Corvette Performance The Technical Bits
by Greg Raven
The Corvette, a.k.a. “America’s Sports Car,” got off to a bumpy start in 1952 when General Motors Art and Colour Studio chief Harley Earl first proposed using the then-new glass-reinforced plastic (fiberglass) construction for the body of a new sports car he had in mind. With the initial goal of the project being to discover the applicability of fiberglass automotive body panels, Chevrolet’s ambivalent approach to the sports car was shown in their selection of a straight-six motor found for 1953 and 1954 Corvette. Derived from the 235 CI Blue Flame six, output was bumped from 105 hp to 150 hp through the use of a high-lift, long-duration camshaft, higher compression (at 8.0:1), and three Carter type YH side-draft carburetors on a special aluminum intake manifold.
The resulting performance was less than thrilling, and at the end of 1954 Chevrolet was ready to throw in the towel on the Corvette project. The only thing that prevented them from doing so was the existence of the Ford Thunderbird. The rivalry between Ford and Chevrolet was intense during this period, and Chevrolet was not about to concede the “sports car” market to Ford. Something needed to be done to revive Corvette’s flagging sales, however, and in 1955 that something turned out to be a 195 hp V8. The straight-6 was still available (with an increase in horsepower to 155), but few were sold. Compared to the V8 Corvette, the Thunderbird had better sales and creature comforts, but the Corvette led where it counted; it was quicker from zero-to-sixty.
So for the first three years of its life, the Corvette image wandered around as if in search of a reason to exist, other than as an answer to the much more popular Thunderbird.
It wasn’t until 1956 that Corvette started to expand into the niche that it would eventually come to fill, when engineer Zora Arkus-Duntov decided that if the Corvette looked the part, it should act the part, as well. In addition to the minor cosmetic and creature-comfort changes seen in the 1956 Corvette, the output of the V8 was increased to 210 hp (with a single four-barrel; 225 hp with dual four-barrels, and 240 hp with the four-barrels and a special Duntov-designed high-lift camshaft). With a standard 3-speed transmission, 0-to-60 times of 7.5 were possible, as were quarter-mile times of 16 seconds at 90 mph. Sales picked up, and the Corvette was saved from extinction.
Arkus-Duntov wasn’t finished with the Corvette, though. Even with no external changes, the 1957 Corvette was big news by virtue of the five engine options: a single four-barrel with 220 hp, two dual four-barrels offering 245 hp and 270 hp, two with fuel injection that produced 250 hp and 283 hp, and a racing version with fuel injection that was also rated at 283 hp. As you might imagine, this early Rochester “Ramjet” fuel injection was less than reliable, but Chevrolet’s marketing department had a field day with the fact that the Corvette had the first mass-produced engine that developed one horsepower per cubic inch.
The 283 block was arrived at by taking a 265 block and boring it out, but there were other small changes that made it a better performer all around. Wider bearings meant the 283 had a stronger bottom end, and larger ports meant better breathing. The compression ratio was higher, as was the lift on the camshaft. Carbureted motors received hydraulic lifters for reduced maintenance, while all models featured oil-control rings on the pistons, longer reach spark plugs, and carburetor-mounted fuel filters.
This year also saw the introduction of a 4-speed gearbox, which when ordered with the 4.11:1 rear axle and mated to a fuel-injected 283 hp engine was capable of reaching 60 mph from a standing start in less than six seconds. The quarter-mile came up in 14.3 seconds at 96 mph.
Nineteen fifty-seven also was the year in which Chevrolet began to address complaints about braking and handling. Where the previous model sported cast iron drums and 158 square inches of lining area, the 1957 ‘Vette had ceramic-metallic linings and finned, ventilated drums. This upgrade was part of the RPO 684 suspension package that also included a front anti-roll bar, heavier front springs, heavier rear springs, and larger, firmer shock dampers. Additionally, the RPO 684 package improved steering response with a high-ratio steering adaptor that retained the same turning radius but changed the number of turns lock-to-lock to 2.9 (compared to the previous year’s 3.7 turns).
Between the engine options and the suspension changes, the Corvette was not only selling better, but it was also becoming a potent force at the track. Production Corvettes finished 12th and 15th overall at Sebring, taking first and second in the GT class and handily beating their nearest competitor, the Mercedes 300SL, by 20 laps.
