GM'S LS9 V8: Life Under Pressure
Moving from the LS7 to the LS9 required a lot of detail work, and surprising few new parts.
Sam Weingarden, executive director, GM Powertrain Engineering, says his goal with the LS9 motor was "to make Corvette chief engineer Tadge Juechter's life miserable by giving him more power than the chassis could handle." Doing so was not going to be easy as pulling more power from the small block V8 would demand forced induction, a potential packaging problem given the Corvette's tight underhood area. For this reason, as well as that of heat rejection, turbochargers were considered and abandoned, leaving supercharging as the only answer. Despite carrying over 76% of the LS9's parts from the Corvette Z06's LS7 engine, the 638-hp engine required 100 new parts. Surprisingly, 25% of those were from other small block applications.
Eaton's Twin Vortices Series supercharger has twin four-lobe rotors twisted 160 instead of the expected three lobes twisted 60. This creates a smoother flow into the engine at higher volumetric efficiency for more boost in less space. Each rotor is extruded from aluminum bar stock and cut to length before a proprietary abradable powder coating is added. This coating improves rotor mesh as it wears away. Due to the low hood, the supercharger's discharge is brought up through the center of the intake and moved into separate right and left plenums, before heading down the intake ports. A dual-brick intercooler is integrated into the housing.
Cylinder pressures in the LS9 are about 13 MPa, or about 1,885 lbf/in.2, which requires a stout structural package. "The block is 319 T5 aluminum. It's the same casting used on all 6.2-liter blocks," says Dean Guard, chief engineer, GM Small Block/Big Block engines. However, small refinements in the bulkhead windows-since added to all 6.2-liter V8s-increased strength 20%. Each block is both deck-plate and bore-plate honed for improved piston fit and ring sealing.
The LS9 engine is validated to 100,000 miles and has over 6,800 test hours behind it. It has undergone 24 consecutive hours of track testing, and the last test engine put through a general durability run on a dyno went 841 hours without incident. Each production engine is balanced to within ½ ounce of zero on cold test to balance the front and rear of the motor. Since there is no dipstick due to the dry sump, 10.5 quarts of oil is added by weight to the 529.9-lb engine.
In order to supply the squirters, a larger dual-gerotor oil pump is used in concert with a larger dry sump system. An increase in the car's maximum lateral capability meant the LS7's system was no longer adequate, but a lack of mounting room resulted in the addition of a piggyback tank about half the size of the main tank. A computer analysis of data gained from hot laps around GM's Milford Road Course allowed engineers to watch the oil through an entire lap, but was so data-heavy that it wasn't possible to create more than a few seconds of the lap per day on the computer. "It took us the better part of a month before the model was to the point where we could do the full lap on-screen," says Guard.
The forged pistons are beefier than those found on the naturally aspirated 505-hp LS7 engine, which affects bearing film thickness, engine speeds, and combustion loads. High-strength titanium connecting rods with floating pins and a polymer wear reduction coating are part of the package. Squirters pumping oil to the underside of the piston are critical to keeping the piston crown cool. Their location in the block necessitated a redesign of the piston stuffers to keep them safe during piston installation. The head gasket has four active layers and three non-active layers. That allows it to handle the 20 microns of movement between the block and head. The head bolts are 12-mm stainless steel that, says Guard, "approaches tool steel grade." That's because clamping loads rose from the LS7's 68 kN to 87 kN on the LS9
The LS9 engine produces 638 hp and 604 lb-ft of torque. It pushes the Corvette ZR1 to a top speed of 205 mph, from 0-60 in 3.4 seconds, and through the quarter mile in 11.3 seconds. While Sam Winegarden, executive director, GM Powertrain Engineering admits that, “most of the time car guys are pretty proud of going from zero to 60 in seven seconds,” he says the ZR1, “will have hit 100 mph by that time.”
The cylinder heads are made from 356-T6 aluminum, and each mold is rotated as the molten metal is poured. "This," says Guard, "gives nominally better mechanical properties, reduced variation, and the best mechanical properties on the deck face." The heads are sent to GM's Performance Build Center in Wixom, MI, in individual plastic bags for assembly.
The forged crankshaft is similar to the LS7's, though it uses nine bolts and a keyway to secure the flywheel instead of the LS7's six bolts. "The keyway stops any micro-motion and keeps the flywheel from spinning off if the full clamp load isn't reached for some reason," says Guard. "It's a best practice we'll be applying to all our engines going forward." It's doubtful that those engines will use the proprietary shaping method found on the LS9, which puts 708 hp through the crank. Each 50-lb. crank is forged flat, picked up at each end, and twisted into its final shape.
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