The C7 Corvette Z06 was always gonna happen. Back in 2008, even when the U.S. economy was down a few cylinders and the C6 ZR1 had yet to start production, Tadge Juechter, Corvette chief engineer, knew he wanted an engine above LT1 for a track package on the C7 Corvette. By 2009, the project was underway. The development team, led by Jordan Lee and John Rydzewski, considered all options, but the lightweight, compact, power-dense small-block was a shoo-in. It was the best option for packing enough power under the Corvette’s low-slung hood, and the recent addition of direct injection opened up even more room to make power.
Because the engine was destined for the Z06, engineers were trying to replace the LS7, so the initial target, shaped in part by 0–60 and quarter-mile goals, was a bit more modest than 650 hp. When the supercharger development came through with tremendous airflow numbers, engineers knew they’d be able to blow the LS7 away and spent time tuning the camshaft to fatten up the powerband, where drivers spend most of their time. The result is an engine that beats the LS7’s peak torque by 1,500 rpm, beats the 911 Turbo’s 520hp peak output by around 4,000 rpm, and has a 50–75-lb-ft advantage over the C6 ZR1’s LS9 for much of the powerband. Right out of the box, in 2013, the Z06 was breaking C6 ZR1 lap times on GM’s test track before suspension and tire tuning even got into full swing. When it was happy with the results, Chevrolet announced the Z06’s supercharged LT4 produces 650 hp at 6,400 rpm and 650 lb-ft of torque at 3,600 rpm. Here are the engineers and the components that got them there.
SHORT-BLOCKAccording to John Rydzewski, assistant chief engineer for small-block V8s, the Gen 5 LT1 was a great foundation for building the LT4. “The block is very robust, we didn’t have to change much.” In this regard, the LT1 and LT4 sharing architecture with GM’s trucks is an advantage. In a truck towing a trailer, an engine could be at wide-open throttle (WOT) for 10 minutes at a time or more, and the blocks are designed to handle that sort of stress. How long could a Z06 go at WOT without running out of road or attracting a lot of law-enforcement attention?
One issue that faced engineers was equalizing the pressure across different parts of the crankcase. The Gen 5 block has bulkhead breathing cavities just above the cross-bolts on the nodular-iron main caps to allow air to circulate between cylinder banks, yet that proved to be insufficient on the LT4. Because all oil drainback feeds through the center of the block, differences in crankcase pressure can prevent oil from draining properly, causing the PCV to suck up oil rather than crankcase gases. “It’s critical that we get oil out of the valley,” says Alan Rice, the design responsible engineer for ventilation and lubrication. The solution was simple: two holes, roughly 5⁄8 inch in diameter, were drilled into each lifter valley. A PCV separator, which is unique to the LT4, was developed to keep oily air from being drawn through the intake, where it could end up collecting on the back of the intake valves. A little bit of oil on a port-injected engine can help lubricate valves, but because all Gen 5 V8s are direct injected, there’s no fuel washing the back of the intake valve. That means oil in the PCV system can end up sticking to the back of the hot intake valves impeding airflow and eventually preventing the valves from seating properly.
We spoke with Mike Garza, design system engineer for the block, crank, cylinder head, valvetrain, and lubrication system, who told us that a forged LT1 crankshaft would fail during extreme dyno tests when subjected to LT4 levels of power, so they engineered a unique crankshaft. Compared to the LT1, it has increased rolling loads (the amount of force applied to the crank fillets). Steel wheels are rolled over the fillet at an angle to extend the fatigue life. Because the cranks were failing at the same spot at the rear of the block, the lightening hole at the last rod journal was eliminated and tungsten slugs were added to the No. 8 counterweight to compensate.
CYLINDER HEADS AND VALVETRAINThe LT4’s cylinder heads are very similar to the LT1 because the two were developed in tandem. More than 6 million hours of engine analysis went into the Gen 5 engine, with Computational Fluid Dynamics (CFD) used to design the intake and exhaust ports digitally before they were ever cast in aluminum. The results are ports and combustion chambers that are efficient and flow a tremendous amount of air, even more than the LS9. The design of the combustion chamber gives the charge-air-mixture motion that ensures combustion starts in the center of the chamber. Because of the increased air volume added by the supercharger, compression is 10:1 compared to 11.5:1 in the LT1.
The valve angles are the same as the LT1, at 12.5 degrees for the intake and 12 degrees for the exhaust, allowing the LT4 heads to use a lot of the same machining as the higher-volume V8s like the 5.3L and 6.2L Ecotec3 truck engines found in the Chevrolet Silverado, Tahoe, and Suburban, as well as their counterparts at GMC. Simply put, without the volume of the truck V8s and the economy of scale that affords the Corvette team, the C7 would have been a lot more expensive. What makes the LT4 head different are its valves, valve seats, and composition. Unlike the LT1 heads, which are cast in 316 aluminum like the block, LT4 heads are rotocast using 356T6 aluminum alloy to better withstand the increased combustion loads. The LT4 also uses titanium intake valves and corresponding valve seats to get reciprocating mass down.
