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.
Finally, the LT4 features a floating piston wrist-pin design, as on the LT1, but with a unique, diamond-like coating for added durability and lower friction.
The LT4 features a standard dry-sump system with a 10.5-quart capacity—the same setup that’s included on LT1 cars equipped with the Z51 package. (The LT4 does get a higher-capacity oil cooler.) Otherwise, the basic oiling configurations of the two engines are similar, which means they use a dual-pressure-control, variable-displacement vane pump driven by the crankshaft. This unconventional design ensures that the engine always receives a sufficient supply of oil, regardless of rpm.
GM recommends its Dexos semi-synthetic motor oil for use in all Gen V small-block engines; 5W30 is the recommended weight for the LT1 and LT4.
The LT1 and LT4 differ more dramatically in the valve-actuation attributes of their respective hydraulic roller camshafts. The naturally aspirated LT1 cam specs include 0.551/0.524-inch intake/exhaust lift, 200/207 degrees duration at 0.050-inch lift and a 116.5-degree lobe-separation angle (LSA).
The LT4’s camshaft, on the other hand, is designed to help process a much greater amount of airflow under boost, with a wider, 120-degree LSA that reduces overlap and opens the exhaust valves just a little earlier. It holds it them open longer, too—for 223 degrees of crankshaft rotation—and delivers a lift rating of 0.551-inch. The intake specs are actually reduced somewhat from the LT1: 189 degrees of duration and 0.492-inch lift.
Overall, the LT4’s camshaft is relatively mild, thanks to the torque-enhancing benefits of the supercharger. The result is smooth operation at low engine speeds, particularly at idle. The wider LSA contributes to that smoothness, while also helping to flatten the torque curve and hold power higher in the rpm band.
Both the LT1 and LT4 feature dual-equal camshaft phasing (variable valve timing), which works with Active Fuel Management to enhance fuel economy and maximize engine performance for given demands and conditions. A vane-type phaser is installed on the front of the camshaft to change its angular orientation relative to the sprocket, thereby adjusting the timing of valve operation on the fly. “Dual equal” denotes that the system adjusts both intake and exhaust valves at the same rate.
The different cam specs mean different timing and rates of phasing between the engines, but the mechanics of the system are shared.
Cylinder head and valvetrain
The LT4 features the same combustion-system design as the LT1, to support the direct fuel injection, but it uses stronger, more heat-resistant Rotocast A356-T6 aluminum cylinder heads in place of the LT1’s conventional 319-T7 cast-aluminum units. The Rotocast manufacturing process involves rotating the mold during casting to eliminate porosity and ensure a denser, more accurate component.
Given the LT4’s pressurized induction, special attention was paid to sealing its heads. Upgrades here include thick, seven-layer stainless steel head gaskets—similar to those used on the LS9—and 12mm head bolts made of hardened stainless steel.
To help reduce compression and better process the supercharger’s massive intake charge, the LT4 features 65.47cc combustion chambers, as opposed to the LT1’s 59.02cc chambers. The LT4’s 10:1 compression ratio is nevertheless high for a forced-induction engine, but the greater combustion control of the direct injection system helps prevent detonation.
In both engines the valves are held at 12.5 degrees (intake) and 12 degrees (exhaust), and splayed 2.61 degrees (intake) and 2.38 degrees (exhaust); this configuration reduces shrouding to enhance airflow. The LT4 uses lightweight, 2.13-inch, solid-titanium intake valves, which feature exceptional heat resistance and superior high-rpm strength as compared with the LT1’s identically sized hollow units. The 1.59-inch, sodium-filled exhaust valves are the same for both applications.
Valvetrain components for both engines include high-rpm, beehive-type valve springs; 1.8-ratio roller-pivot rocker arms; and large, 8.7mm (outside diameter) pushrods.
The LT4’s direct injection system is similar to the LT1’s, but it features a new, higher-pressure engine-driven pump capable of delivering fuel at an astounding 2,900 psi. The LT1 pump is capable of “only” 2,175 psi. The LT4 also features a unique, higher-capacity fuel-rail assembly designed to accommodate the supercharger manifold. It’s fitted with injectors rated at 1.5 liters per minute, while the LT1 uses 1.2-lpm squirters.
GM’s Active Fuel Management cylinder-deactivation system carries over to the LT4. It’s capable of temporarily deactivating four of the engine’s cylinders at light loads to reduce fuel consumption. The transition between four- and eight-cylinder power takes less than 20 milliseconds and is virtually imperceptible.
Besides the supercharger and its complementary accessory-drive system, the LT4 also uses stainless-steel exhaust manifolds in place of the LT1’s conventional iron ones. Equipped with smooth flow passages and equal-length runner geometry, their flow performance is comparable to that of the high-flow manifolds used on the LS7 and LS9 engines.
Finally, a lightweight aluminum balancer stands in for the LT1’s cast-iron unit.