum pistons and hollow camshafts. Thanks to the flat plane design, the crankshaft does not require a counterweight, which further reduces mass.
The block itself is aluminum, and while the bore spacing is Chev’s traditional 4.4 inches, the cylinders themselves measure 104.3mm by 80mm. The large bore and short stroke are 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 tradition with every Corvette engine, every LT6 is handcrafted 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 its offering as a cash unit in the not too distant future. On an unrelated note, does anyone have an unwanted MX-5 chassis that we could borrow?
Four quick facts
To the moon
There is an Easter Egg hidden in the LT6. Informed observers noticed that small rockets were cast into the castings of the engine block. It’s a nod to the project’s original code name, Gemini, which it shares with the US space program. The Atmo V8s aren’t rocket science, but it’s a nice touch.
BMEP for your money
If you want to get super cheesy comparing naturally aspirated engines, we have to use Brake Mean Effective Pressure (BMEP) – actually a complicated measure of internal combustion chamber pressure. All you need to know is that the higher the BMEP, the better. The Aston Martin Valkyrie’s Cosworth-built 6.5 liter naturally aspirated V12 spins at 11,100 rpm and produces 735 kW / 740 Nm. It has a BMEP of 14.5 bar at peak power and 13.1 bar at maximum torque. Z06? Exactly the same, just without the million dollar price tag.
Chevrolet’s solution for the hot side of the engine is to equip the LT6 with a pair of 4-2-1 manifolds of equal length on each row of cylinders. Not only does the flat-plane crank design give even firing order, it delivers the thrilling, high-pitched engine note similar to that of the Ferraris of yore. The Z06’s exhaust is anything but loud and lazy.
In the strange valley
The mid-mount of a relatively large capacity V8 engine resulted in a number of packaging issues for Corvette, especially when its customers still demand a trunk that can accommodate a set of golf clubs. As a result, the LT6’s starter motor was placed directly under the intake manifold and the alternator placed in the cylinder rows. Hot-vee? Not when the maintenance time comes.