The LT4 has the same 6,600-rpm fuel cut-off as the LT1, so the two engines use the same valvesprings. However, because the intake charge is under boost, the LT4 uses less lift, 0.472 versus 0.511 inch on the LT1. The supercharger ensures the engine is ingesting much more air and fuel compared to the LT1, so the exhaust got a 17 percent boost in lift, from 0.472 to 0.551 inch, and more duration to ensure it has enough time to escape the cylinder. The 0.551-inch lift on the LT1 intake and the LT4 exhaust is no coincidence; it’s the limit of the Active Fuel Management (AFM) lifters. The AFM system allows half of the engine to shut off by controlling oil to the four sets of lifters. With the lifters collapsed, the cam’s lift isn’t transferred to the valves and fuel is also shut off for those cylinders while the rest of the engine continues to run as a V4. The LT4 was even tested to operate in V4 mode under boost, but it was not found to be more efficient that way. With a small frontal area, low-drag bodywork, and a tall cruising gear in both the seven-speed manual and eight-speed automatic, the Z06 is able to maintain highway speeds and up to 3,000 rpm in AFM mode for fuel economy unheard of in a car at its performance level.
LUBRICATIONCalibrated with a spring that determines the pressure curve, an active displacement oil pump unique to the LT4 helps increase efficiency. For normal operation, the pump provides 44–58 psi of oil pressure. As engine speed goes up, bearings require more oil, so pressure increases to the 65–72-psi range. In case of spring failure, the pump defaults to the higher pressure.
Adding power adds heat, and not just in places you’d expect it. Oil squirters help cool the piston from underneath, which means more heat is absorbed by the oil. The LT4’s oil pan has a cooler, bypassed at low oil temps, that’s fed from the radiator’s output. Compared to the LT1, the LT4’s oil cooler has more cross-sectional area for a 20 percent increase in cooling. The LT1 is also offered with dry-sump oiling, and the LT4’s system is similar with both engines using a salt-core, sand-cast pan with oil passages cast in place. The LT4 uses a single pickup for the scavenge side that runs to the scavenge pump. From the pump, oil is routed to the tank, where the lift tube has a centrifugal oil/air separator. Oil control is always critical to engine longevity, and due to the Z06’s ability to create up to 1.2 g’s in lateral acceleration on some tracks, the development team put the dry-sump oil pan through grueling tests. Before the engine was ever put on a track or a test stand, however, the pump parameters, bearing sizes, bearing clearances, and flow restrictions were all modeled in a computer.
SUPERCHARGER AND INTERCOOLERIn order to fit the supercharger within the confines of the C7 Corvette and still meet pedestrian safety regulations for vehicles sold in Europe, an intercooler design like the one found on the LS9 wasn’t going to work. There had to be area under the hood available for the bodywork to deflect and absorb impact before hitting anything solid. To keep the sleek lines of the C7 Corvette intact and still give the driver a great view of the track ahead, engineer Dan Hommes was tasked with removing 3 inches of height out of the supercharger and intercooler assembly without losing airflow or power. In engineering terms, that’s quite a challenge, as the LS9 was already very compact.
“The airflow that we were able to produce is just incredible” — Dan Hommes
In the LS9, the intercooler heat exchangers are in the lid, the only place to put them because the Eaton TVS 2300 supercharger rotors take up a lot of room in the lifter valley. The rocker covers weren’t going to change, so that meant the supercharger couldn’t get any wider. The solution was to use supercharger rotors 10mm smaller in diameter. The existing TVS 1650 rotor size was not quite large enough for the power levels the LT4 team was targeting. To get more air volume, Eaton built a longer set of TVS 1650 rotors that displace 1,740 cc per revolution, a size unique to the LT4. The smaller-diameter rotors mean less air per revolution, but the reduced mass and inertia allow them to turn 34 percent faster than the 2300 rotor set. In the LS9, max supercharger speed was 15,080 rpm; on the LT4, the rotors spin at 20,150 rpm at max engine speed. An added benefit of the smaller supercharger rotors is that less torque is required to drive them.
The smaller rotor set allowed the intercooler heat exchangers to move out of the lid and down between the rotors and the rocker covers. The LS9 intercooler heat exchangers were made using a tube and fin construction that featured flattened tubes without any internal structure, like a radiator. For the LT4, Chevrolet used intercoolers made by Dana Thermal Products that use clamshell-stamped plates that use a turbulizer plate—essentially a fin inside the plate—to vastly increase the surface area the coolant comes into contact with. The heat exchanger has 15 fins per centimeter—that’s 50 percent higher density than on the LS9. So even though it’s 24 percent smaller, the intercooler rejects 10 percent more heat than the LS9. On a typical 68-degree day, the discharge air from the supercharger can reach 248 degrees. By the time it passes the heat exchanger, those charge air temps are down to less than 120 degrees.
We asked how the LT4’s intake operates when it’s not under boost, and whether or not the intercooler impedes airflow. Dan told us he initially thought the small plenum volume might be an issue, yet even when not under boost, distribution isn’t a problem and the front cylinders fill just fine. As for the heat exchanger, we learned that in order for the exchanger to efficiently cool the air, there has to be a pressure drop. At peak boost, the pressure-drop requirement is only 0.5 psi, and when it’s not under boost, there’s little restriction because there’s just less air.
The intercooler packaging wasn’t the only detail that was improved over the LS9’s design. The intake airflow path was developed using computer simulation, with a goal of minimizing restriction. The end design has volumetric efficiency of 92 percent, up from 89 percent on the LS9. Combined with the cylinder heads ability to flow more air, the LT4 is able to make more power than the LS9 with less boost in the middle of the powerband, while peak boost is similar at 9.71 psi. Remember, boost is just an indication of how much restriction the supercharger is facing when trying to move air through the engine. In total, the LT4’s supercharger is more efficient, more powerful, more compact, and 20 pounds lighter.