When the LS7 V-8 in the current Z06 Corvette debuted a couple of years ago, it was deemed to be the pinnacle of what could be achieved from a normally aspirated small block. The displacement had been boosted to 7.0L with an output of 505 hp and 470 lb-ft of torque. Although more power is certainly possible, it would have come at the expense of long-term durability, and since GM won't release an engine for production unless the design is validated for at least 100,000 miles, the LS7 approach was deemed a dead end for now.
Getting past that 500 hp wall with this architecture would require forced induction provided by a new Eaton Rootes type supercharger. However, the pressures generated by going much past 500hp would have been too much for the 7.0L block. In order to get the necessary robustness to make the engine last, the GM Powertrain engineers reduced the displacement back to 6.2L in order get the cylinder wall thickness they needed. The block was also beefed up in other ways. Fillet radii (a fillet is the curved surface at the joint where two other surfaces meet) were increased in the block bulkhead area improving strength by 20% even as passages were opened up to improve fluid flow.
When cylinder heads are bolted to an engine block and torqued down, the cylinder bores undergo some deformation. That's part of why pistons have to be smaller than the bore, and spring loaded rings are used to follow the cylinder wall and provide a seal for the combustion chamber. The production tolerances inherent in manufacturing all these parts leads to loss and inconsistency of power delivery.
To help overcome this problem, a deck plate is used during the boring and honing process of the block. A steel plate is bolted to the block and torqued down to distort the bores of the cast iron cylinder liners prior to machining. When the plate is removed the block relaxes, but the distortion caused by installing the cylinder heads restores the bores to their condition when they were machined. The end result is bores that are much closer to being parallel and round allowing a tighter piston fit and more consistent performance. The bore and stroke of the LS9 come out to a nicely over-square 103.25mm x 92mm.
The heads themselves are based on the design from the L92, LS3 and LS6 engines with a few modifications. A wing cast into the intake port induces some swirl in the air/fuel mixture as it passes the 55.0mm titanium intake valves. The spent gases exit past sodium filled exhaust valves on their way to the same fabricated dual wall stainless steel headers used on the LS7.
The heads themselves are made from a premium A356-T6 aluminum alloy that is rotocast. The rotocasting process rotates the mold as the aluminum is poured in. This helps get gas bubbles out of the mold and the molten metal, reducing porosity and increasing ultimate tensile and yield strength. The material change provides a 15% increase in strength with a further 9% coming from the rotocasting. The same process is used on the heads for the 2.0L turbocharged engine used in the Sky Red Line and other vehicles.
Like the block and heads, the reciprocating parts of the LS9 have to withstand tremendous pressures. The LS9 has a 9.1:1 compression ratio along with 10.5 psi of boost pressure. The pistons are forged from aluminum with polymer liners on the skirts to reduce friction. The tops of the pistons are sumped to allow clearance for the valves without the use of valve pockets. The continuous surface helps to improve the strength of the piston, which is further enhanced by anodizing. More titanium shows up in the connecting rods, which add strength with lower reciprocating mass. At the bottom end, a forged steel crankshaft is held in place by six bolt steel main bearing caps.
Generating power in an engine requires air and fuel. More power takes more air and more fuel. In this cast, the extra air comes courtesy of the latest 6th generation Eaton Rootes supercharger. The new rotors now have four lobes and there is significantly more twist. The old unit had 60 degrees of twist while the new version has 160 degrees. The result is much improved efficiency and quieter operation.
The LS9 supercharger has 2.3L of displacement and at its maximum speed of 15,000 rpm draws about 80 hp as compared to 120 hp for a gen 5 supercharger. The radiated noise from the blower case has been reduced by 10 dBa.
This is the first known use of the 6th generation Eaton supercharger by an OEM in a production application. Roush uses the unit on some of its vehicles and also offers various sizes in the aftermarket, including the 2.3L unit from the LS9, a 1.9L and a 0.9L for smaller engines.
The fuel side of the equation is taken care of by a dual pressure fuel system. Getting smooth low speed operation out of such a powerful engine requires a lot more dynamic range from the injectors than is currently available. At maximum output, the fuel system needs to be able to supply 58 g/s. Injectors that can supply enough gasoline to feed an engine with this much power tend to be hard to control at the low flow rates required at idle.
