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755hp from the inside out, supercharged C7 ZR1 LT5

Deep Inside the LT5: The C7 ZR1 Packs the Most Powerful—and Power-Dense—Small-Block Ever

Barry Kluczyk Mar 6, 2018
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You can opt for a 420hp engine in a Tahoe, but it’s worth remembering that it was only a little more than 10 years ago when the C6 Z06 stunned the world with 505 race track-derived horsepower from the 427-inch LS7 engine. And it has been less than 10 years since the C6 ZR1’s supercharged LS9 engine brought down the house with 638 horsepower.

Time and technology march on. Today, the 2019 Corvette ZR1 offers a stunning 755 horsepower and equally impressive 715 lb-ft of torque in a take-no-prisoners package that rivals the Millennium Falcon in hyperspace jumping capability. Along with trumping its predecessors as the most powerful production engine ever for Corvette, the new, supercharged LT5 represents the pinnacle of small-block engineering and performance.

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“The small-block’s legacy is rooted in more than 60 years of continuous engineering advancements that have brought performance achievements that couldn’t have been dreamt of when it was conceived,” says Jordan Lee, General Motors’ chief engineer for the small-block. “And while there have been plenty of great small-block engines over the decades, the new LT5 tops them all in terms of output, engineering and technology. It’s the ultimate small-block for the ultimate Corvette.”

When originally offered in the 1955 Corvette, the optional 265-cubic-inch small-block was rated at 195 horsepower, for a power density ratio of 0.73—or 0.73 horsepower for every cubic inch. The new LT5’s ratio is 2.00. That’s 275 percent greater than the original small-block.

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Hell, the 1990 ZR-1’s DOHC LT5 engine’s 375 horsepower netted a power density ratio of “only” 1.07, which was pretty much as good as it got back then. Of course, the 1955 small-block and the 1990 LT5 were naturally aspirated, while the all-new LT5 receives a figurative and literal boost from forced induction.

“It builds on the successful supercharging legacy established with the LS9 and furthered with the LT4,” says Lee. “Advanced technologies such as direct injection and supercharger efficiency improvements have enabled us to make the most of what forced induction can offer, thereby expanding the performance range of the engine to deliver exceptional power delivery across the rpm band.”

In a nutshell, the LT5 goes bigger with the blower to push more air into the engine. It was as if the engineers watched the classic Saturday Night Live skit after making their calculations for the engine’s target output and told Lee, “We need more cowbell.”

03 Chevrolet Corvette Lt5 Engine 4/23

That’s exactly what they got: an all-new, more efficient supercharger based on the same, effective four-lobe design as the LS9 and LT4 air compressors but larger. At 2.65 liters in displacement, it is 56 percent larger than the LT4’s 1.7-liter compressor and pumps out more boost.

Significantly, the larger compressor makes more boost while spinning a little slower than the LT4’s blower. That’s important because the pressurized air charge doesn’t get as hot before it hits the heat exchangers of the intercooling system, reducing the overall temperature all the way to the combustion chambers.

As for the intercooling system itself, approximately 30 percent larger “bricks” contribute to about twice the capacity of the LT4’s system. The larger supercharger combined with the larger bricks mounted above it, however, soak up some real estate and the consequently taller LT5 necessitates a taller hood. In fact, the engine cover mounted to the supercharger/intercooler assembly rises through an opening in the hood. And, yes, it torques from side to side with the movement of the engine.

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Of course, all that cool, dense air force-fed into the engine requires a commensurate amount of fuel to maintain a nice, happy and detonation-free equilibrium in the combustion chambers. And because they got what they wanted with the bigger blower, the engineers went back to Jordan Lee and asked for an extra set of injectors. Not just a replacement set, you understand, but an additional set of eight conventional, port-injection-style injectors to supplement the engine’s standard direct injection system, for a grand total of 16 injectors in the engine.

For most driving conditions, the engine operates solely on the direct injection system, with the secondary port injectors supplying additional fuel under heavy loads, particularly at wide-open throttle. The direct injection system simply maxes out at WOT and the engineers couldn’t find a suitable higher-capacity injector that met their needs. It’s the first dual-fuel system of its kind ever in a GM automotive engine.

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A second engine control module oversees the operation of the port injectors. There are more unique features in the engine, too, including the largest throttle body ever on an LS or LT engine, an electronically controlled bypass for the supercharger, specific main bearings and more, while the bottom end and cylinder heads are largely derived from the LT4. There’s also the requisite dry-sump oiling system pushing Dexos2 synthetic oil through the engine’s veins.

It’s all to support the Corvette ZR1’s supercar-slaying capability, with Chevy claiming 0-60 in less than 3 seconds, the quarter-mile in 10-second and a top speed of 212 mph for the coupe model. Regardless of the car’s real-world performance, the all-new LT5 engine establishes a new high-water mark for small-block engineering and specific output.

