The racing rules behind Cheetah and Z/28

In 1963 Bill Thomas was building a car to beat the Cobra. The Cheetah came out of a small shop in Anaheim, California, and it had a quiet friend inside General Motors: Vince Piggins, the Chevrolet performance man who would rise to head of product performance, was covertly supporting the project. Chevrolet had officially withdrawn from racing. Piggins spent the next decade finding ways for it to race anyway.

The Cheetah

Thomas had been tuning Corvettes since 1956 and had his own shop, Bill Thomas Race Cars, by 1960, when GM came to him for development work on drag cars and the Chevy II. The Cheetah began as a proof of concept, a way to show GM what the shop could do, and became a Cobra-fighter he raced for promotion. It was small, brutal and genuinely fast, and for a moment it looked like Chevrolet's back-door answer to Carroll Shelby.

The production question has two kinds of evidence. In Anthony Young's 1981 Automobile Quarterly interview, Thomas recalled a rise from 100 to 1,000 cars. The same account says he could not complete the first hundred quickly enough and had to race in the big-bore Modified class. The FIA's 1964 Appendix J instead separates a 100-car GT minimum from a 1,000-car touring minimum. It does not document a blanket 1964 GT increase. Later production estimates also count different things: completed cars, unfinished cars, chassis or bodies.

1964 FIA production thresholds depended on category

That distinction matters. A driver's recollection of a production obstacle and a dated rule for a particular category answer different questions. They leave the precise timing and application of the reported change unresolved.

The memo

The Camaro's later Trans-Am package had its own development and eligibility history.

The SCCA had started a new series for sedans in 1966, the Trans-American Sedan Championship, and its over-two-liter class capped engines at five liters, or 305 cubic inches. Chevrolet had nothing that fit it: the new Camaro's V8s were a 327, a 350 and a 396, and every one of them was over the limit. In August 1966, a month before the Camaro reached showrooms, Piggins sent Chevrolet management a memo proposing a Camaro package built to be eligible for Trans-Am. Enthusiast historians have called him the Z/28's father ever since.

The engine was the neatest piece of rule-reading in the whole program. Chevrolet took the 327 block, with its 4.00-inch bore, and fitted the 283's crankshaft, with its 3.00-inch stroke. The result displaced 302 cubic inches, three under the limit. Chevrolet rated it at 290 horsepower, a number almost nobody believed; the company's own 1968 dealer sheet said it was "actually much stronger than that."

A car built backwards

Everything else on the Z/28 followed from the same logic. The Z-28 option sheet in the 1967 press material lists the 302 with dual exhausts and deep-tone mufflers, special suspension, a heavy-duty radiator, quick-ratio steering, 15 x 6 wheels on red-stripe tires, a 3.73 axle and stripes on the hood and deck. A plenum air intake and tuned headers could be added, shipped loose for the owner or dealer to fit. Buyers also had to order the close-ratio four-speed, power brakes and front disc brakes. It was a race car's parts list with a warranty.

And almost nobody knew it existed. Chevrolet did not advertise it by name at first. Early cars are sometimes described as a Sport Coupe with the Z/28 Special Performance Package, and the name itself was an order code, buried in a book. You had to know to ask for it.

Six hundred and two

SCCA nominally wanted a thousand of those too. Chevrolet built 602 in 1967 and SCCA accepted it anyway.

Ate Up With Motor reports that Piggins had the regular SS350 Camaro homologated and persuaded officials to consider its sales alongside the Z/28. That is an attributed account of Camaro eligibility, rather than evidence that an identical rule was applied differently to the Cheetah. Chevrolet's volume-production Camaro and Thomas's small-run sports car were separate programs, competing in different classes and circumstances.

What the Z/28 produced

It worked. Roger Penske's Sunoco-sponsored team ran a Z/28 in 1967, including an acid-dipped, drastically lightened car known as "The Lightweight," and Mark Donohue took his first Trans-Am wins. In 1968 he won 10 of 13 races and Chevrolet took the manufacturers' championship. In 1969 it won again.

The showroom followed the racetrack. For 1968 the Z/28 became a regular, orderable package, and production jumped to 7,199. For 1969 it grew again, to somewhere between 19,014 and 20,302; the sources have never settled on one figure. The "DZ" suffix stamped on the 1969 engines gave the car its enthusiast name, the DZ 302. When Trans-Am's 1970 rules let teams de-stroke a larger production engine to fit the unchanged 305-cubic-inch limit, Chevrolet dropped the 302 and gave the Z/28 the Corvette's LT-1 350. The rule had changed, so the car did.

The five-liter limit shaped the Z/28's original engine package. Its name then lasted through later Camaro generations with different engines and competition roles. That documented relationship between a class limit and an engine is firmer than treating the Cheetah's end as the cause of the Z/28.

Thomas went on too. After the Cheetah he worked with Nickey Chevrolet in Chicago on its 427 Camaro conversions, and his shop sponsored first-generation Camaro race cars. The dealer-tuner trade is partly what he did next.

Read more about Bill Thomas, Vince Piggins, the Trans-Am years and the first generation.

Did you know

• The Z/28 nearly carried the Cheetah's name. Automobile Quarterly's 1981 history says Piggins and Bill Thomas coined "Cheetah" from "cheaters," said in a southern accent, and that Piggins later proposed it to Chevrolet management for the high-performance Camaro. He was voted down, and the option code Z-28 became the name.
• In the 1981 Automobile Quarterly interview, Thomas recalled both trouble completing the initial hundred cars and a later rise to a thousand. The article describes Cheetahs racing in the big-bore Modified class. The date and category behind his reported increase remain unresolved against the 1964 FIA text.
• You could buy a Cheetah for the street. For just under $8,000, Automobile Quarterly says, you could walk into Thomas's shop and drive out in one. Thomas recalled selling several road cars, including one to the wife of Seattle dealer Alan Green, who took it to drag strips, and another to a member of the Mamas and Papas who bought it on the spot from a dealership display.
• The 302 was an old hot-rod recipe before it was a Chevrolet engine. The Camaro Research Group says racers had long made the same displacement by boring a 283 out to 4.000 inches, and that USAC sprint cars used the combination. Chevrolet's version put the 283-style crank in a 327 block with its 4.00-inch bore.
• Chevrolet's homologation plans had already fallen short once. Before GM's February 1963 racing edict it planned 125 lightweight Grand Sports for homologation; only five were built.
• The 1966 Trans-Am rule was more than the 305-cubic-inch cap. Ate Up With Motor says the sedan class also limited wheelbase to 116 inches, which the Camaro's 108-inch wheelbase cleared easily.

Sources for these details

• Anthony Young, "Bill Thomas' Fast-Moving Cat," Automobile Quarterly, 3rd quarter 1981 (reprinted on cheetahevolution.com) Period magazine
• Hagerty Media, "Rare Bill Thomas Cheetah is up for grabs" Reference
• Chevy Hardcore, "The Wild and Wonderful Bill Thomas Cheetah" Reference
• Camaro Research Group, "The First Generation Camaro Z28" (CRG Research Report, 2026) Club/registry
• Ate Up With Motor, "First, Foremost: The First-Generation Chevrolet Camaro Z/28" Reference
• FIA 1964 Appendix J, Articles 257 and 266: distinct touring and GT production minima Period governing-body regulations

Sources and method