GM details Corvette’s new and mighty LT6 V8
GM’s Small Block engine experts pull out all the stops for the world’s most-powerful normally aspirated V8.
If the end of the internal-combustion epoch truly is within sight, General Motors intends to see it out with a bang. It’s latest — who’s to say whether it’s the “last?” — clean-sheet V8, internally designated LT6, arguably is its best, considering GM touts it as the world’s most-powerful normally aspirated V8 ever fitted in a production vehicle.
Earmarked for the 2023 Z06 high-performance variant of the Chevrolet Corvette, the LT6’s most-illustrious design feature is its flat-plane crankshaft, but engineers in mid-February 2021 laid out for media the numerous other unique engineering elements that led to the thunderous SAE-certified output of 670 hp and from a comparatively moderate — in V8 terms — displacement of 5.5L. Over its eight-year development that saw the first prototype in 2015, engineers from GM’s Small Block engine team dubbed the engine “Gemini” due to its many “twin” architectural elements.
Just don’t call the LT6 a “small-block” V8, even if it was developed by the engineering team bearing that name. Apart from its 4.4-inch (111.76-mm) bore-center spacing – the single architectural dimension that connects all five generations of GM’s legendary V8 introduced in 1955 – the LT6 has virtually no connection to small-block V8s, declared chief engineer Jordan Lee. Not only does the 90-degree LT6 have the significant difference of a dual-overhead-cam (DOHC) layout rather than the overhead-valve (OHV) arrangement that’s a hallmark of the small-block family, Lee confirmed the LT6 shares not one part with any previous pushrod small-block. To further drive home the point, he added, “In today’s day and age, it’s hard to define an engine by one dimension [the 4.4-in. bore centers]. This is a unique engine on its own.”
Less mass, better throttle response
To fully exploit the flat-plane V8’s intrinsic quick throttle-response and revving ability redline is a dervish 8600 rpm — lightweighting and reciprocating-mass reduction were key LT6 priorities. At 364 grams, its forged-titanium connecting rods are 21% lighter than the titanium rods for the LS7 supercharged 7L V8 in the sixth-generation (2006-13) Z06 Corvette, pointed out design release engineer Thomas Halka.
The LT6’s forged-aluminum pistons are 8% lighter than the LS7’s. A major weight-saving contributor is the flat-plane crankshaft itself, thanks to fewer counterweights compared to a cross-plane crankshaft. But the LT6’s crank also has extraneous mass drilled out around the main bearings and other areas. Design release engineer Clarence Smith said the engine’s crankshaft is a full one-third lighter than that of the LT2 6.2L V-8 that powers the standard Corvette. In all, said Gardner, despite having 175 hp more than the standard Corvette’s LT2 V8, the LT6 is just 1 kg (2.2 lb.) heavier.
Optimizing around flat-plane
To stay within an optimum displacement range to make the LT6 suitable for worldwide endurance racing and to also help address the vibration issues that can afflict V8s with flat-plane crankshafts, the LT6 fronts a short stroke of just 80 mm compared to its bore of 104.25 mm. The flat-plane V8, a longstanding preference of Ferrari (a recent example of which GM purchased from a salvaged model) allows for improved exhaust-scavenging behavior and is a key enabler of enhanced volumetric efficiency and high-rpm power.
The Small Block engineering team initially set a maximum-output goal of 650 horsepower. After some analysis of the engine’s characteristics, “We quickly realized we had 650 [horsepower] in the bag, said assistant chief engineer Dustin Gardner. And Yoon Lee, design system engineer, noted that the production engine makes more power than the racing version of the LT6 (endurance-racing rules require intake restrictions that limit power output). “I think it’s going to stand the test of time of being one of the true icons,” Gardner added.
The LT6 is comprised of a two-piece aluminum block mated at the crankshaft centerline and topped by aluminum cylinder heads. The short stroke delivered a deck height some 23 mm (0.9 in) lower than the standard Corvette’s LT2 V8. The 12.5:1 compression ratio is abetted by forged-aluminum pistons which have startlingly short skirts and are fabricated by a racing vendor.
The new engine’s intake system perhaps is the most visible manifestation of the Gemini concept, its prominent dual-plenum design encasing an approach to air-management that enhances torque production. The system, extensively detailed here , uses an arrangement of three valves to manipulate air movement between the plenums. The result is a variable volume of intake that optimizes induction resonance while boosting torque output to its maximum 460 lb-ft (624 Nm) and at the same time making it more consistent across the rpm range (see chart).
The complexities of the intake variables are one element comprising some 41,000 adjustable calibration values for the LT6, which “starts to ‘tune’ effectively at 3,000 rpm and above,” explained chief engineer Lee. The engine’s power and torque chart bears out Lee’s assertion, showing a distinct rise and leveling to its torque peak between 3000-4000 rpm. Lee summarized by calling it “an incredible amount of power for this displacement.”
The LT6’s valvetrain,lubrication and thermal-management circuits are immensely optimized subsystems designed to handle the punishment of racetrack abuse and deliver day-to-day robustness uncommon to many high-strung performance engines. Most surprising about the valvetrain is its use of solid (i.e. mechanical) valve lifters chosen to reduce moving mass and assure valve performance at high rpm. One of a wide selection of specificallysized steel lash caps is robotically fitted for each valve after automated measurement of 32 different tolerance areas.
Phil Baranek, design release engineer, assures that the fixed arrangement is good for the life of the engine. That guarantee may be supported by the unique actuation arrangement of a diamond-like carbon coating for the roller-finger followers and an integrated oil jet for each finger follower to assure constant and consistent lubrication at this vital interface.
Variable valve timing of course is part of the equation. The intake valves have 55 degrees of crankshaft authority while the exhaust valves are variable over 27 degree of crankshaft authority. The valve timing scheme plays a significant role in the LT6’s emissions profile and surely enhances overall efficiency, but GM did not detail fuel economy figures or confirm if the engine will be labeled a gas guzzler.
The lubrication system for the LT6 is comprised of no less than six scavenge pumps and one supply pump. These are driven by lobes on an intermediate shaft situated above the crankshaft that also is fitted to drive a secondary chain for the camshafts. The intermediate shaft also has lobes to energize both the direct-injection fueling system’s 5000-psi fuel pumps. Four of the scavenge pumps serve each of the unique sealed bays that see pairs of opposing cylinders closed from the other cylinders.
The sealed crankcase bays, say GM’s engineers, reduce piston pumping losses as well as crankcase windage losses. Oil scavenging is also improved thanks to the clever “scraper” design at the bottom of the crankcase and the high degree of crankcase vacuum. Meanwhile, the dry sump for the eight-quart oil supply for the LT6 (this is 1.75 quarts less than required by the LT4 V8 of the seventh-generation Corvette Z06) now is attached directly to the engine. It is rated for 1.25 g of lateral force and astonishingly retains 96% of the oil supply in the tank at redline, compared with 68% for the LT4 V8.
The 2023 Corvette Z06’s cooling system has five radiators – a new central radiator and supplemental radiator are added – that help deliver a 50% increase in total cooling capacity compared to the standard Corvette with the Z51 performance package. The Z06’s side openings are larger to enable 20% more airflow and the front fans are 50% more powerful. Engineers said the increased thermal performance comes with no loss of package space. The system’s capacity allows customers to run at the racetrack with the air conditioning engaged in 100-degree ambient temperature, the engineers boast.