“There’s nothing like the shove in the back induced by a supercharged small-block,” says Lee. “And the new LT5 hits like a sledgehammer when you nail the throttle.” Vette

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The LT5 uses the same aluminum block as the LT1 and LT4, featuring cast-iron cylinder liners with 4.065-inch bores. Their 4.400-inch center distances—the measurement between the centers of the bores—has been a small-block design cue since the very first engine in 1955. The main caps are nodular iron and they’re complemented with LT5-specific H14 lead-based main bearings, which support the high loads and crankcase pressure generated by the supercharged engine.

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Like all other LT engines, the LT5 features jet-spray oil squirters, which douse the bottom of the pistons with oil to help reduce temperatures for optimal performance and durability.

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The 3.622-inch-stroke crankshaft’s steel forging is unique to the LT5, while the crankshaft-to-damper interface includes a new keyway to transfer the higher loads from the supercharger’s operation.

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Attached to the crankshaft is the LT5’s complementing and unique damper, which features a nodular iron hub with a steel inertia ring and grooves for an 11-rib drivebelt (the LT4 has a 10-rib belt).

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The forged aluminum pistons, used with floating wristpins, are the same as the LT4, delivering a 10:1 compression ratio, which is comparatively high for a supercharged engine. The precise nature of the direct injection system helps stave off the chance of detonation. A polymer coating on the skirts reduces friction and bore scuffing.

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Like the pistons, the connecting rods are the same as the LT4, but they feature specific bearings with a high-heat, high-wear G488 coating.

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The camshaft is designed to support high-rpm power under boost, but just as importantly, enable good driveability and power at low rpm, when the supercharger isn’t producing much boost. The lift and duration specs are 0.551/0.524-inch intake/exhaust and 200/207-degrees at 0.050-inch lift. The lobe separation angle is a comparatively wide 116.5 degrees. As with the LT1 and LT4 engines, it also features dual-equal phasing.

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Premium A356 T6 aluminum is used for the rotocast cylinder head casting, which eliminates porosity by rotating the head mold as the molten aluminum cools. The combustion chambers displace 65.47 cc, while the titanium intake valves measure 2.13 inches and the sodium-filled exhaust valves measure 1.59 inches. Hearty 12mm bolts clamp the heads to the block.

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This cutaway view of the engine shows the location of both injectors for one of the cylinders (arrows). The lower arrow indicates the injector for the primary direct injection system and the upper arrow calls out the supplemental port injector mounted in the supercharger manifold, which kicks in at higher engine loads.

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Here’s the new “R2650” supercharger, which is exclusive to the LT5. Displacing 2.65 liters, it produces 14 psi (0.9 bar) of boost. That’s about 4.5 psi more than the LT4 engine, but achieved with a slower, 15,860-rpm maximum rotor speed, which helps keep down the pressurized air charge’s temperature. The LT4’s 1.7-liter blower spins up to 20,000 rpm.

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Compared to the rotors in the LT4’s supercharger, the LT5’s are larger in diameter and have a new, 170-degree pitch for the four-lobe design versus the LT4’s 160-degree pitch. The higher pitch angle enhances the blower’s efficiency at high rpm, helping it sustain max boost through the top of the rpm band.

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With the lid removed, a look inside the supercharger case shows a truly massive, electronically controlled bypass valve, which manages the amount of pressurized air that enters the engine. It offers greater boost control and torque management than a conventional mechanical bypass. And talk about clever parts sourcing, it’s actually the 72mm throttle body used on the 6.6-liter Duramax turbo-diesel.

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A higher-capacity charge-cooling system matches the larger supercharger and reduces the pressurized intake air charge by about 140 degrees F (60 C). Like the LT4, it features a pair of heat exchanger bricks mounted longitudinally above the supercharger. They’re cooled by their own coolant circuit, with remote pumps and a 5.0-liter heat exchanger mounted at the front of the vehicle. The bricks are 30 percent larger than those on the LT4 engine, supporting approximately double the cooling capability of the LT4’s system.

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Only slightly smaller in diameter than the Lincoln Tunnel, the LT5’s 95mm electronically controlled throttle body is the largest ever used on an LS or LT engine.

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A couple of things to note here. First, the camshaft-driven, high-pressure fuel pump for the direct injection system supplies the injectors at a staggering 2,175 psi (150 bar). Secondly, the drive gears for the supercharger are visible and they’re thicker, heavier-duty gears than the LT4’s gears.

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The LT5’s exhaust manifolds are the same as the LT4 and are constructed of cast stainless steel. They’re designed on the principle that low restriction and less pumping loss equals more horsepower and torque. Analytical data was used to determine the proper sizing and length of the pipes.

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It’s not perhaps a cast of thousands, but literally dozens of engineers and other supporting staff contributed to the design, testing and validation of the 755-horsepower LT5 engine, which took about 3 years. They created the most powerful Corvette production engine in history, so hat’s off to the whole lot of them.

Photography courtesy of General Motors, Photographer by Tony Hufford

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