If you've got an airbox or...
If you've got an airbox or after-cat system already, the next step is often a supercharger, like this Magnuson unit bolted on a stock 6.0L LQ4. These roots-style superchargers are perfect for this engine because they deliver approximately 5 psi of max boost and can add a real 100 hp.
In early performance combinations, the aluminum version of the Gen III/IV V-8 had shown itself to be able to stand only so much in street-going applications. Some numbers to put it in context: The stock aluminum block has been known to handle a constant 650 to 700 hp without issue-and this is with the factory torque-to-yield head bolts. Some design changes would allow more, but GM is more concerned with lightweight, fuel-efficient, and emissions-friendly engines. As an aside, there are nitrous Vettes on the street with 550hp engines and 150hp shot systems that are providing reliable service.
The cast-iron blocks appeared in the '99 fullsize GM trucks. They weigh 88 pounds more than the aluminum blocks and have been used as the foundation for engines making more than 1400 hp. Near-stock cast-iron blocks (GM PN 12551366) have been known to handle up to 850 hp in "drivers."
Other than being a bargain from GM Performance Parts, the advantages of the cast-iron blocks are numerous. These blocks are common with the aluminum blocks in their acceptance of components, they can be bored out 0.030 [the 4.8 and 5.3L blocks can be bored to 99 mm and the 4-inch (101.6mm) bores can go 0.030 of an inch over] and offer increased strength at a very low weight penalty. Simply put, just combine a 4-inch bore cast-iron block with a 4.125-inch-stroke aftermarket forged crank and you've got a 403ci bad-boy Gen III that can handle 1,000 hp with few problems.
Swapping the cam on a Gen...
Swapping the cam on a Gen III LS1 can be accomplished quickly. This is because unlike the previous small-blocks, the oil pan does not need to be dropped to remove the front cover, and the lifters can be left in the engine.
The stock LS1 Gen III V-8 cast crank, powdered metal rods, and hypereutectic, cast-aluminum pistons are all commonplace large-scale production OE components. None of these would be the first choice of a hot-rod enthusiast, but all the pieces do an amazingly good job of handling 500-plus hp in a performance application. Beyond that, you'll need forged, floating-pin pistons (the stock pistons use a pressed-on pin up to '04), and if you're going with forged pistons, you might as well go with new rods.
Most enthusiasts will go up in stroke if they spend the money on an aftermarket crank. This is one of the great advantages of the Gen III V-8: The engineers at GM raised the cam and left plenty of space in the cylinder block for a little more than a 4-inch-stroke crank. So, no grinding or special machinework is required to go up in arm to gain cubic inches.
A more involved change, but...
A more involved change, but worth its weight in power, are long-tube headers on the Gen III LS1 V-8s. Issues such as fitment, cost of replacing the entire exhaust, emissions compliance, and the need for a different calibration will require your attention, but double-digit power increases will usually result.
In an effort to simplify the...
In an effort to simplify the large-scale assembly process, the Gen III V-8 uses torque-to-yield head bolts. This means the bolts actually stretch beyond their reusable limit, or to their plastic yield, to provide a consistent clamping load on the cylinder heads. Notice the different lengths: The head bolts look like this for engines built from 1997 to December 2003. After that, the bolt lengths were standardized to the 55mm shorter versions in January 2004.
The four-head-bolt-holes-per-cylinder...
The four-head-bolt-holes-per-cylinder bore on the block are blind, which means they are closed off at the end. For this reason, it is critical you get all fluid or solid material out of these holes before torquing down the head bolts. Most engine shops use dry, compressed air to blow the holes out (shown) and sometimes brake cleaner to loosen up any material in the holes before blowing them out. If you torque down the bolts without doing this, there is a good chance the block will crack, making it junk.