What happened next would have been a disaster for most other cars. First, in 1958 Chevrolet decreed that the Corvette start showing a profit. That meant the clean design of the 1957 model gave way to chrome excesses of the period. Second, General Motors joined the Automobile Manufacturers Association ban on race support. Lastly, barely out of adolescence, the Corvette started putting on weight.
Longer, wider, and heavier. But even with the ban on corporate racing involvement, the engine options for the Corvette continued to please enthusiasts. Again five engines were available, including a 230 hp single-barrel, 245 hp and 270 hp dual four-barrels, and 250 hp and 290 hp versions with a reworked, more reliable fuel injection.
For 1959, the motor options stayed the same, but underneath, radius rods had been added to help reduce axle tramp, and the RPO 684 option was revised to include stiffer springs. Brakes, still a problem, could be upgraded with the RPO 686 option of sintered metallic linings.
As it turned out, 1959 was the calm before the storm. Corvette engineering had been working overtime on improvements, including four-wheel independent suspension and a rear-mounted transaxle, both of which appeared first in the so-called Q-model Chevrolet prototype, which was later taken over as the Q-Corvette. Unfortunately, the Q-Corvette never jelled into a production model (Chevrolet balked at the cost of the transaxle), so Corvette buyers had to wait until 1963 for independent suspension.
What they did not have to wait for was a Duntov-revised suspension that relied less on stiff springs and more on anti-roll bars front and rear — the first American car to be so outfitted. To decrease weight, the 1960 Corvette made greater use of aluminum. It worked well in clutch housings and radiators, but not so well in the cylinder heads of the fuel-injected cars. After a small initial run showed the problems inherent in early alloy heads, cast iron again became the material of choice.
If the average Corvette buyer did not appreciate these features, the Corvette racer did. Each change made the Corvette more and more competitive, finishing first in class at Sebring for the third year since 1957, and finishing 8th overall at the 24 Hours of Le Mans. Additionally, Corvettes captured both the C Sports/Racing and B-Production championships in SCCA competition.
Incremental refinements of the 1959 Corvette resulted in the 1960 Corvette. The aluminum radiator became standard equipment, and aluminum replaced iron in the transmission case. The top-rated engine produced 25 hp more than the previous model, with 315 hp on tap. With short gearing, 60 mph came up in 5.5 seconds from a standing start, and the standing quarter-mile flashed by in 14.2 seconds at 99 mph.
Even at that, 1961 marked the end of an era for the small block 283. In 1962, bore and stroke were increased to make room for 327 cubic inches and even more horsepower. The base model came with 250 hp, but engine options offered buyers a choice of 300 hp or 340 hp with carburetors or 360 hp with fuel injection. With more horsepower came better performance: 15-second quarter-mile times at over 100 mph. Handling, braking, and driver comfort problems had been suitably resolved as well, making the 1962 Corvette a major-league player in the world of sports cars.
Engine offerings for 1963 were identical to those of 1962. The big changes were in styling, chassis, and suspension. The styling department had spent considerable time in the wind tunnel refining the shape of the rebodied Corvette. In those years, “refining” meant reducing drag, and they did a creditable job of that. What they did know at that time was the new low-drag Corvette body generated a substantial amount of lift at high speeds. The hidden headlights had not been seen since the 1942 DeSoto, although Corvette engineers made certain the mechanicals on their system were better than their inauspicious predecessors. Also hidden within the 1963 Corvette was a stronger steel support structure for the body, the result of time spent with computer modeling of the frame.
The lessons learned from the gone-but-not-forgotten Q-Corvette finally made it into production with the 1963 model year, as well, with three-link independent rear suspension replacing the live rear axle. Double-jointed driveshafts, control arms, and trailing radius rods completed the picture, while a single transverse leaf spring provided suspension travel. With the space limitations in the Corvette, the transverse spring (reminiscent of the Ford Model T) was the only design that worked.
It is nice to think that the Corvette would have received independent rear suspension by this point anyway, but the motivating factor for Chevrolet was Carroll Shelby’s new Cobra. Originally built to beat Ferrari, the Cobra was not as refined as the Corvette and did not the sales numbers of the Corvette, but it could and did beat Corvettes at the race track.
