LS3-Based 418ci Engine Build - Big Power, Small Budget

When first introduced in 1998 in our beloved Birds, the 346ci LS1 was the proverbial tip of the cubic-inch iceberg. Much like PMD's classic engine that began life as a 287 and evolved to a 455 with both bore and stroke enlargements, GM has taken the Gen III LS1 and LS6 engines and developed a host of Gen IV block and head designs that are offered in both OEM platform engines and GM Performance Parts crate engines. Everything from a 4.8L LR4 (293 ci) through a 7.0L LS7 (427 ci) can be bought over the counter. For enthusiasts looking for max performance, the cast-iron LSX block and the LSX tall-deck block can be purchased and built to achieve up to 482 and 500-plus cubic inches, respectively.

Things started to get real interesting when GM launched the LS7 in the '06 Corvette C6 Z06, which featured high-flow rectangular-port cylinder heads. GM then quickly developed a street-based hybrid of the LS7 rectangular port head in 2007-the L92. As compared to the previous cathedral-port heads such as the LS2 (260 cfm on the intake and 180 cfm on the exhaust at 0.600 lift), the L92 head in stock form had tremendous head flow (316 cfm on the intake and 189 cfm on the exhaust) and excellent velocity. The L92/LS3 cylinder head is currently used on both the 6.0L L76 engine in the G8 (362 hp with 391 lb-ft of torque) and the 6.2L LS3 in the C6 Corvette (rated at 436/428 with optional sport exhaust). For the '09 model lineup, Pontiac enthusiasts will be able to get an LS3 powered G8 GXP that's set to produce 402 hp with 400 lb-ft of torque.

Not long after the L92 block and heads were introduced along with the L76 intake manifold assembly, the aftermarket started bolting the heads and intake combination onto 6.0L or larger blocks (4.0-inch bore minimum for L92/LS3 head) and developing aftermarket stroker engine packages and cylinder head-porting programs to exceed LS7 power numbers (505 hp/470 lb-ft torque) with alarming regularity. According to Jason Mangum of Texas Speed and Performance, "The L92 and LS3 blocks and heads are now homogenized into 'LS3' part numbers. Whereas the most popular stroker combinations were an LS2-based 402 or a 6.0L iron 408, the market has shifted to the L92/LS3 aluminum-based 6.2L block and LS3 heads in the majority of applications, except for all-out drag-racing, where the GMPP LSX iron block has the most demand."

The L92/LS3 is an excellent block for the street, as the casting is derived from the LS2 and incorporates numerous revisions to increase strength. Most notably, the redesign of the "windows" between each cylinder's "bay" in the crankcase strengthened the "webs" by 18 percent, according to GM. These windows enhance bay-to-bay breathing in the interest of oil control, along with reducing parasitic losses from crankcase windage.

In stock form, the engine has a 4.065 bore (LS2 4.0) and a stroke of 3.622, which yields 376 ci. Due to the thickness of the cast-iron cylinder liners, the LS3 block can be bored and stroked to 418 or 427 ci configurations. With a set of our ported LS3 heads and an aggressive cam, we can achieve 650 hp with the 418 and over 675 hp with the 427."

Retailing at $4,095, the TSP short-block is offered in both 418 and 427ci versions (PN 25-418LS3LB or 25-427LS3LB) and is a solid foundation for a serious LS-series build. For hobbyists looking to purchase a long-block, TSP retails them at $7,399 (PN 25-418LS3LB or 25-427LS3LB). The GMPP LS7 (PN 19165058) crate engine retails for $14,195, and the TSP 418 in crate-engine trim is approximately $9,607, making the latter a great choice for the F-body, GTO, or G8 enthusiast looking to generate serious power on a budget.

A 6.2L (376ci) aluminum L92/LS3...        read full caption A 6.2L (376ci) aluminum L92/LS3 block is the foundation for both the 418 and 427ci configurations. In stock form, the block carries a 4.065 bore and 3.622 stroke. To create a 418ci LS3, the bore will be opened up to 4.080 and the stroke elongated to 4.0 inches.	GM LS3 main caps are torqued...        read full caption GM LS3 main caps are torqued to spec in order to maintain load conditions on the block before machining operations begin. The block is then set up on a Rottler F-67A three-axis CNC machine by Heber Ramirez of Sunset Racecraft to both deck and bore it. The deck height is measured to determine how much machining is necessary to ensure a flat deck. According to Ramirez, "A production LS3 block has a stated deck height of 9.240 inches, and traditionally we see 0.003-0.005 of variability across the deck surface." In this case, the variability was quite low, so a total of three passes were taken at 0.001 per pass, and the deck height was set to 9.238 inches.	After the bores are measured...        read full caption After the bores are measured for size to begin the boring process, the critical next step is to determine the center of each bore, as shown. If the top of the bore isn't what GM states, then each cylinder is carefully measured with a dial-bore indicator to determine the actual bore from top to bottom.
The boring head is attached,...        read full caption The boring head is attached, and the CNC machine is programmed with the bore center, deck height, boring stroke, and the amount of material to achieve the desired bore. Since the bore specified was 4.075 inches, a single pass on each cylinder brings them to the final spec. Dial-bore gauge measurements are taken to ensure the desired bores are obtained.	In order for the longer connecting...        read full caption In order for the longer connecting rods (6.125 inches versus stock length of 6.098 inches) to clear the bottom of the bore with the planned crank/rod combination, a carbide bit is used to remove 0.006 inch of aluminum at the bottom edge straight across each bore, after the block is mounted to a traditional engine stand. Once the aluminum is removed, a sanding roll is employed to smooth and radius each cut to ensure that a piston won't nick the area on the downstroke.	At the Sunnen CV616 Cylinder...        read full caption At the Sunnen CV616 Cylinder King honing machine, machinist Rodney Pope takes over. A metal bar (fixture) is bolted through the crank journal, and a bank of cylinders is set up in preparation for machining.
After cleaning the cylinders,...        read full caption After cleaning the cylinders, a cone-shaped sanding disc is used to add a slight bevel at the top of each one, and the block is cleaned again.	The final bore size that the...        read full caption The final bore size that the honing process achieves is a combination of the piston bore (4.075 inches) and the piston-to-wall clearance (0.0050 inch). A BJH 4.080-sized torque plate is utilized with a Cometic MLS gasket, and the 10 15mm bolts are torqued in sequence to 60 ft-lb.	To achieve a hone of 0.005...        read full caption To achieve a hone of 0.005 inch, a rougher set of stones is affixed and 0.003 inch is taken off each cylinder. A second pass takes off an additional 0.001 inch, before the stones are changed out to the final finish hone, and the bore is verified to be 4.080. According to Rodney, "Honing stones range in grit from 70 to 280, and the final finish is dependant upon the type of rings that will be employed."