It’s official: the 2015 Corvette Z06 is GM’s most powerful production car ever, rated at 650 horses and 650 lb-ft of torque. Comparisons are inevitable, but while it’s tempting to contrast the new LT4 with the C6 ZR1’s supercharged LS9 engine, the differences between the Gen IV and Gen V small-blocks—and in particular their combustion systems—make this less of an apples-to-apples match-up than it might at first seem. Suffice it to say that with the same displacement, a smaller supercharger and a slightly higher compression ratio, the LT4 makes more power and torque than did the mighty LS9.
More to the point is how General Motors’ powertrain engineers adapted the naturally aspirated LT1 foundation to generate some 40 percent greater output using forced induction. An all-new, ultra-efficient version of the Eaton twin-vortices supercharger design (see sidebar) delivers the air, but supporting the boosted airflow and the crankcase pressures it generates required a comprehensive makeover inside the cylinder walls—not only to help produce the power, but also to ensure that it is delivered with the civility and durability this daily-drivable supercar requires.
Cylinder block and rotating assembly
The LT1 and LT4 share the same 319 cast-aluminum cylinder block with 4.06-inch bores, as well as the small-block family’s signature 4.40-inch bore centers. Additionally, piston oil squirters are employed in both applications. The two engines also use a tough, 1538MV forged-steel crankshaft with induction-hardened journals and intermediate pin drills, but the LT4 features ground pin collars and heavy-metal (tungsten) balancing material.
Components differ among the parts attached to the crankshaft as well. Like the LT1, the LT4 uses 6.125-inch-long forged, powdered-metal connecting rods, but they’re highly machined for greater strength and reduced reciprocating mass. They have the same balance mass as the LT1 rod but a higher load capability, with exclusive features including machined lightening slots and a “stepped” pin end with a premium bushing.
The LT4 also uses unique forged-aluminum pistons with a structure designed to cope with the more intense cylinder pressures that come with forced induction, including strength-enhancing internal ribs similar to those used on the LS9 pistons. A flat crown, or head, helps reduce compression from the LT1’s 11.5:1 ratio to a more boost-friendly 10:1.
Other LT4-piston exclusives include a unique ring pack designed for greater wear resistance and durability under extreme conditions. It comprises a PVD-coated top ring, a chrome-coated second compression ring and a nitrided oil-control ring. The bottom ring land also features eight oil drains. Friction-reducing skirt coatings are used on both the LT1 and LT4 pistons.