Vehicle development—and in particular the development of high-performance cars such as the Corvette—was profoundly impacted by 1970 amendments to The Clean Air Act of 1963, as well as subsequent regulations flowing from the U.S. Environmental Protection Agency (EPA). Automotive engineers were tasked with meeting increasingly stringent emissions rules, and engine output suffered as a direct result. It would be more than a decade before computer-management systems and other advanced technologies enabled carmakers to conform to emissions requirements while also advancing performance, but in 1970 those responsible for the Corvette made one last stand with “old tech” in the form of an option package labeled RPO ZR1.
The heart of the ZR1 option was its LT-1 engine. Displacing 350 cubic inches and rated at 370 horsepower, the LT-1 benefited from all of the power-producing tricks in Chevy Engineering’s arsenal. These notably included a solid-lifter camshaft delivering 317 degrees of intake and 346 degrees of exhaust duration with 96 degrees of overlap, and valve lift of 0.45 to maximize power higher in the engine’s operating range. Free-flowing cylinder heads were fitted with 2.02-inch intake and 1.60-inch exhaust valves, while breathing was further aided by a dual-plane aluminum intake manifold topped with Holley’s vacuum-secondary 850-cfm carburetor.
The engine exhaled through a dual exhaust system featuring 2.5-inch diameter pipes—the same size as the setup employed on Chevy’s monster big-block lineup. Domed aluminum pistons, impact extruded for added strength, yielded a stout 11:1 compression. For increased bottom-end durability, the engine’s connecting rods and crankshaft were forged from high-strength steel. Four-bolt main bearing caps anchored the crank, increasing durability at extreme engine speeds. A long list of other parts in the LT-1, ranging from piston pins and pushrods to valve springs and the oil pump, also came from the heavy-duty parts bin.
Though Chevrolet called RPO ZR1 a “Special Purpose Engine Package,” it was actually far more than just a powerplant upgrade, which makes sense given that the LT-1 was also available as a stand-alone option. The engine by itself cost $447.60, while ZR1 added $968.95 to a 1970 Corvette’s MSRP, so what exactly did the buyer get for that $521.35 extra?
Every ZR1 was fitted with a Muncie M22 close-ratio four-speed transmission. Nicknamed the “Rock Crusher” because of the noise its straight-cut gears made, the M22 could stand up to far more torque, power, and punishment than the standard M21 four-speed. All ZR1 Corvettes also got a Delco transistor ignition rather than the more pedestrian breaker-point unit found on most other Corvettes of the era. The transistor setup was more precise, especially as the LT-1 neared its 6,500 rpm redline, where mechanical ignition points tend to bounce off one another and cause erratic timing.
To help keep the engine out of the danger zone, even on a racetrack, all ZR1s came equipped with a high-capacity Harrison aluminum radiator. This was the same unusual radiator previously used for 1965 396-equipped Corvettes, and all 1967-69 Corvettes fitted with the mighty L88 427. And unlike the more mundane ’70 Vettes, which came with plastic fan shrouds, all ZR1-optioned cars used a specific metal fan shroud.
Beyond the engine and drivetrain, ZR1s got a host of suspension and brake enhancements designed to increase their speed and durability on a road course. The Corvette used the same basic chassis from 1963 through 1982, so it’s not surprising that virtually all of the ZR1 upgrades were developed previously and used on the high-performance variants that came before, most notably the L88-powered cars.
For added stopping power, ZR1s got the same heavy-duty brake package that was mandatory with 1967-69 L88s and ’69 ZL1s, as well as being available as a stand-alone option called J56 from 1966-69. This package included power assist, a pressure-proportioning valve, and an extra brace for each front caliper. The brake-pad friction surfaces were made with a metallic compound and bonded, rather than riveted, to the backing plates. In the front, the backing plates were made from a heavier-gauge steel and secured to the calipers with two pins instead of just one. All four calipers got special ceramic heat insulators for their pistons to help keep the fluid from boiling.
As with the brakes, the beefed-up suspension system included with RPO ZR1 was previously required with L88 and ZL1 cars; it was also available as a solo option called F41 from 1966-69. While the exact specifications for F41 differed between cars, the parts it included were very similar to the race-oriented suspension that came on the ’63 Z06, as well as the 1964-65 F40 package. In all cases, higher-rate front coil and rear transverse leaf springs were installed. Stiffer Delco hydraulic shocks all around and a larger diameter front stabilizer bar were also included. A rear stabilizer bar, which was installed on all big-block Corvettes beginning in 1965, was also part of the F41 option and incorporated into the ZR1 package.
According to published production figures, only 25 of the 17,316 Corvettes manufactured in 1970 came with RPO ZR1. In retrospect, this low build total should not be terribly surprising. To begin with, the $968.95 cost for the package added 18.7 percent to the $5,192 base price of a coupe, and just shy of 20 percent to the $4,849 tab for a convertible. That alone surely dissuaded some people from considering it. Another factor that undoubtedly turned off some buyers was Chevrolet’s refusal to allow the ZR1 to be built with air conditioning, power windows, power steering, a radio, and most of the other luxury features otherwise available as options for Corvette in 1970.
A third reason so few ZR1s were built is perhaps the most important: The primary purpose for these cars was road racing, and by the dawn of the 1970s the majority of individuals campaigning Corvettes favored ’68 and ’69 models because they generally cost less and were a little bit lighter. As a result, someone who wanted to go racing with a Corvette in 1970 was more likely to buy a one- or two-year-old Stingray rather than a pricey, new ’70 model.
A True Original
Of the 25 ZR1s put together in Corvette’s St. Louis assembly plant, the Laguna Gray coupe featured here is probably the most original to survive. It was delivered new by England-Cook Chevrolet in Lansing, Michigan; is well-documented with its original tank sticker, Protect-O-Plate, and dealer invoice; and when photographed for this feature had traveled just a little bit more than 16,200 miles since day one. The body has never been damaged and still wears most of its original lacquer paint. It’s virtually untouched under the hood, with factory finishes on all surfaces and correctly numbered and dated components throughout.
Inside, the car sports its factory-installed interior, which interestingly is the optional Custom Interior Trim, one of the few luxury or appearance options that could be coupled with RPO ZR1. This upgraded interior includes leather seat covers; plush, cut-pile carpeting; wood-grain accents on the door panels and center console; and a strip of carpet with bright edge trim on the bottoms of the door panels. As with the body, engine compartment, and interior, the chassis of this beautifully preserved Corvette is nearly all original, with factory finishes, inspection marks, and parts stickers visible throughout.
The incredible originality of this ZR1 has been recognized by the National Corvette Restorers Society (NCRS), which has a special category for unrestored cars called Bowtie judging. A team of judges with specialized knowledge evaluates four areas, including the car’s interior, exterior, mechanical parts, and chassis. The judges offer their opinions as to whether each of these areas is original and unrestored, as well as in good enough condition to be of historic and educational value. For each of the areas where a car is adjudicated to meet these criteria, it earns a star, and the very few cars that earn a star in all four categories are given a Bowtie award. To nobody’s surprise, this beautifully preserved Corvette was awarded a prestigious 4-Star Bowtie award at the 2015 NCRS National Convention in Denver.
Tom Flannagan, the car’s present caretaker, is carrying on the habits of those who came before. He drives his ZR1 very sparingly, covering just enough mileage to ensure all the moving parts stay moving. In addition, he proudly displays it at NCRS events and local shows, helping ensure that this rare piece of Corvette history survives to give future generations the same joy it provides us today.