Even at that, Duntov had to save money wherever he could to make room in the budget for his long-hoped-for independent suspension. To simplify the rear suspension (making it less expensive), Duntov used the half shafts as load-bearing members, as he had in the first Chevrolet Engineering Research Vehicle (CERV I). Up front, Duntov used existing pieces from a full-size Chevrolet to further help keep costs down, but the geometry required by the Corvette made the car extremely sensitive to wheel alignment. A hydraulic steering damper proved to be the cure, and Duntov got his rear suspension.
For 1964, the fuel-injected 327 engine produced 375 hp and 11.0-second quarter-mile times. In 1965, the Corvette received four-wheel disc brakes, after years of attempts to come up with a system that would work properly on a car with the high weight and low frontal area of the Corvette (a larger frontal area would have slowed the car more due to wind resistance, reducing the demands on the brakes).
Where many had tried and failed, Delco engineers came up with a novel approach: they did not worry about retracting the brake pads away from the discs when the brakes were not being applied. This improved brake response, obviously, but more importantly it gave them the latitude to design a caliper that would do the job. In 1962 when they took over the project from Girling, they were told it couldn’t be done. Three years later, they had a four-piston caliper with internal fluid passages, internally vented rotors, and enough strength to turn on a little stopping power where it was needed. For good measure, they also added insulators to help keep the brake fluid cool in competition.
Until 1965, Duntov’s philosophy about the Corvette had been one which stressed high technology over brute power, to use his own words. With the introduction of the 427 CI Mark IV motor (a sneak preview of which was seen in 1964 with the 396 motor), the high tech approach was left at the side of the road by the marketing types. The new, bigger motor was cheaper to produce than the fuel-injected small-block 327, and it made gobs of horsepower. The pure performance of the 427 was made customers happy with 0-to-60 times of 4.8 seconds, and it made the bean counters at GM happy by allowing Chevrolet to drop the expensive and somewhat troublesome mechanical fuel injection in favor of carburetors.
All along, Shelby’s Cobra goaded Corvette engineers on to higher heights. While the 1963 lightweight version of the Corvette (called the Grand Sport) acquitted itself well for a while (thanks in part to the 377 CI engine), it was never produced in sufficient numbers to dominate, and by 1966 it was again outclassed by the Cobra. Duntov’s answer in 1966 was the L88 427 big block with 560 horsepower.
Still, the lighter Cobra proved to be a formidable opponent in the A-Production class. That left endurance racing, where the robustness of the heavier, sturdier Corvette carried it to the finish line. Unfortunately, GM Corporate had again pulled back from race support, leaving a long list of competition suspension and brake system options for the Corvette but not much else. The rest was up to privateers such as Jim Hall and Roger Penske.
Things on the engineering side carried on without many changes through 1967, and for that matter, on through the body change in 1968 as well, although the 1968 and 1969 Corvettes had so many teething problems that they are generally held in low esteem. No matter what one’s opinion of the 1968 re-style, it did show the results of yet more time in the wind tunnel. Although the Cd was still high by modern standards, the amount of lift generated by the body at high speeds was reduced enough to keep the front wheels firmly on the ground, something that could not be said of the earlier body.
In terms of pure design, the body itself was the last descendent of a line that started with 1964 production-car based Mako Shark II in 1965, extended through the 1966 version of the Mako Shark II, and eventually made it into production with the 1968 production Corvette, with changes to make it more amenable to street driving.
Through 1969, racers could still order either the 350 or 427 engines with anywhere between 300 horsepower to 435 horsepower, not counting the 500 horsepower racing versions (L88 and ZL1). In 1970, however, the 427 gave way to a yet larger engine: the 454 CI Mark IV. Nineteen seventy was, in fact, the only year in which the 454 could be had with high compression. The very next year, compression ratios dropped in anticipation of no-lead and low-lead fuels. Looking at the numbers, the drop in compression ratio looks worse than it was due to the fact that published horsepower ratings were expressed in net figures (with accessories and accessory drive belts installed) rather than in gross figures.
