Engine Lowdown: 2015 Corvette Z06's 6.2-Liter Supercharged LT4

See All 16 Photos16

When it arrives in showrooms next spring, the 650-horsepower, $78,995 Corvette Z06 will be the most powerful production vehicle ever offered by General Motors. As of this writing, its development is in full swing but is far from complete.

"Z06 development is coming along," said Corvette chief engineer Tadge Juechter at a recent eight-speed Hydra-Matic presentation. "When we introduced it at Detroit earlier this year, we said the car's not going to be available until the end of this year, because we have a lot of work to do. We've been working through the spring and summer, and that will continue into the fall."

Still, that doesn't mean everything -- such as its 6.2-liter LT4 V-8 engine -- is a secret. The nucleus of the LT4 is GM's naturally aspirated, 455-horsepower, 460-lb-ft, 6.2-liter LT1 from the Stingray. The task of beefing up the LT1 was left to GM's chief small-block engineer, Jordan Lee, and his team of enthusiast engineers.

They had six objectives to hit when creating the LT4:

• Had to be small, compact, and light
• Needed to meet Euro NCAP PED PRO requirements
• Needed the ultimate in horsepower and torque outputs
• Substantial low-end torque was a must
• Had to use entire suite of Gen 5 small-block technology, i.e., direct injection, active fuel management, etc.
• Needed to be durable enough to withstand hard track duty

Lee mentioned the LT4 is about the same size at the LT1, growing about an inch in height at its rear end (26.1 versus 25.3 inches). Width remains 27.6 inches, but the LT4 is a tad longer (27.5 versus 25.3 inches).

See All 16 Photos16

"The LT4 is about the same size as the LT1. And the LT1 has gotten a lot of acclaim for being very small and compact. Small and compact is a hallmark of the small-block architecture, and it's a hallmark that we continued with the Gen 5 small-blocks and we continued it with the LT4. It's extremely important that the engine is small, lightweight, and compact to fit under the hood of the Corvette -- a very low hood line, front-engine car."

His team considered other methods of power production, but realized that "if we did dual overhead cams and twin-turbo V-8s like a lot of our competitors, the engine gets huge. It's very large, the hood gets taller, the center of gravity of the car is all messed up, and it doesn't have the handling characteristics that Tadge wants out of the Corvette. So, it's very important that the small-block maintains its compact nature."

While the engine is tightly packaged, it continues to use the LT1's fuel-saving technologies. "We're applying the entire Gen 5 suite of hardware. Direct-injection is a huge enabler for increased low-speed torque, fuel economy, and high-speed power, primarily because it allows us to raise the compression ratio. The LT4 is running a 10:1 ratio (versus the LS9's 9.1:1). Variable displacement oil pump and active fuel management - we do run in four-cylinder mode. As strange as it seems, Corvette owners do, indeed, brag about their fuel economy. Many of them won't buy the car for fuel economy, but they brag about the fuel economy."

They also brag about power, which the LT4 isn't lacking. "If you take a look at the low-end torque, it's a significant increase compared to the LS9, which is why the car does not feel sluggish compared to the LS9. It's just instantaneous torque, instantaneous power even at the lowest rpms. We're making 350 horsepower at roughly 2,400 rpm."

See All 16 Photos16

To get to that output level, engineers changed a long list of items. To start, the new four-lobe Eaton R1740 TVS-series supercharger has a 160-degree helix rotor design (same as the LS9) but is 20 pounds lighter than the LS9's R2300 and revs at a maximum 20,150 rpm (compared to the R2300's 15,080 rpm). The custom-made rotor's diameter (100.2 versus 111.7mm) and length (200.2 vs. 211mm) are smaller, and its center distance is significantly shorter (67.3 vs. 75mm). To boot, it has a 3-inch slimmer profile compared to the R2300's.

Reworked intercooler bricks have a 23-percent smaller (by volume) clamshell design (versus typical tube and fin) with internal turbolizers that help increase heat rejection by 10 percent. Peak pressure is lower than before (9.2 versus 9.4 psi), but boost application is spread more evenly across a wider rev range than before. The supercharger fits deeper within the engine's V, said John Rydzewski, assistant chief engineer for GM small-block engines.

Getting enough fuel into the engine required the use of a completely redesigned fuel pump -- one of the biggest in the world, Rydzewski claims -- operating at 2,900 psi (20 MPa). The LT1, in comparison, operates at 2,175 psi (15 MPa). Fuel injectors are bigger, too, at 25cc/second flow.

Intake and exhaust ports are carryovers from the LT1, but the chambers have a 10:1 compression ratio volume (premium fuel required). There are titanium intake valves with adjusted seats and cast stainless (versus the LT1's non-stainless) exhaust manifolds. There's a different cam profile (longer duration, especially on exhaust side), yet the geometry of the valves are the same as in the LT1. Its forged pistons employ DLC (diamond-like coating) pins and lightweight machine-forged metal steel rods. The crankshaft is made from a 1538MV steel forging and has a tungsten insert crank balance. As for lubrication, all Z06s will have dry sump setups with 14-stack oil coolers in place of LT1's 12-stack.

Every LT4 engine will be built at either GM's Tonawanda Engine Plant or Bowling Green Performance Build Center. By the time production begins, the engine will have already been subjected to Lee's "brutal" test regimen.

See All 16 Photos16

"We have brutal durability tests for the LT4 and the LT1," he explained. "We went wide-open throttle for literally hundreds of hours to ensure the structure was durable, the components were durable, and that nothing breaks. In addition, we run abusive thermal cycling tests where we cycle the cooling of the engine from ambient temperature up to max operating temperature and back down to ambient temperature, running hot and cold fluid through the engine while it's running wide-open throttle, then back to idle. Some of those tests take as long as 12 weeks to complete. It's a very exhaustive suite of durability tests. And we are proud of the fact that the small-block has a history -- a legendary history -- of being very durable in the field.

"If you think it's easy to make sure structures are capable of maintaining 650 horsepower, wide-open throttle, for hundreds of hours, it is brutal. It's a brutal test on the components. We found out that it's even more of a brutal test on the dyno hardware. We broke a lot of parts on the dynamometer, and we joked amongst ourselves that it became not only an engine durability test, but it became a dynamometer and a dynamometer setup durability test."

When asked about its performance capabilities, Juechter mentioned just one thing: "We expect the vehicle to be extremely quick. It set the lap record out here on the MRC (Milford Road Course) on its very first outing. When you experience the car, this car is a marvel of low-end torque -- 650 foot-pounds of torque instead of 604 for the outgoing ZR1 engine. After you get in the LT4 for a while and get back into the ZR1, it feels kind of sluggish, which is kind of hard to imagine given the ZR1's performance capabilities. But it really does feel that way."

We can't wait to feel it ourselves.

See All 16 Photos16