2023 Chevrolet Corvette Z06 LT6 V-8 Deep Dive: The World’s Most Powerful Naturally Aspirated V-8
Not only is the 2023 Chevrolet Corvette Z06’s LT6 engine the largest flat-plane crankshaft V-8 in the world, it’s also the world’s most powerful naturally aspirated production V-8, leapfrogging the 622-hp 6.2-liter M159 engine that powered the Mercedes-Benz SLS AMG Black Series from 2013-2014 and producing 670 hp at 8,400 rpm. How did Chevy more than manage to find “a replacement for [745 cubic centimeters’ worth of] displacement” relative to that mighty Benz? With better breathing and by spinning the engine a lot faster—horsepower is, after all, just torque times rpm divided by 5,252. Also note that at 460 lb-ft this is not the world’s torquiest V-8 (the Benz above made 468). Here are five ways Chevy accomplished this.
Airflow Uber Alles
Two huge, 87mm throttle bodies feed a separate intake plenum for each cylinder bank of the Z06 engine. Altogether it holds 11 liters of air in total—twice the cylinder displacement. Fun fact: the C7 Z06 was fed by a single 87mm throttle body. Inside these plenums, each cylinder inhales through its own unique intake trumpet, angled in slightly different directions to optimize cylinder filling. The team would love to see the aftermarket develop a clear plenum that allows folks to see these trumpets, but it’s a structural part and it’s yet to be determined whether the plastic part can be reliably painted in Edge Red to match the cam covers, as shown on the display engine.
Resonance Supercharging (Free Boost!)
In a flat-plane-crankshaft V-8, the firing order always swaps from one bank to the other, lacking the syncopation cross-plane engines get from successive firing on a single bank (that’s what makes them burble). The real intake-manifold magic that leverages this potential volumetric efficiency windfall is a series of three valves connecting the two intake plenums that open in different combinations to vary the degree to which pressure waves communicate within or between the LT6 engine’s plenums.
When each of the Corvette Z06’s 32 individual intake valves slams shut, it creates a pressure wave that reflects back up the intake runner. When the timing’s just right, that wave reflects back down a runner while its intake valve is open to help cram extra air in. This is called resonance supercharging, and it’s based on the same Helmholtz resonance effect that creates sound when you blow across the top of a bottle. This type of supercharging exacts no horsepower penalty on the engine.
The three intake-plenum valves all start out closed. Then as revs rise, one opens at around 2,000 rpm, then all three open for a short time around 5,800 rpm, after which they all close again. (That’s the strategy in Sport and Track modes—different strategies are employed in the Stealth and Tour modes to optimize for quiet running.) This allows the LT6 engine to achieve a volumetric efficiency rating of greater than 100, meaning it can naturally inhale a volume of air greater than the static size of its cylinders. This is rare in naturally aspirated road-legal production engines.
Ultra-Light Rotating Mass
A major key to the LT6 engine’s high-revving nature—in addition to the inherently lighter flat-plane crank (which employs no counterweights)—is its oversquare bore and stroke (104.3/80.0mm), which greatly reduces peak piston speed at high rpm. Short-skirt lightweight forged aluminum pistons, forged titanium connecting rods, and an aluminum harmonic balancer all help minimize rotational inertia. And up in the dual-overhead-cam cylinder heads, we find lashed-for-life mechanical valve lifters that eliminate slop and friction in the valvetrain. Each roller follower is supported by one of 40 different sizes of lash cap individually selected after high-precision robotic measurements. The resultant tolerances are so tight and precise that adjustment is never needed. Oh, and the titanium intake and sodium-filled exhaust valves are closed by dual-coil springs.
Free-Flowing Center-Exit Exhaust
The exhaust exits the LT6 engine through equal-length header pipes, then flows through a completely new exhaust system. The challenge with a flat-plane-crankshaft exhaust system is that the exhaust signature is like that of two four-cylinder engines. This can sound rather blatty, and connecting the banks via an X-pipe or crossover tube tends to cancel and deaden the sound. The Corvette team discovered the best result was to locate the mufflers in the corners of the car and let the pipes exit in the center. Now with the loud valve open, adjacent tubes deliver the bark of each cylinder bank, allowing the exhaust pulses to combine in a less cacophonous fashion.
