Key notables for the LS1 include a switch to a more powerful PCM in 1999 which is more tuner-friendly for you computer hackers out there. In 2001, they then received the freer-breathing LS6 intake manifold and the LQ4 camshaft for a total of 350 horsepower. The 2001-2004 ZO6s all had great heads, cams, and engine/drivetrain packages, making them the most desirable and as expected, the most expensive. Over time the 5.7-liter LS1s and LS6s have proven to be incredibly reliable, efficient, and powerful. The main thing to look for is weak oil pressure on early LS1s and signs of abuse, which typically show up as oil leaks and odd engine noises. These cars hardly ever spew a drop of lubricant but dances with the rev limiter, missed shifts, and general lack of LSX respect will also result in piston slap on cold start and lifter noise. Documented maintenance records help, while frequent oil changes with quality synthetic lubes promote engine life.
Don't be scared of modifications such as long-tube headers, a mild cam and some tuning as these mods are pretty much considered bolt-ons and won't greatly affect the life of the car. Once you get into superchargers and nitrous, things change. We'd recommend asking a good technician to see the quality of the work that is on a prospective car that you're looking at. LSX engines can easily handle a moderate boost in power with a power adder but elevated power levels, no matter how sexy they may sound on paper, require a careful eye to catch potential problems.
The rest of the C5's mechanicals are equally unique in design and engineering. The transmission, mounted directly to the rear transaxle, sits behind the driver to offer near-perfect weight distribution and thanks to a close-ratio T56 with two overdrive ratios and a 3.42 ring and pinion, taking advantage of the LS1's broad powerband is just a slick shift away. Automatic versions received a four-speed automatic with a single overdrive gear and either 2.73 or 3.15 final drive ratios.
T56 transmissions can become rough shifting over time, but it does not mean the car was abused. Because the C5 uses such a long input shaft within its torque tube, the synchronizers are asked to do a lot more than a regular car as it has to match the heavy input shaft speed to the selected gear's speed. Some cars have been completely cured by the use of GM's Synchromesh fluid while other cars with over 75,000 miles may need a transmission rebuild.
Automatic models with the 4L60E generally last until 100,000 miles without a problem so long as it is serviced with fresh fluids over its life. Be cautious of cars that have been fitted with aftermarket torque converters and/or have been programmed via the PCM to remove the torque management feature, which normally limits power in between shifts. Generally, transmission life can be cut in half (or more) if these modifications have been made together.
Rear differentials can sustain 500 horsepower all day long and failure is relatively low, even when modified. Gear swaps are not exactly cheap, so make sure a gear change is what's best for your engine combination before making the commitment. Generally, the 3.42s work best for mild application but 3.90s and 4.10s love high-revving naturally-aspirated engines in front. When you approach the 600-rwhp level, consider a hardened shaft upgrade and even a transaxle brace kit.