And so with power levels down (they would drop even further over the next few years) and the body essentially unchanged, the Corvette metamorphosed from a study in evolution to a static display, waiting along with everybody else for the effects of the Arab oil embargo of 1973 to dissipate.
This is not to say that absolutely nothing of interest happened between 1968 and 1983. The 1968 Corvette, for example, was outfitted with fiber-optics that allowed the driver to monitor the functioning of running and brake lights (these only lasted until 1971, however). The 1969 Corvette was available with a 500+ hp 454 CI aluminum ZL-1, and the next year, if a 370 hp 350 CI LT-1 motor was not enough, you could opt for a 454 CI engine with either 390 hp (street trim) or 465 hp (race trim).
Nineteen seventy-three saw the inclusion of the soft nose bumper, with the rear getting a similar treatment the very next year. A year later in 1975, Corvette introduced the bladder-style fuel tank, and breakerless ignition to replace their already delightful points-and-condenser set-up. Perhaps most significant from these years, the 1981 Corvette pioneered composite chassis springs by replacing the 33 pound rear transverse leaf spring with a “plastic” leaf spring that weighed all of seven pounds.
Then came 1984.
With fifteen years on the old body style, everyone knew a new ‘Vette was on the way. When it arrived, it was better than could have been hoped for.
Underneath the new skin remained the same throttle-body injection of the previous two years, but with a more exciting exterior, the Corvette team seemed to come back to life. Power levels again started climbing, and a host of small improvements made their impact.
Like the 1968 Corvette, the 1984 Corvette needed some sorting out. As nice as the car was, the ride was harsh. The skidpad numbers were high, thanks to the fat, low-profile tires now standard, but as one reviewer said, “I drove the car for two weeks and never saw a skidpad once.”
As the base suspension was refined, Corvette added the Z51 suspension for the benefit of those racing the Corvette, a list that turned out to include a lot of people. Whether the racing was in SCCA Pro Solo or in Showroom Stock, Corvettes were a force to be reckoned with. Faithful Porsche racers campaigned normally-aspirated 944s unsuccessfully for the first season, confident that the arrival of the 944 Turbo would turn the tide.
This was not to be a repeat of the Corvette/Cobra duel of old, however. By the time the 944 Turbo was able to take its place on the starting grid, the Corvette had moved forward with better brakes, better suspension, and better engine management computers. In short, it dominated the competition, until in 1988 Chevrolet decided to pull out of Showroom Stock racing and hold a special Corvette Challenge race — naturally for Corvettes only.
One unusual aspect of Corvette engine development is that all the engines discussed thus far have been normally aspirated. (Some might feel it strange the Corvette had to wait so long to get an OHC engine, but given Chevrolet’s success with pushrod V8s, this is to be expected.) Duntov experimented with turbocharging and supercharging in the CERV I car back in 1960, but for the most part forced induction (which must include nitrous oxide injection, too) was the province of aftermarket tuning shops.
With the appearance of the 1984 Corvette, Corvette engineering decided to try their hand at turbocharging. Thirty prototypes and countless dollars later, they had a ton of data but no positive results. The Corvette engine compartment is crowded, making the going tough right out of the gate. The big problem they kept running into, however, was the low heat tolerance of the fiberglass body material.
Enter Reeves Callaway of Callaway Engineering in Connecticut. Corvette engineers had a chance to drive an Alfa Romeo with a Callaway turbo system and were very impressed. They contacted Callaway and asked if he would be interested in turbocharging the Corvette. Reeves had already considered the project but rejected it as being impractical. But, with the delivery of a couple of prototype vehicles, some seed capital, and the stack of data, he decided to give the project another look.
The result, which first appeared in 1987, was the Callaway twin-turbo Corvette, a monster of a machine producing 375 horsepower where the standard L98 engine managed 240 hp. Torque was up as well, yet both figures came at lower rpm than the stock engine, maintaining the car’s driveability. As impressive as a stock 1987 Corvette is, in the Callaway Corvette the 0-to-60 time is on the order of 4.6 seconds, with a quarter-mile time of 13.2 seconds at 109.1 mph.
For an encore, Callaway spent nearly a million dollars improving on their 1987 design in coming up with the 1988 Callaway Corvette, which is capable of 382 horsepower with greater reliability than the previous year’s car.
