In 1990, regular production option ZR1 was a special performance package intended to transform a base Corvette sport coupe into the fastest production car that could be sold in the U.S., and it was nicknamed “King of the Hill.” Federal exhaust emission regulations started horsepower numbers falling in 1971, and when they bottomed out in 1975 the Corvette’s base 350 was left with 165 hp before slowly making a comeback, reaching 245 hp in 1989. But in 1990, power-hungry buyers had a choice. The ZR1 package included the 375hp LT5 which would reach 405 hp in 1993, all out of the same 350ci displacement. At the time, however, it was thought it took four camshafts, 16 fuel injectors, and 32 valves to stay within current federal emission standards while making this kind of power. Articles on the ZR1 started appearing in 1988 (anticipating a 1989 introduction), but when its launch was pushed back to 1990, instead of losing interest, power-starved horsepower junkies like us just got more excited, and for 10 years this author owned the 39th ZR1 ever built.
Lotus in Hethel, England, was part of GM at the time, and they designed the LT5 based on their stillborn Etna 4.0-liter V8 architecture, with CPC (Chevrolet-Pontiac-Canada) Powertrain division developing the induction system in Warren, Michigan. CPC Powertrain Engineering was also rethinking their 30-plus-year-old pushrod V8 to determine what it was going to take for it to produce the DOHC LT5’s 405 or more hp. They started by redesigning most of its major components. This brought the second-generation LT1 350ci V8 up to 300 hp in 1992, with an optional 330hp version, the LT4, coming out in 1996. But to make further gains required an entirely new architecture that all later generations have been built on. The 1997 Gen III 345hp LS1 shared nothing with the past except 4.40-inch cylinder bore centers. The LS1’s 90-degree aluminum block had cast-in iron cylinder liners which reduced its displacement to 346ci, with four-bolt main bearing caps cross-bolted through deep side skirts. The heads were also cast aluminum with equally-spaced identical ports topped by a composite intake manifold flanked by individual ignition coil packs over each cylinder on cast aluminum rocker arm covers.
The LS1 was introduced with the fifth-generation Corvette coupe, which was followed by a convertible in 1998 and a hardtop in 1999 that became a dedicated extreme performance model in 2001, which was named after the 1963 Sting Ray’s Z06 racing package. Unlike the 1963 Z06, this version included a hopped-up engine, the LS6, that easily bested the 1990 double-overhead cam, 32-valve LT5 by 10 hp, and matched its 1993-and-later 405 hp in 2002 with four fewer cubic inches, a single camshaft, 16 valves, and a pushrod valvetrain. The innovative architecture of the C5 Corvette’s chassis and the Gen III small-block V8 were an entirely new beginning, and after two generations of upgrades and refinements their basic layout remains in production today. When we asked Dave Hill, the Corvette Engineering Director from 1992 to 2005, about the similarity of the C5 and C6 Corvette chassis he responded, “Why start over again when you can build on success?”
A forced-induction engine first appeared on the Corvette’s option list in 1987 with the Callaway twin-turbo package available through Chevrolet dealers, but not installed by GM. It boosted the L98’s output from 240 to 345 hp. This system was effective, but bulky and hard to package, while Eaton would later develop a more compact, belt-driven roots-type supercharger with finned intercooler tubes over its rotors in the same housing.
This is the type of supercharger currently available on production Corvettes, but the first supercharged engines to be installed on the Corvette Bowling Green assembly line were LC3 444hp Northstar V8s in the two-seater Cadillac XLR-V in 2006. The Corvette got its first Eaton blower three years later on the Gen IV 376ci LS9, rated at 638 hp for the Gen VI incarnation of the ZR1. The seventh-generation Corvette will eventually have two supercharged fifth-generation small-blocks in its engine lineup: the 650hp LT4 introduced with the 2015 Z06, and 2019 ZR1 will have a whopping 755hp from the newest LT5.
HOT ROD staff editor Brandan Gillogly concluded his article about the LT4 in the December 2014 issue with this comment about the 650hp Z06 Corvette: “Chevrolet had every right to call this car the ZR1, yet they didn’t. What do you think the Corvette team is working on now?” At the time this article was written, Jordan Lee, chief engineer of the small-block engines’ team of engineers, had been working on the second coming of the LT5 for over a year, and the Corvette it was going into would be the next ZR1. Assistant chief engineer John Rydzewski led the team that squeezed an additional 15 percent—or 105 hp—out of the already supercharged LT4, which is no small feat with an emission-controlled engine of the same displacement.