To help achieve this, a separate fuel system ECU is mounted next to the fuel tank. Based on demand it can adjust the fuel pressure to either 600 kPa at high speeds or 250 kPa at low speed. With the lower pressure, the flow rate at the injectors can be more easily modulated. By way of comparison, the LS7 system only provides 40 g/s of fuel flow.
This same issue is why the the LS9 is not currently set up for flex-fuel capability. The high octane characteristics of E85 would be well suited to an engine like the LS9. However, according to Sam Winegarden, the higher fuel flow rates required to make up for the lower energy density would have had too much of a negative effect on low speed drivability.
Packaging was a major challenge for the LS9 engineers. They only had one extra inch in height to work with in the ZR1 engine compartment and less than that in length. In order to make everything fit, the air to liquid inter-cooler uses two heat exchangers spread apart with the air flow going horizontally and then back down into the intake ports.
The inter-cooler lowers the temperature of the intake charge by 140 degrees F. Because of the limited space, there was no room to add a separate drive belt for the blower, so a revised two-belt setup was devised. An 11 rib belt drives the blower, water pump and power steering pump. To withstand the added loads, the water pump also gets a beefier bearing.
The pressures generated by the blower and combustion displace the cylinder head by up to 16 microns. Since each of the two active layers of the multi layer head gasket in the LS7 can take up 5 microns, the LS9 gets a gasket with 4 active layers. Also helping to keep the cylinder heads tightly sealed to the block are larger 12mm head bolts (the LS7 uses 11mm bolts).
Keeping all the metal parts moving smoothly requires a lot of lubrication. The LS9 keeps the LS7's dry sump system but with some enhancements. An extra gravity-fed 2.75 qt reservoir ensures that the oil pickup never goes dry even under the most severe cornering conditions that the ZR1 can generate.
A dual gerotor oil pump carries over from the LS7 with one rotor pumping oil to the engine while the other scavenges the sump, all the while oil temperatures being regulated by an oil cooler mounted on the side of the sump. A new feature of the LS9 is the block-mounted oil squirters. These units mounted below each cylinder barrel shoot a spray of oil toward the underside of the pistons. This has the dual effect of cooling the pistons and increasing the lubrication between the piston and cylinder reducing both friction and noise.
All these precision machined parts give the LS9 a red-line of 6,600 rpm and an output of at least 620 hp and 600 lb-ft of torque. That's enough to push a 3,340 lb ZR1 to 200 mph and beyond. The use of the supercharger gave the engineers more flexibility in tuning the cam-shaft to improve low end drivability of the LS9 compared to the LS7.
The LS9 cam has less lift and 27% less overlap than the LS7, which provides a steadier idle. The twin-plate 260mm clutch has more than enough capacity to handle the LS9 prodigious output while requiring the same or less effort than the single plate 290 mm unit in the Z06. The Tremec 6060 six-speed gearbox has a 2.29:1 first gear ratio and 3.42:1 final drive, a combination that allows it to make the all important 0-60 mph run without having to shift to second gear. The revised shift linkage takes 20 percent less effort to move over a 12 percent shorter distance.
The LS9 won't be getting its final certification until about March, but so far GM has put over 6,800 dyno hours on the new engine. They have run an engine for 100 hours continuously at wide open throttle and done both simulated and real 24-hour track cycles similar to what the GT1 C6.Rs run on the dyno and in the car.
Thanks to the extensive simulation and modeling done prior to building any parts, the engines have been remarkably reliable. To date, through three years of development, they have never had a failed piston, connecting rod or crankshaft in testing. The simulations have gotten so accurate that the first prototype engine on the dynamometer ran within 3.5% of the simulation throughout the entire power and torque curve.
The LS9 may very well represent a high water mark for production Corvette engines. According to Powertrain VP Tom Stephens, new fuel economy and CO2 regulations may prevent GM from going much beyond what has been achieved here, but for now it's too early to say. At some point the LS9 could see the race track, as well.
For now, though, all the LS9 engines will be built and hot-tested at the Performance build center in Wixom, MI before getting shipped down Bowling Green KY to become the new heart of the King.
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