There are a lot of reasons to consider building a fresh engine. Sometimes the need arises from practical considerations, such as replacing a powerplant that is just plain tired and worn-out. Sometimes the motivation may be the search for more power or larger displacement-both worthy goals in our book. In the case of the stroker LS2 featured here, the reason for its existence was simply because the previous engine just never lived up to expectations.
This build is based on a new...
This build is based on a new factory LS2 block, which was machined slightly oversized. The block was detailed by fully blending all the oil gallery junctions, clearancing for the 4.00-inch stroke crank, and tapping for earlier LS6-style knock sensors.
Steve Smith found himself in this situation with the 407ci LS Chevy engine in his '02 C5 Z06 Corvette. Looking to increase the already potent power of the factory LS6-equipped supercar, the engine was replaced with what looked like a very promising combination based on a bigger-bore LS2 block, coupled with a 4.00-inch stroke crankshaft. This 402 combination featured power parts that really emphasized top end output, with large-ported LS7 cylinder heads, and a long duration camshaft well into the 250s (at .050 inch). Sure these parts can be optimized for a big number up top, but Smith, a high-performance driving instructor, definitely favors crisp throttle response and a fatter power curve with instant torque off the corners. This car was never intended to be a high-rpm dragstrip machine. In fact, the car's application is primarily aimed at the street, where the engine just seemed to fall short. As Smith described it, he felt like he needed to call ahead to the engine room every time he wanted to nail the throttle.
Clevite H-Series bearings...
Clevite H-Series bearings were used in the main saddles, with the bearing clearance already checked to provide .0018-inch running clearance. ARP studs are the fasteners of choice, ensuring the factory main caps are clamped tight.
The final irony of the situation was that the cam and valvetrain in the original combination simply did not deliver the stability needed to pull high rpm. The bottom line was the engine was soft down low, intractable on the street, and then fell over just when it was supposed to get good, showing a maximum of 480-490 hp on the chassis dyno. To remedy the situation, Smith enlisted the aid of Tony Mamo for a new bullet, a man known in LS circles for stout performance combinations, and the chief cylinder head designer at Air Flow Research. The build goals were to create an engine combination that would improve the driveability, bottom end, and responsiveness. We'll cut to the punch line and say that the engine proved to do all that and more. The kicker here is it cranks far more top end power than the old engine, punching out a solid 540 hp at the rear wheels.
Tony Mamo selected a Callies...
Tony Mamo selected a Callies Compstar crankshaft featuring reduced diameter 2.00-inch rod journals instead of the factory 2.100-inch dimension. The smaller journal increases clearance with the longer stroke, and reduces bearing speed and frictional losses. Mamo says it is a "win-win."
The original engine was sold off in parts, and Mamo started with a fresh LS2 block as the foundation for the replacement. LS2s come at a 4.0007 bore as OEM, and Mamo will normally have the block honed to 4.005 inches with a torque plate, but in this case Smith had already ordered the pistons at 4.022 inches in anticipation of freshening the old engine. The pistons are top-of-the-line Wiseco forgings cut for their GFX 1.2, 1.2, 3mm ring package, which is an outstanding setup in terms of low drag and sealing. The bores in the fresh block were machined to match the pistons, and then Mamo followed with a complete detailed prep, including full custom porting and radiusing of the oil gallery junctions, and clearancing for the longer-stroke Callies Compstar crankshaft. The crank features 2.00-inch journals, with the reduced size cutting down the bearing speed and providing more stroke clearance in the crankcase. The rods are Callies H-beam Compstar units, at 6.125 inches center-to-center length.
Looking for improved driveability, the camshaft selection played an important role in the final characteristics of this engine. COMP ground the custom hydraulic roller 'shaft at 242 degrees duration, a specification more than 10 degrees shorter than the cam in the previous engine. The cam was cut with 114 degrees of lobe separation, and it provides .610-inch lift via a set of Terra Yella Ultralite 1.7:1 ratio rockers. Mamo installed the Cloyes timing set to park the camshaft at 111 degrees, intake centerline angle. The combination provides ample lift to tap into the flow potential of the cylinder heads, while the moderate duration and wide separation help keep things tame enough for a street performance application.