What has increased in displacement is the LT5’s supercharger. It grew from the LT4’s 1.7 liters to a newly-developed R2650 Twin Vortices 2.65-liter supercharger which was largely designed by GM’s Small-Block Group working with Eaton’s engineers. Scott Halsall has been the supercharger design release engineer since the LT5 reached the gamma level two years ago, and the blower housing which mounts directly to the cylinder heads was almost entirely designed by GM’s Global Propulsion Systems Engineering Center in Pontiac, Michigan. The LT5’s four-lobe compressor rotors are larger in diameter and longer with a tighter 170-degree helical twist than the LT4’s 160-degree rotors. The LT5 produces 14 pounds per square inch of boost, compared to the LT4’s 9.4 psi. The passenger-side rotor is driven by the crankshaft pulley through an 11-rib belt—three ribs more than the LT4—with a pulley ratio of 2.4:1 for 15,860 rpm compared to the LT4’s ratio of 3.1:1. A wider pair of spur gears also drive the LT5’s second rotor.
In order to sell the C7 Z06 Corvette in Europe, it had to meet stringent regulations. This meant taking three inches off the height of the LT4’s supercharger, which was an unacceptable compromise for the new “King of the Hill,” so the 2019 ZR1 won’t be available in Europe and the LT5’s massive supercharger is 2.5 inches taller than the LT4’s. The R2650 blower is under a carbon-fiber dome mounted to its housing cover that protrudes through the ZR1’s hood and visibly moves as torque reaction rocks the LT5 on its motor mounts, making this the Corvette’s first “shaker” hood.
Air is drawn into the LT5’s voluminous supercharger housing through a newly-tooled electronically controlled 95mm throttle body—Chevy’s biggest ever. The air increases in temperature as it is compressed going through the rotors. The hot, high-pressure air goes up into a plenum chamber in the top of the supercharger housing cover and then turns downward into a pair of stamped aluminum plate-and-fin intercoolers which are 30-percent larger than the LT4’s, with coolant running through them in tubes lowering the temperature by 140 degrees. The intercooler’s cooling system is similar to the engine’s, with both its heat exchanger and the radiator grouped together near the front of the car, with pumps circulating their coolant through them and back to their heat sources. The LT4 has a butterfly bypass valve controlled by a vacuum diaphragm through a mechanical linkage to reduce excessive boost pressure, while the LT5’s bypass valve is an electronically controlled throttle-body sourced from the L5P Duramax turbo diesel. This approach more precisely manages the amount of pressurized air that reaches the intake ports, particularly at idle and low engine speeds. This makes for better torque management and improved throttle response.
The 1990 LT5 had 16 intake valves with eight small primary fuel injectors spraying into its primary intake ports, and eight larger secondary injectors for its secondary ports which were only available when the power or “valet” key was turned on. The 2019 LT5 has only eight intake valves, but it also has 16 fuel injectors, eight large primary cylinder injectors that it shares with the LT4, and eight smaller injectors that spray fuel into the intake ports from the base of the supercharger housing. The engine normally runs on the direct-injection system with the port injectors providing additional fuel only when the DI system cannot keep up with demand. The PFI system is supplied by the fuel tank’s electric pumps at a pressure of 58 psi and for fuel injected directly into the cylinders delivery pressure in increased to 2,900 psi by a high-pressure pump. This mechanical pump is driven by an extra lobe on the camshaft in the same location as the Gen I cam’s distributor drive pinion and was first used on the 2014 naturally aspirated LT1. This pump was bored and stroked to increase its capacity for the supercharged LT4, and now the LT5.
Dustin Gardner is the design system’s engineer for the LT5 duel fuel system and most of the internal components below it, many of which carry over from the LT4. Nevertheless, there are some necessary upgrades, starting with the crankshaft and torsional damper. The crank is forged from higher-strength steel and its damper pulley has an additional groove for the 11-rib supercharger belt with a nodular cast-iron hub and a steel inertia ring. It is driven by the crank through a stronger key to handle the higher loads. The crankshaft runs in new tri-metal main bearings to support the higher loads, and the LT4-sourced forged powdered steel connecting rods also have new coated bearings to withstand the additional heat and pressure. All Gen V V8s except the LT5 and manual Camaro ZL1 with the LT4 have valve lifters that can be deactivated on half of their cylinders by an active fuel management system, while the lighter conventional hydraulic roller lifters used on the other cylinders activate all 16 of the LT5’s valves.
