When it comes to making power, there is little need for convincing when the subject of GM LS-Series engines enters the conversation. Right from the jump, these are some of the most powerful OEM engines ever delivered by the Motor City, and that’s just the start. The healthy aftermarket support means the sky is the limit when looking to breathe even more horsepower into these already stout powerplants. Since its inception as an all-aluminum 346-cube unit in 1997, the LS-series has been produced in a wide range of OEM configurations, using both aluminum and cast-iron blocks. OEM displacements range from 4.8-liter (290 cid) using a 3.78-inch bore, to the substantial 4.125-inch bore, 7.0L configuration of the LS7. Any of these LS-family engines can swallow a meaty stroker crank, however the bigger bore engines are naturally better endowed when outright power, torque, and displacement are the goal.

While the 4.125-inch bore blocks offer the most displacement potential, their limited production makes them an unlikely candidate when considering a budget build. On the other hand, the iron 6.0L truck blocks offer an ample bore size of 4.00 inches, right in the sweet spot for a big-cube stroker. The abundant production of the 6.0-liter ensures that they are plentiful in the salvage yards. To Bret Bowers of Racing Engine Design, this seemed like a reasonable basis for a hot LS engine build. The engine featured here was built to compete in the AMSOIL Engine Masters Challenge, our annual engine-building competition. Stretched to 426 ci and producing 643 hp, this combination illustrates the potential of the 6.0L LS.

Configuring The Cubes

The starting point for this build is a ’04 6.0L truck engine core, and as Bowers explains, these units are virtually bulletproof in a performance application: “We have used this block in very high-horsepower applications; because it’s iron, the cylinder walls are very stable, as is the main structure in the crankcase. It doesn’t change shape with heat as much as an aluminum block, and that’s why we tend to use an iron block for a pure horsepower application.” Owing to the robust OEM configuration, the bottom end really needed a minimal amount of modification, and, in fact, it is remarkably stock. As Bowers tells us: “I really didn’t change anything in the block besides the crank and reciprocating assembly. I left the oiling system and oil pump stock, just like the truck. We added some extra drain back in the windage tray to control the oil a little better. For the most part, all we needed to do is blueprint the block and torque-plate hone it.”

The iron 6.0L truck engine is already packing ample cubes for a small-block engine, using a bore of 4.00 inch and a stroke of 3.622 inch for a stock displacement of 370 ci. This is where Bowers made a major departure from the OEM configuration, upsizing the cubes big time with a 4.100-inch stroke Manley crank. The long-stroke combination is just the setup for a broad powerband with a huge increase in available torque. Bowers tells us that the long arm drops right into the 6.0L block: “You don’t have to do a thing to make the stroker crank fit. There is no clearancing or anything needed.” With a finished bore size of 4.060 inches and the 4.100-inch stroke crank, the volume is pumped up to 425 cubes.

Now, don’t get the idea that it takes exotic, one-off parts to put together a similar short-block. Bowers used readily available, off-the-shelf parts throughout. “Everything in this engine was off the shelf; nothing was custom. We used Manley pistons and Eagle lightweight 6.125-inch rods. We didn’t change anything in the rotating assembly; we just balanced it. You can readily order any of these pieces to build this engine.”