New GM V-8 Proves Pushrods Still Rock

For decades, auto pundits and enthusiasts have dissed cam-in-block pushrod engines as "old-school" and "low tech." Indeed only a few survive: Chrysler's venerable Hemi V-8 (a past Ward's 10 Best Engines award winner) and GM's six-decades-old-and-counting "small-block" V-8 (and related V-6) family.

And, guess what? The latter's all-new, all-aluminum Generation V small blocks retain that configuration – for very good reasons.

Think about this: If all engines since the beginning of time had used overhead camshafts to activate their valves, and someone came up with the idea of doing that from a single cam buried in the block of a V engine, it would be heralded as brilliant.

Why? For starters, it would reduce a same-displacement engine's size, weight, cost and complexity. It would be quicker and cheaper to build and service. It would also lower the engine's center of gravity, very important for sports cars and race cars, such as Corvettes.

"I agree completely," says Jordan Lee, chief engineer and program manager for GM's small-block engines. Yet he says designing a modern pushrod V-8 is much more challenging.

"I can state that unequivocally, because I have done both," Lee says.

The cylinder-block design of a CIB engine is much more complicated, with extra oil galleries, lifter bores and machining, he says. Plus, the structural integrity required for the different load parameters, including springs, valvetrain components, cam bores and main bearings that have to be fed oil, and all the lubrication and ventilation-system work that's required, make it very complex.

The cylinder block for a DOHC V-8 is simple in comparison, Lee says. “It has one oil gallery feeding the mains and you dribble a little oil up into the cylinder head for the overheads. That's about it."

He adds that Corvette chief engineer Tadge Juechter wanted the smallest, lowest, most power-dense engine possible for his seventh-generation car.

"We looked at every possible option," Lee says, "including a DOHC V-8. But with 4 ins. (102 mm) more engine height, the car's center of gravity would have been higher, and it would not have performed the way he wanted. We ended up back with the proven 90-degree small-block design because it achieved the power, torque and fuel efficiency in a very small package size."

On the negative side, overhead cams offer higher rpm capability and peak output from a given displacement. Four valves per cylinder provide greater air flow than the Gen V small-block's two. But GM partially has overcome those disadvantages with variable valve timing (VVT) and Active Fuel Management (AFM) cylinder deactivation, which is easy to do in a CIB engine by hydraulically disabling individual pushrods. Cylinder deactivation is more difficult with an OHC layout.

"We're easily able to get 6,600 rpm," Lee says. "And we have very efficient airflow paths to make a lot of power. The LT1's specific output is less than, say, a 4.4L twin-turbo V-8 making similar power. But displacement is just one tool to achieve our goals. If our LT1 is physically smaller and lighter but makes more power and torque with better fuel efficiency, who cares that it's 6.2L relative to a (dimensionally) larger, heavier, thirstier 4.4L?"

Not WardsAuto editors, who unanimously rave about the ’14 Corvette's 460-hp V-8, calling it silky smooth, with an exhaust note like "a sip of 15-year-old single malt."

Adds editor Drew Winter: "If we are going to have a fire-breathing V-8 on the list this year, this should be it, not just because it is a beast but because it is a very fuel-efficient beast."

WardsAuto editors also heap praise on the 6.2L siblings, the 4.3L V-6, 5.3L V-8 and 6.2L V-8 EcoTec3 which power GM's all-new ’14 pickup trucks and ’15 SUVs. A fifth variant, the mighty 625-hp 6.2L supercharged LT4 V-8, will arrive later this year in the ultra-high-performance ’15 Corvette Z06.

While it pumps out a pavement-wrinkling, SAE-certified 465 lb.-ft. (630 Nm) of peak torque in the standard ’14 Stingray, this new LT1 V-8 is surprisingly fuel efficient at 17/29 mpg (13.8-8.1 L/100 km) city/highway.

"We had very stringent efficiency targets for both the trucks and the cars," Lee says. "And we wanted the highest-performing Corvette possible. Tadge challenged the engine team to leapfrog in output from one generation to the next. So the base engine has similar performance to the last-generation Z06, and the Z06 will have the performance of the last-generation ZR1."

He adds that the manufacturing system for these engines also is new and less automated than in the past. "All of the machining operations are done by CNC equipment, and a lot of assembly operations are done by manual operators. We have a lot more craftsmen putting them together, yet we're able to do it more efficiently."

Efficiency Targets Toughest Challenge

Key suppliers include Nemak for the aluminum blocks and head castings; Eaton for valves, lifters and the LT4's supercharger; Federal-Mogul for the pistons; and Stanadyne for the high-pressure fuel pump.

The toughest challenge was meeting the efficiency objectives. "We had lofty goals and achieved them with a lot of painstaking work and analysis. The classic image of a V-8 is big and thirsty, so we wanted to dispel those notions by making our Gen V engines extremely efficient. Refinement was a big deal for us as well."

Lee adds that direct injection, the key technology that all the others were built around, allowed engineers to raise the compression ratio to improve power, torque and fuel efficiency. "Coupling that with AFM," he says, "we can run in 4-cylinder mode a lot more than in past generations because we have a significant improvement in low- and midrange torque."

And he asserts GM is one of the very few manufacturers in the world to combine DI, VVT and AFM in all variants of an engine family. "We were adamant that we needed all three technologies in all of the engines because we wanted to offer the best fuel economy without customers having to pay extra for it.

"The hallmark of the small-block engine is its power density in a small package size, and we are extremely proud to have one of the smallest and lightest 460-hp engines in the world today. Compared with the BMW 4.4L twin-turbo V-8, for example, our LT1 is 40 lbs. (18 kg) lighter and 4 ins. (102 mm) shorter in height, with more power and torque and better fuel efficiency."

And he says there is a lot of "headroom" left in this architecture for future improvements in both output and efficiency: "We're working on new designs and technologies as we speak, including higher compression ratios, further combustion-system improvements, the ability to blend more AFM time and maybe even more cylinders in AFM."