Like the LT4, the LT5 will be assembled by hand in GM’s Performance Build Center at the Corvette assembly plant in Bowling Green, Kentucky, and both engines will be built on the same aluminum cylinder block. This Gen V block is also used for the LT1 and has 4.06-inch bore cast-iron cylinder liners that, with the 3.62-inch stroke crank, add up to 376 cubic inches. The block features nodular-iron six-bolt main bearing caps and has a provision for oil-spray piston cooling. The forged aluminum pistons are designed to withstand the pressure of forced induction, and their flat crowns have a recessed bowl and other topography to guide the directly injected fuel and encourage it to mix with the air for combustion. The piston pins float in the rod bushings and pin bores, which reduces stress, and both V8s have a 10:1 compression ratio, which is unusually high for supercharged engines, thanks to the precise fuel control of direct injection.
Dry-sump lubrication has been a feature of ultimate high-performance Corvette engines since the 2006 427ci naturally-aspirated 505hp LS7, and both supercharged 2019 engines will be equipped with a similar but more sophisticated dry-sump system. In this set-up, oil draining back after lubricating the engine is scavenged from the pan and returned to a remote sump tank which for the LS7 was an eight-quart cylindrical tank mounted vertically on the passenger’s side of the firewall. The LS9-and-later oil tanks are in the same location and hold 10.5 quarts, with a gerotor scavenge, and on Gen-V engines, a variable displacement vane pressure pump driven by the nose of the crank in the same housing. Jets in the crankcase spray oil on the underside of the pistons for cooling, and on the surrounding cylinder walls, with the oil temperature kept within the optimal range by the engine’s cooling system through an external cooler.
Engine oil pressure also controls a cam phaser on the front of every Gen-V camshaft which changes its angular relationship to the sprocket, allowing the valve timing to be advanced for idle and retarded for maximum power. Large lightweight valves are used in the forced-induction heads with 2.13-inch titanium intakes and 1.59-inch hollow steel sodium-filled exhaust valves. These valves, looking at them from the front, are inclined inward 12 degrees for exhaust and 12.5 degrees for intake. Viewed from the side, intake and exhaust valves converge slightly toward the centerline of the cylinder, forming a twisted-wedge 65.47cc combustion chamber, and by comparison the naturally-aspirated LT1’s 59.02cc chambers give it a higher 11.5:1 compression ratio. These heads are rotocast with the mold rotated as the molten A356 aluminum alloy cools (a T6 hardness is spec’ed), and are topped by the distinctive Gen-V rocker arm covers with their ignition coils mounted between internally baffled domes. An integrated positive crankcase ventilation system is incorporated into these covers that separates oil and air from the crankcase vapors. The LT5 has a center-feed version with a revised oil vent system that has additional holes for a faster oil return.
Six months is the average amount of time it takes for GM’s dynamometer laboratory to develop a new version of a high-performance engine, but lead dyno development engineer Gary Price Jr. and his team spent nearly a year getting the most out of the LT5. Dyno cell D116 took most of the beating as test engines started producing enough power to overheat its exhaust system and take in more air than the combustion air handling system could provide, thus lowering the barometric pressure in the cell. With these problems overcome and special ultra-high flow fuel carts, LT5s were run 16 hours a day for months, and occasionally around the clock. This was to complete particular tests tuning and calibrating every system with a lot of attention given to spark and fuel. The program wrapped up with 105 hp over the LT4.
We asked Jordan Lee if, with GM spending billions on electrification and autonomous vehicles, the LT5’s 755 hp at 6,300 rpm and 715 ft-lb of torque at 4,400 rpm was the high watermark for GM’s internal combustion engines. We were very relieved to hear that development of new combustion engines is far from over or even winding down, and that the small-block pushrod V8 has a bright future.
This is what Jordan Lee’s boss Dan Nicholson, vice president of GM’s Global Propulsion Systems, has to say about the end results of the Small Block team: “The LT5’s horsepower puts Chevrolet and our small-block over the 700-horsepower threshold for the first time, but just as important, that power is very driveable in the ZR1. Painstaking engine integration with a dynamically capable vehicle enables the use of all 755 hp. The sensation behind the wheel of this dual fuel-injected, blown small-block is something hard to find elsewhere in a lifetime.”
Powered by WPeMatico