Normally, the efforts of a tuning firm would be at most an interesting footnote to a story about a company such as Chevrolet. In the case of the Callaway Corvette, however, it must be pointed out that Chevrolet was so impressed with the quality of workmanship that they have assigned the twin-turbo its own RPO (regular production order) number, the only time in Chevrolet’s 75-year history that an RPO has been assigned to an outside vendor, and the first project they have ever offered to share the warranty on.
Meanwhile, back at the ranch, Corvette engineers were cooking up some magic of their own for the Corvette. The brakes, suspension, and tires were up to the task for the racing crowd, but the high-performance street enthusiast was left with the decision of whether to go with the relatively soft stock suspension or the decidedly stiff Z51 suspension. In 1987, Corvette offered the Z52 suspension to bridge the gap, and it may be the best all-around Corvette suspension ever.
By this time, the Corvette was at a level of development where it again became proper to speculate as to whether the Corvette actually could be classified as a “world-class” car. The numbers were all there (acceleration, deceleration, and cornering ability), so that while the fit and finish were not up to the standards of, say, Porsche, the Corvette was certainly a logical comparison against the overrated Ferrari.
That’s when Corvette engineers made a move calculated to blow virtually every other “high-performance” car into the weeds.
In conjunction with Lotus Engineering of Great Britain (the majority shareholder for which is none other than General Motors), Corvette engineers embarked on a program to develop a state-of-the-art engine for the Corvette.
The result, of which prototypes were available to the press in late 1988, was the ZR1 Corvette, featuring the Lotus-designed, DOHC, four-valve eight-cylinder LT5 motor. With 380 horsepower and a compression ratio of 11.25:1, the ZR1 covers the quarter-mile in 12.86 seconds at 116 mph, with a 0-to-60 time of less than 5.0 seconds. Before you hark back to the 1965 L76 motor with similar output, remember that this car meets all emissions standards, and is fuel-efficient enough to avoid having to pay the gas guzzler tax.
To handle the output, the ZR1 has an all-new six-speed transmission, along with wider, lower-profile 35-series tires on 17 X 11-inch rims in the rear, replacing the scrawny 40-series tires on 9.5-inch rims mounted on the standard Corvette. For safety’s sake, a switch on the center console allows the owner to limit the output of the engine to “just” 200 horsepower before handing over the keys to valet parking attendants.
Looking back over the trail of Corvette concept cars and their impact on production vehicles, it is difficult to guess what the next generation of Corvette will look like. For now, the best clue we have comes in the form of the Corvette Indy, a mid-engine, four-wheel-drive, 2-passenger coupe that made its first appearance in 1986. The specification sheet released by the Chevrolet public relations department reads like a sneak preview of the 1989 Corvette ZR1; the same bore and stroke, the same dual overhead cams, the same cam drive, the same number of valves per cylinder, roughly the same power output, the same wheels and tires, and the same projected performance.
Where the Corvette Indy differs is in engine location (mid-ship, like the CERV I — perhaps the third time will be the charm), full-time four-wheel-drive (ala the CERV II), active suspension, video cameras instead of rear-view mirrors, four-wheel steering, “drive-by-wire” throttle control, and traction control (an add-on to the anti-lock brakes to control wheelspin … in a four-wheel-drive car!). As with all production Corvettes, the Corvette Indy promises a composite body, consisting of carbon fiber, Nomex, and fiberglass in a sandwich, along with reinforced plastic. In the Corvette Indy, the chassis gets a similar treatment, with plastic, carbon fiber, and Nomex panels bonded to a carbon-fiber tub.
Corvette chief engineer Dave McLellan, one person who is in a position to know the future direction of the Corvette, says that although they will no doubt get inspiration from the Corvette Indy car (as they have in the past from the other mid-engine Corvette prototypes including those with rotary engines and the Aerovette), he expects only spin-offs; the Corvette Indy is simply too far out, he claims.
Looking at the Corvette Indy, the lines are sleek enough to suggest that this is a more realistic prototype than others that have preceded it. The most far-out aspect of the Corvette Indy was always the high-output engine, and that is now a reality.
Far out, indeed.
Published in Autotech magazine?