Conversion Machine

Also from Issue 82

  • 2014 Stingray Convertible
  • The Last C6: 2013 427 Convertible
  • Interview: Dave Tatman
  • Buyer’s Guide: C5
  • Amelia Island Sting Ray Salute
  • Profile: Dave McLellan
  • 1962 Restomod
  • Corvette Racing: Data Acquisition
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Instead of increasing engine output, however, Chevrolet’s first step towards increasing C5 performance was to reduce weight and increase chassis rigidity. It accomplished this by offering a fixed-roof model, which was essentially a convertible with a permanently affixed hardtop. By going this route, Chevy was able to trim 80 pounds off the car and increase stiffness by 12 percent. This change, combined with the standard fitment of the Z51 package, made the new ’99 hardtop the best-handling Corvette available. By fitting it with the least amount of luxury equipment and making it available with only a 6-speed manual transmission, Chevy was also able to make it the lowest-priced Corvette.

While the new hardtop became an underground hit with the autocross and track-day crowd, the general Corvette-buying public took a pass. In 1999, Chevy sold only 4,031 hardtops, compared to 18,078 standard coupes and 11,161 convertibles. The following year, this number dropped by half, with a mere 2,090 hardtops being sold. Chevy realized that in order to sell more of its high-performance model, it would need to increase engine output. This thinking resulted in the LS6 V8 and the Z06 which it propelled.

By bringing back these two hallowed designations from the Corvette’s past, Chevy was setting a bar high indeed. In 1971, ordering the LS6 option got you a 7.4-liter V8 that pumped out 425 horsepower. It was the last high-output big block that GM ever offered. Instead of adding displacement, this time around Chevy engineers utilized the time-honored hot-rodding techniques of bumping up compression, improving breathing and increasing fuel delivery to extract more power from the existing 5.7-liter LS1.

To begin with, they made some minor but significant changes to the alloy block. Instead of machined holes in the bulkheads between the cylinders, the LS6 has cast-in windows. By improving bay-to-bay breathing, parasitic horsepower loss was reduced. In addition, this setup sped up oil return to the engine’s sump.

The LS6’s higher compression was achieved with a new cylinder-head casting that featured smaller pent-roof combustion chambers than the LS1 heads. By reconfiguring the intake and exhaust ports, engineers were able to drastically improve the flow of air into and spent gases out of the heads. To take advantage of this, a new billet steel camshaft with increased lift, shorter duration and increased overlap was developed. On the intake side, plenum volume was increased, while the LS6’s increased fueling needs were addressed with larger injectors.

The result of all this work was an engine that generated 385 horsepower at 6,000 rpm and 385 lbs-ft of torque at 4,800 rpm, making it the highest-output small-block V8 in Corvette history. In order for the car to earn its Z06 designation, however, Chevy engineers couldn’t stop with the powerplant; they had much more work to do.

The original ’63 Corvette Z06 featured a 360-hp L84 engine, a limited-slip differential and a 4-speed transmission—items that could be ordered on any ’63 Corvette model. However, it also had a number of exclusive upgrades, including special brakes and stiffer suspension pieces. The ’01 Z06 went down a similar path, but had even more extensive upgrades. First of all, it was given more aggressive gearing. As a result, the ’01 Z06’s 171-mph top speed was actually 4 mph lower than a standard C5 coupe’s. It was no contest when it came to acceleration, though. The Z06 blasted from zero to 60 mph in 4 seconds flat and dusted the quarter mile in 12.6 seconds, while the base car went from zero to 60 mph in 4.7 seconds and did the quarter in 13.4. The Z06 engineers may not have achieved big-block power output, but they certainly achieved big-block acceleration.