MILFORD, MI – With several months of engineering work remaining on the ’15 Chevrolet Corvette Z06, the sports car already owns the fastest lap at the General Motors Proving Ground. In the coming weeks, more records are expected to fall when it travels to Germany’s fabled Nurburgring for testing.
The supercharged, 638-hp Corvette ZR1 owns GM’s quickest turn of “The ’Ring,” posting a 7:19.63 in 2012. That’s about 3.5 seconds ahead of the previous-generation Z06, which still commands a top-15 time at the track from 2011.
But the redesigned ’15 Z06 boasts an all-new 650-hp 6.2L LT4 V-8, which like the ZR1 with its 6.2L LS9 V-8 uses an Eaton Roots-type blower for added power over base-model Corvettes.
Jordan Lee, chief engineer and program manager for the small-block V-8 at GM, says the LT4 crushes the performance benchmark set by the LS9.
“The LT4 makes the ZR1 feel sluggish,” Lee says during a recent backgrounder on the engine here. “It’s brutal in a raw sense, but a thrill.”
The LT4’s calling card is maximum power almost instantly. The engine, based on the fifth-generation small-block LT1 6.2L V-8 architecture powering the ’14 Corvette Stingray, makes 457 lb.-ft. (619 Nm) of torque just off idle, Lee says, and 625 lb.-ft. (847 Nm) at a scant 2,800 rpm.
The LT4 also maintains 90% of its peak torque, or 592 lb.-ft. (802 Nm), from 2,500 to 5,400 rpm. The full brunt of the LT4’s 650 lb.-ft. (881 Nm) of thrust comes at 3,600 rpm and gives it nearly 8% greater peak grunt than the ZR1.
GM has not published a top speed for the Z06, but given the power output it would almost certainly exceed the 205 mph (330 km/h) of the ZR1. GM has said the ’15 Z06 with its new 8-speed automatic will do 0-60 mph (97 km/h) in 3.7 seconds and cover the quarter-mile (0.4 km) in 11.9 seconds, clear evidence this engine indeed is loaded with low-end torque.
Supercharger Key Component
The rubber-shredding numbers of the LT4 are made possible largely on the strength of updates to the LS9’s supercharger.
The new R1740 TVS 1.7L supercharger atop the LT4 spins at up to 20,150 rpm, according to GM. That’s 5,000 rpm more than the 2.3L LS9 blower. Like its predecessor, the LT4 supercharger contains a pair of 4-lobe rotors, but they are shorter in length and smaller in diameter, which makes them capable of higher rpm to deliver more boost earlier.
The new supercharger is more efficient, too, thanks to a more direct discharge port that creates less turbulence, which reduces heat and speeds airflow into the engine, GM says. The supercharger’s charge-cooling system also gains more efficiency, offering 10% greater heat rejection through 23% reduced volume compared with the LS9, the automaker says.
GM shrunk the overall size of the supercharger and intercooler assembly to retain a low hood profile and still meet pedestrian-protection regulations globally.
It lost 20 lbs. (9 kg) off the LS9’s supercharger, while adding a negligible 1 in. (25.4 mm) to the height of the LT4 compared with the naturally aspirated LT1 found in the Corvette Stingray.
The supercharged LT4 uses many of the same components making up the LT1, although a few technical changes were made to accommodate forced induction.
The LT4 direct-injection fuel system, which was a first for the small-block on the LT1, receives a higher-flow fuel pump; higher-flow fuel injectors with a unique spray to optimize combustion under boost; and a higher-capacity rail to handle the capacities of the new pump and injectors.
Rotocast A356-T6 aluminum cylinder heads are added to the LT4 because they are stronger and handle heat better than conventionally cast aluminum heads.
Combustion chambers are bigger on the LT4 than the LT1, and lightweight, solid titanium intake valves replace hollow units for greater strength and resistance to heat.
The LT4 also swaps in lighter, stainless steel exhaust manifolds compared with the iron manifolds of the LT1.
Rotating assembly parts are unique to the LT4 to support higher cylinder pressures of the boosted engine, as well its lower compression ratio of 10:0.1 versus 11:5.1 on the LT1.
The oiling system of the LT4 remains essentially the same as the LT1, but it receives a larger oil tank and a larger oil cooler as well as unique Dexos 5W30 lubricant.
And while GM builds the LT1 in Tonawanda, NY, the LT4 gets assembled at the automaker’s Performance Build Center, which recently moved to the Corvette’s Bowling Green, KY, assembly plant from a skunk-works facility in Wixom, MI.
Despite all the power, expect respectable fuel economy. The LT1 achieves an EPA-estimated 29 mpg (8.1 L/100 km) on the highway. Much of that is owed to cylinder deactivation and variable-valve timing technologies, also appearing on the LT4.
In fact, Tadge Juechter, Corvette chief engineer, expects an improvement to 30 mpg (7.8 L/100 km) for ’16 on the base Corvette. “We’ll try again next year,” he vows.
However, the LT4 consumed its share of fuel during development. It averaged 35 gallons (132.4 L) per hour on the dynamometer.
