Chevrolet/Illustration by R&T
Recently, GM hosted an event where engineers from the Chevrolet Corvette and Small Block programs gave a three-plus hour presentation about the LT6, the engine that powers the new Corvette Z06 C8. The most common reference point is the LT4, the supercharged 6.2-liter V-8 used in the C7 Corvette Z06, and currently found in the Camaro ZL1 and Cadillac CT5-V Blackwing. It’s a killing machine, although from the start, it’s plagued with errors.
Namely, overheating. In track use, early C7 Z06s had a habit of hitting high engine temperatures and going into limp-home mode, something that led the owners to file several class action lawsuits against GM. Chevy fixed the problem by raising the supercharger cover starting with the 2017 Z06, but GM’s reputation for building durable track cars has been tarnished. It was a rare glitch for the Corvette team, and while no one directly mentioned the LT4 issue in the LT6 presentation, the subtext was, “we know where we screwed an LT4-powered C7.”
(We should note that the LT4 hasn’t shown any heat-immersion issues in the current Camaro ZL1, 3rd generation Cadillac CTS-V, or Blackwing CT5-V. Cadillac’s media launch for the CT5-V Blackwing was held at the Virginia International Raceway last summer, and the cars had no problems all day in the 95 degree heat with the A/C on.)
LT6 engineers spent a lot of time talking about how they achieved impressive numbers with this all-new 5.5-liter V-8 engine: 670 hp at 8400 rpm, 460 lb-ft of torque at 6300 rpm, and a redline of 8600 rpm. But they dedicate a lot of time to explaining the cooling system, and how the engine is made to be strong.
It started with how LT6 was built. The block is made of two separate aluminum castings: one for the cylinder, water jacket and top of the crankcase, the other forms the bottom crankcase and holds the oil exhaust system. This was a very unusual design—most V-8s made since the ’30s used a single casting for the block and crankcase.
This design is used to maximize the efficiency of the dry-sump lubrication system. Separate crankcase casting allows Chevy to create four enclosed crankcase chambers, which help keep air out of the system, allowing the crankshaft to rotate more freely and reducing pumping losses. Each crankcase has what engineers call a “gun gap” to allow oil to enter the individual scavenger circuits. “It’s very important in high-performance engines not to aerate the oil and blow it out of the crankshaft,” said Jordan Lee, chief engineer of GM Small Blocks. Aerated oil causes bearing damage, and a new crankcase design and dry body system are designed to keep oil free of air.
There are six oil scavenger circuits in total: one for each crankcase, one for getting oil from the front cover, and one for the cylinder head. The oil tank itself is directly above the pump at the front of the engine. The engineers took great pride in the efficiency of the system, noting that at 8600 RPM, 96 percent of the engine oil—eight liters of 5W50—remained in the tank. That outperformed the LT4, which held only 68 percent of its oil in the tank at the redline. There’s also a bottom-mounted oil cooler, which engineers say has 85 percent more cooling capacity than the LT4. Lee says the goal with the LT6 was to keep oil temperatures below 248 degrees F on track, and in track testing, temperatures rarely went above 230. The system can handle 1.25g of lateral or longitudinal acceleration, so a lack of oil shouldn’t be a problem. .
Of course, the lubrication system is very important in making a long-lasting engine, but it doesn’t stop there. Compared to the C8 Stingray, the Z06 has two additional radiators, one in the center of the front grille, and one behind the driver’s side for oil cooling. The two outboard front radiators each get a new fan and a new compressor. For track driving, the front license plate bracket is easily removed, increasing cooling airflow to the center radiator. The LT6 gets an even bigger compressor and A/C pulley compared to the LT2 on the Stingray, for that 100-degree track day.
Chevy loves engine materials, both for strength and weight. The forged titanium rods are the lightest in the Z06, as are the forged aluminum pistons. The pistons and mechanical finger retainers get a diamond-like carbon finish to reduce friction. Without hydraulic tappets, the finger-follower valvetrain has “lifetime” cracks and crevices, and after 100,000 miles of testing, engineers saw “negligible” wear on these components. They also claim to not see unusual oil consumption in track testing, and say the vacuum created by the new crankcase design actually helps keep the piston rings in place.
The LT6 is a radical departure from GM’s small block tradition, incorporating engineering solutions commonly associated with limited-production cars without demanding special treatment from the engine. “You get an exotic engine, but not an exotic ownership experience with an incredible maintenance bill and incredible maintenance frequency,” said Corvette chief engineer Tadge Juchter. “No special treatment,” added Jordan Lee. “Change your oil and filters, and change your spark plugs after 100,000 miles.”
It’s clear that the Corvette and Small Block teams are looking to win back those who were disappointed by the issues with the last Z06. The Z06 is supposed to be the most track-focused Corvette, and those early LT4 issues really ruined the reputation of the C7 version. With its C8 Z06 and LT6, GM engineers looked for a way to fix the error. It’s a tantalizing promise: the most powerful naturally aspirated road car V-8 ever, with the reliability and durability we expect from a small block.
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