The 2015 Corvette Z06’s LT4 V8

650 hp is the New 505 hp

Brandan Gillogly Aug 20, 2014 0 Comment(s)

FUEL SYSTEM

Yoon Lee was the only member of the team developing the LT4 that also directly worked on the LS9. His focus was the fuel system, and he’s proud to have engineered the largest direct-injection (DI) gasoline pump in the industry. Yoon’s description of how they got the pump to deliver extra fuel to feed the increased demand from the LT4 was right up our alley: “We bored and stroked it.”

Chevrolet LT4 Yoon Lee 1/38

Yoon Lee holds the LT4’s DI fuel pump, the largest of its kind for a gasoline engine. It was “bored and stroked” compared to the version used on the LT1. It’s so precisely machined that the plunger doesn’t use any sort of rings.

The DI fuel pump has a 26 percent higher displacement, thanks to a 1mm-larger-diameter plunger and 0.3mm more lift on the cam. The development team considered a high-pressure fuel pump driven off the accessory drive, but that idea was shelved in favor of a more compact, more reliable cam-driven pump. On direct-injected DOHC engines, there are a lot of places to mount a pump, but the single-cam Gen 5 small-block didn’t leave many options. Mounting a pump at the front of the block would have meant extending the block due to the position of the timing chain and cam phaser. That’s not something you consider lightly, as the compact design is a small-block trademark. Instead, the fuel pump was mounted at the rear of the block, and there’s a hole in the lifter valley right where you’d find a distributor in a Gen 1 small-block. The billet-steel pump is bolted to the block with 8mm bolts and uses a roller follower just like a valve lifter. There’s even a set of dual springs with a beehive outer spring. In fact, the VVT cam phaser mechanism had to be tuned differently on the LT4 because spring pressure on the pump created an additional load on the valvetrain.

The in-tank electric pump pushes fuel to the cam-driven, direct-injection pump at around 70 psi. Fuel goes through a pressure damper, then into the pump where a plunger increases the pressure up to 2,900 psi. Fuel exits the pump and goes to the feed pipe, then to the crossover, and into each fuel rail. Compared to an LT1, which operates at 2,175 psi, the crossover pipe and supply pipe are a little larger in diameter to compensate for pressure pulsations. The injectors are the largest GM uses in DI applications and flow 14 percent more fuel than the injectors used in the LT1. The injectors use two O-rings to seal against cylinder pressure and feature a burly injector clip keeping it in place. “The mounting bosses are much beefier.” Unlike a diesel engine that can use multiple injections during the compression stroke, the spark-ignition direct-injection LT4 uses one injection on the intake stroke. The high fuel pressure helps make a fine, uniform spray that more evenly atomizes the fuel and is aimed at the bowl at the top of the piston, timed so that mixture is right at the spark plug.

Lee explained they’ve found that three lobes on the fuel pump cam seem to be optimal. With more than three lobes, there isn’t enough time to fill the pump. There’s also not a lot of room to add more lift, and if there were coil bind causing the plunger to bottom out, that force will transfer directly to the camshaft.

We asked Lee how much power was left in the factory fuel system. He told us that the limit is the DI pump, not the in-tank pump. Because the factory tune has to keep emissions components longevity in mind, they enrich the air/fuel ratio a bit, meaning there’s some “cushion” beyond the current 650hp rating, although there’s not a whole lot of room for a factory tune to eke out more power. However, he knows there are ways, adding “the aftermarket will do what the aftermarket will do.” By the way, the folks at Lingenfelter are already privy to the part number of the LT4’s Stanadyne direct-injection fuel pump.

When posed with the same question, Jordan Lee told us there’s still some power to be had: “I think there’s more in that supercharger.” John Rydzewski, who admitted that some early iterations of the LT4 were producing around 680 lb-ft of torque, was also free to admit where to find more power: “Open up the exhaust system and work with the camshaft.” If it weren’t for the stringent emissions requirements, “There’s a lot of room to move around on the camshaft.”

650 HP AND BEYOND

With the 707hp Challenger claiming the bragging right for the baddest supercharged V8 from the Big Three, Chevrolet fans might be looking for a hint at more to come to retake the throne. Just keep in mind that the 650hp LT4 is what happens when Chevrolet needed to replace a 505hp engine. Chevrolet had every right to call this car the ZR1, yet it didn’t. What do you think Tadge and his team are working on now?

Chevrolet LT4 Steve Kiefer 2/38

Steve Kiefer’s not new to engines or the Corvette; he worked for Pontiac 30 years ago and built the LS7 that’s in his current car as a part of the Corvette Engine Build Experience.

STEVE KIEFER

Our visit at GM’s Powertrain facility in Pontiac, Michigan, included some face-to-face time with Steve Kiefer, vice president of global powertrain for General Motors. Keifer started working for GM in the very same building 30 years ago. Back then, he was focused on fuel efficiency and emissions on four-cylinder Pontiac engines. He’s now responsible for GM’s engine and transmission development around the world.

We asked Kiefer how GM plans to keep the small-block alive, and he reminded us that it has remained competitive because engineers keep reinventing it and adding new technology. The challenge, Keifer said, will be to keep pushing the envelope in efficiency. In low-volume niche applications, it will continue to have tremendous performance, while the high-volume engines will deliver efficiency. “There’s good life for the small-block,” Keifer said.

“By the time we’re done with this upgrade, this company will have the best powertrain development facilities on the planet, by far.” — Steve Kiefer

What Keifer was most excited about was the upcoming changes to GM’s Global Powertrain Engineering Headquarters in Pontiac, Michigan. The huge facility is already home to a vast array of engine dyno cells, tilt stands, and about 3,800 employees—and more are coming. In early 2013, GM announced plans to expand the facility to house the majority of their racing development. Almost every form of motorsports that GM competes—Cadillac in Pirelli World Challenge Series, C7.R, COPO, NASCAR, everything but IndyCar, really—will be under one roof.

Keifer would like the change to make the whole facility feel more like a race team. GM has long been rotating its race team engineers into the teams that develop the next generation of production engines, and moving the race development and production engine development together will allow more opportunities for engineers to cycle through the race program. While it won’t be a requirement to work on a race team, the program helps GM draw talented engineers to their programs. “I really want to see a lot more of our young engineers cycle through racing,” Keifer said. “Imagine getting hired right out of school and getting the opportunity to travel around the country with a race team.”

Chevrolet LT4 Jordan Lee Yoon Lee John Rydzewski Steve Kiefer 24/38

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