Conversion Machine

Four hundred and five horsepower for roughly $20K—the C5 Z06 is a car that turns buyers into true believers.

Photo: Conversion Machine 1
May 3, 2013

Four years ago, Barry MacKinnon was not a Corvette enthusiast. Sure, he admired the model, but only from a distance. He was a car nut while growing up in his native New Zealand and had owned some performance machinery over the years, but felt that his sports-car days were behind him. The last one he had owned was a 280ZX, back in the ’80s when Nissans were still called Datsuns. The San Francisco Bay Area resident was resigned to the fate of driving boring, transportation devices; his ride at the time was a Prius. Buying any Corvette, much less the 405-horsepower beast on these pages, was the last thing on MacKinnon’s mind.

A guy named George Robinson changed all that, steering the mild-mannered physicist on a path that not only led to C5 Z06 ownership but driving said car flat-out on racetracks. You see, Robinson is somewhat of a Corvette evangelist, serving as president of the Northern California Corvette Association, the oldest Corvette club in the nation. Wanting to convert new members to the fold, he often barrages them with generosity.

In MacKinnon’s case, it was the offer of driving Robinson’s C6 convertible in an Oakland autocross, the Chariots of the Coliseum no less. The two had gotten to know each other as members of an Autodesk user group. Though their political views were divergent, the pair found plenty of common ground, especially when it came to talking about cars. Still, Robinson’s kind offer came as a surprise to MacKinnon; beyond the fact that the two hardly knew each other, MacKinnon had never autocrossed before, nor had he driven a Corvette. Though MacKinnon demurred, Robinson insisted on the baptism—and it worked.

“The car was like a rocket,” says MacKinnon of his experience behind the C6’s wheel. “The way it handled was unbelievable to me. It didn’t even lean—cornered flat as a tack.” He was also impressed with the car’s balance. The hopped-up Holdens he had owned back in New Zealand were prone to oversteer when pushed, but this Corvette remained dead neutral no matter how hard he flogged it. After a few short blasts around the cones, MacKinnon had been converted.

Photo: Conversion Machine 2

That autocross was held in July 2009; three months later, MacKinnon bought his 2002 Z06 Corvette. Naturally, Robinson was instrumental in the purchase. He had tracked down a suitable machine for his new disciple, one that had been lovingly maintained by a couple who were NCCA members. Make that fanatically maintained. Though Eric and Laurie Brandt hadn’t been afraid to put miles on their Torch Red machine (the odometer showed 52,000 miles at the time of sale) and even autocrossed it regularly, they had some very specific demands for the car’s new owner. “I had to assure them that the car would be kept under a cover, which they included in the price,” says MacKinnon. “They even told me what kind of gasoline to buy.” He acquiesced to their demands and handed over a check for $22,500.

Shortly after buying the Z06, MacKinnon was inducted into the NCCA. “I created a bit of a stir at the ceremony,” recalls MacKinnon. “I was the first club member to trade a Prius for a Corvette.” Nevertheless, he was welcomed with open arms. MacKinnon’s wife, however, was less than enthusiastic about his sports-car purchase, which she referred to as her “worst nightmare.” But then again, her husband, who was pushing 70 at the time, had just gone from a hybrid to a supercar in a matter of months and was now a member a secretive society that speaks in RPO code. She probably wondered if she would have to hire a deprogrammer.

When the C5 debuted in 1997, it represented a huge advancement over the fourth-generation Corvette. Its handling was dramatically improved thanks to a big increase in wheelbase, a significantly stiffer frame and a lower curb weight. For the first time in Corvette history, the base model was powered by an all-aluminum engine. Rated at 350 horsepower, the LS1 small-block V8 represented only a 15 horsepower increase over the previous year’s LT4 engine, but was more refined and offered plenty of room for future tuning.

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.

Photo: Conversion Machine 3

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.

Photo: Conversion Machine 4

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.

Contributing to the Z06’s incredible performance at the drag strip was its light weight. Though the hardtop coupe upon which it was based was already the lightest car in the Corvette lineup, Chevy took further mass-reducing measures. The entire exhaust system rearward of the catalytic converters was constructed of titanium. The Z06 got a special battery that weighed 5.7 pounds less than the standard version, as well as a smaller brake pressure modulator. Chevy even went so far as to fit the Z06 with a thinner windshield and backlight. At 3,130 pounds, it weighed 99 pounds less than an ’01 coupe. To prevent customers from piling on the pounds with options, the Z06 could not be had with fog lights, a head-up display, a telescoping steering column or passenger-side power seat.