The exhaust tips now mount to the fascia and feature parabolic reflectors—reverse megaphones if you will, contoured to reflect soundwaves forward toward the cabin as they expand radially outward upon exiting the pipe. From this central location, exhaust noise has a more direct path to the cabin than it does from the corners of the Stingray.
They sound fabulous. The character is Ferrari-esque, but the sound is in a lower register, reflective of the Corvette’s deeper lungs. This character steps down as you transition from Track to Sport and Tour, all but disappearing in Stealth Mode 1, with the loud valve completely closed and all exhaust transiting the entire muffler system and exiting via the smaller, outer pair of pipes (the openings of which include perforated outer flanges that resemble spark arresters, but are just there to match the apparent size of the inner pair of pipes). Stealth Mode 2, if you were wondering, dims all the nonessential cabin and dash lighting.
Everyone loves those burps and pops on overrun, and Chevy delays closing the direct fuel injectors slightly to encourage a bit of this—especially in the more aggressive drive modes. But they only do so while your foot is moving. As you back off, you’ll get pops, but if you hold a throttle position—even one that has the car slowing—the pops subside.
A brief note about those injectors: in leveraging lessons learned on Chevy’s IndyCar engines, they’re side-mounted under the exhaust valves. This frees up valuable real estate in the valley for that elaborate intake, but it also helps mix the fuel better. Most engines put them under the intake valves where they naturally stay cooler, but this often requires some intake-runner-induced tumble that can reduce volumetric efficiency. The water jacket extends down around the injectors, the exhaust headers are routed up and away from them and are completely sheathed in heat shielding, and considerable airflow from the side inlets cools the engine bay.
Six-Bay Oil Scavenging
The final trick to making a high-strung and complex engine like the 2023 Chevrolet Corvette Z06’s LT6 survive long track sessions is a sophisticated oiling system. Here again, the LT6 borrows heavily from racing, with each piston pair being hermetically sealed off from every other so that the air beneath the pistons simply moves laterally back and forth with minimal pumping losses. A six-bay oil pump that runs the length of the engine scavenges each of these bays plus each cylinder head and takes 14 hp to run. The car calls for 10 quarts of 5W50 oil, and with the large oil cooler in the left rear air intake, track-day oil temperatures reliably hover around 250-265 degrees.
One fun development-drive note about this system: The pump draws a 60-psi vacuum. Upon shutdown, that vacuum can suck air past the crankshaft main seal causing a brief howling noise. A design fix has cured this, but one of our development-ride cars hadn’t been updated to the new design yet.
Bonus: Million-Dollar COVID Exhaust
The Corvette team really hopes you like the exhaust sound because it was expensive. When the pandemic shut things down, spending also stopped. But the computers kept iterating exhaust system designs, resulting in this one with the center-exit. Everyone loved the sound but tearing up the rear fascia design to accommodate the centrally mounted reverse-megaphone exhaust tips cost “millions.” Worth it!
Bonus: Valley of the Ancillaries
This doesn’t necessarily help the engine breath better or spin faster, but it’s worth noting that packaged beneath that 11-liter intake manifold in the valley of the vee are two major components most engines hang off the front or sides of an engine: The starter fits at the rear of the valley and the alternator sits at the front. The two high-pressure fuel pumps reside here as well, driven by a dedicated cam with lobes optimized for 9,000-rpm operation.
Bonus: Project Gemini
The design team dubbed the LT6 engine family “Gemini,” owing to its use of twin cams, twin throttle bodies, twin high-pressure fuel pumps, and more. To pay homage to this unofficial nomenclature, you can find little rocket-ship icons molded or cast into various engine parts
Bonus: Long Live LT6!
With big naturally aspirated engines on our extinction watchlist, we asked Corvette chief engineer Tadge Juechter about the LT6’s future. He assured us that it has been developed to meet every forecast emissions regulation. European regs are somewhat tougher and a particulate filter will be required for export markets to meet Euro 6d. The biggest challenge to legalizing this racing engine: Getting the catalysts to light off located way at the end of the four-into-two-into-one headers, sized to optimize scavenging (the sucking-exhaust-out counterpart of resonance supercharging). But the catalyst light-off fueling strategy seems to work.
Photos by William Walker, Jason Lanham/Astro Creative, and the Manufacturer
Source: Read Full Article