The Z06’s lower mass also helped it in the handling department, but Chevy wasn’t content with simply equipping it with the Z51 package as it had been with the hardtop. Instead, the Z06 got its own unique suspension setup. Compared to the standard Z51 kit, the Z06’s FE4 package featured stiffer front and rear springs, a larger-diameter front anti-roll bar (30 mm versus 28.6) and wider-diameter Bilstein shocks (45 mm versus 36 mm). The Z06’s rolling stock was also upgraded, with forged alloy wheels (9.5 × 17-inch front, 10.5 × 18-inch rear) and specially formulated Goodyear tires (245/45ZR17 front, 275/40ZR18 rear). Because this sticky, unidirectional Eagle F1 SC rubber did not have run-flat capability and engineers were loathe to fit the Z06 with a heavy spare tire, the car got a can of fix-a-flat instead. To optimize the size of the tires’ contact patch, the Z06 received unique wheel alignment specifications with half a degree of additional negative camber. The Z06 could generate a supercar-like 1.03 g of lateral acceleration around the skidpad.

Photo: Conversion Machine 5

To increase braking performance, the size of the rear brake rotors was increased from 12.0 in 12.6 inches, while the addition of air scoops just in front of the rear-wheel openings helped keep them cool. Combined, this long list of upgrades made the Z06 the most track-capable Corvette Chevy had ever offered, one that handily outperformed the C4 ZR-1 at a price point ($47,500) that was $20K lower.

Not surprisingly, the ’01 Z06 was a sales success. Chevy sold 5,773 of them, and would have sold more if wheel-supply problems had not limited production. Yet the carmaker did more than just find a new rim outfitter for the 2002 model year—in this case Speedline, which developed a cast wheel that was just as strong as the forged one—it managed to extract an extra 20 horsepower from the LS6 engine and performed a small suspension tweak.

As it turns out, GM powertrain engineers weren’t quite done with the LS6 when the Z06 debuted for the 2001 model year. They had run out of time to optimize the valvetrain and maximize engine breathing. The first step was to revise the catalytic converters. The new ones reduced back pressure and allowed the engine to flow more air; a larger opening in the air box helped as well. This, in turn, allowed the use of a more aggressive, high-lift camshaft. To keep pace with the cam, lighter, hollow-stem valves were fitted, as were stiffer valve springs.

In the end, the 2002-spec LS6 generated 405 horsepower at 6,000 rpm and 400 lbs-ft of torque at 4,800 rpm. That works out to 70.1 horsepower per liter, which simply blows away the 57.4 horsepower-per-liter figure for the ’71 LS6. In celebration of its accomplishment, Chevy festooned the car’s front fenders with “Z06/405 horsepower” plaques. It also provided the car with revised performance figures: The Z06 could now dash from zero to 60 mph in 3.9 seconds and cover the quarter mile in a fleet 12.4 seconds.

These improvements at the test track weren’t gained through increased engine output alone. By re-valving the rear shocks, engineers were able to reduce wheel hop at launch, helping the tires to hook up more quickly. This revision was also aimed at making the Z06 less jittery on bumpy backroads.

During our brief time behind the wheel of MacKinnon’s Z06, it certainly felt planted in the corners. Its ride was stiff, but never harsh or brittle. Our biggest impression, however, was of the car’s speed; it felt damn fast. Between the incredibly eager-to-rev LS6 and the low gearing, this car flat-out bolted whenever we got on the gas. While the 505-hp 7.0-liter LS7 that powers the C6 Z06 deserves all the accolades it has received, we think the 405-hp LS6 is underappreciated. In some ways, the LS6 is more responsive; it seems that you get into the meat of its powerband more quickly than with the LS7. Taller gearing plays a role in this, something that also affects acceleration. As a result, both Z06s post the same zero-to-60 mph time, and the newer C6 version is hardly any quicker down the quarter. Autocrossers actually prefer the older model.

MacKinnon certainly sees no reason to make any changes to his Z06, nor does he forsee selling it any time soon. He’s put roughly 10,000 miles on the Corvette since he bought it, driving it to work about 50 percent of the time. “It’s a perfectly satisfying car as a commuter,” says MacKinnon. Though he’s gone on a few NCCA rallies, including one to Oregon, what he really loves to do is get his car on track. He learned his chops at Thunderhill Raceway Park, where he completed a couple of Northern California Racing Club driving schools. Naturally, given his line of work, he is rather interested in exploring the physics of the racing line.

MacKinnon is particularly fond of Mazda Raceway Laguna Seca, which is where we caught up with him for our photo shoot. With his pal Robinson offering up the occasional pointer, MacKinnon posted some respectable lap times. He is quick to give credit to the car, however, saying that its abilities still far exceed his own. Though his wife has yet to warm up to the car, MacKinnon has been very pleased with his Z06. “Couldn’t be happier with it,” he says. Speaking like a true Corvette convert, MacKinnon adds, “The C7 looks like a fantastic car, but I would have to give up the C5 to buy one—plus, I’d have to make sure it found a good home.”

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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