Steve Magnante's 1001 Corvette Facts. Steve Magnante
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tachometers.
192 The fuel-injected 1957–1961 Corvettes built with generator-driven tachometers were the milder versions. With their lower compression ratios, relatively conservative cams, and hydraulic lifters, rapid RPM surges weren’t as much of a threat as they were with the top-tier fuelie engines, whose elevated compression ratios, more radical cams, and high-revving solid lifters more easily slipped fan belts, sending delayed and inaccurate engine RPM info to the driver. Moreover, 1961 was also the first year Chevrolet barred the mating of fuel-injected engines and Powerglide automatic transmissions. From 1961 to 1965, every fuelie was backed by a manual transmission.
193 The 3-speed manual transmission offered for the first time on 1955 Corvettes was not a derivative of the Muncie 318/319 unit used in passenger cars since 1940. Rather, it was manufactured by GM’s Saginaw Gear division and featured unique ratios of 2.20:1, 1.30:1, and 1:10. Passenger cars from 1955-on (also) using this new transmission were equipped with 2.94:1, 1.31:1, and 1:10 ratios. The 2.20 First gear ratio offered less torque multiplication than the 2.94:1 passenger-car ratio but was offset by the Corvette’s 500-pound-lighter mass. The benefit of the 2.20:1 Low-gear ratio was that it suffered less of the RPM drop encountered during upshifts, and thus kept the engine closer to its power band when driven with a heavy throttle foot. Of the 3,467 Corvettes built in 1956, only 1,878 were equipped with this new 3-speed manual. Unfortunately, this new transmission lacked synchromesh in First gear.
194 All 1953–1955 Corvettes came with a 3.55:1 rear-axle ratio, regardless of engine or transmission. For 1956–1961, the 3.55 ratio was standard only with the Powerglide automatic. For 1961, manual-transmission-equipped Corvettes came standard with slightly deeper 3.70:1 gears to bolster off-the-line torque multiplication. Optional rear-axle ratios also arrived in 1956 and included 4.11 and 4.56 ratios. For reasons unknown, the overdrive-equipped 3-speed stick available on passenger cars was not available in Corvettes. Although acceleration with 4.11 or 4.56 gears was thrilling, highway cruising at 3,800 rpm could be loud and tiresome. In 1961, a 3.36:1 gear-set option arrived for buyers less concerned with acceleration than with calm highway cruising and improved fuel economy.
195 In 1962, the standard 3.55:1 rear-axle ratio was changed to 3.36:1 because the new 327 delivered an average of 50 extra ft-lbs of torque. Thus, 3.36:1 gears appeared as standard equipment with all engines except for the top-tier, solid-lifter 340-hp (RPO 396) and 360-hp (RPO 582) fuelies. The only exception to the 3.36:1-ratio rule came when the 340- and 360-hp engines were coupled to the 4-speed manual transmission (and most were). Then the standard axle ratio was 3.70:1. The torque-laden 327 also opened the door to Corvette’s highest ratio (low numeric) to date, a 3.08:1 gear set, which was available with all versions of the new 327.
196 The Corvette’s small-block engine was offered in four displacements, 265 (1955–1956), 283 (1957–1961), 327 (1962–1968), and 350 (1969–1996). Unlike the practice employed in the passenger-car lines, where different sizes were offered optionally, Corvettes always came with the largest displacement offered for that particular model year. By contrast, Corvette’s early nemesis, the 1955–1957 Ford Thunderbird, forced buyers to pay extra for larger engines. In addition, unlike Thunderbird, which offered a lowly 2-barrel carburetor on base models, even the most inexpensive Corvette model was fitted with a 4-barrel carburetor and dual exhaust.
197 An open, unshielded distributor cap and wires in any early Corvette can only mean one thing: radio delete. Because the fiberglass firewall didn’t block electronic interference from the receiver and antenna, radio-equipped cars required elaborate stainless steel and chrome-plated stamped-tin shrouding.
The exposed wiring won’t cause any radio static on this Corvette because it was born without a radio. To save a few manufacturing dollars, radio delete Vettes didn’t carry the shielding. Notice the unused screw holes and mounting tabs on the spark-plug wiring loom. The same bits were used in either case.
198 Proving that racing improved the breed, the 1957 model year brought the much-needed Posi-Traction limited slip differential to Corvette’s option sheet. It answered the need for a means of preventing wheel spin on hard acceleration and cornering that was apparent from the start of Corvette’s existence in 1953. After validation aboard 1956 factory Corvette race-team cars, the clutch-type differential was recognized as the best type for mass production. Unlike the harsh on/off ratcheting differentials tested by Arkus-Duntov’s minions at the racetrack and proving grounds alike, the spring-loaded clutches placed within the differential case engaged gently and sent equal power to both rear tires. Chevrolet also added Posi-Traction to the passenger-car option sheet in 1957, and a legend was born.
199 General Motors rightfully recognized Posi-Traction as a technological advance worth bragging about. Just as its Rochester fuel injection warranted special chrome fender emblems, General Motors came up with metal badges touting the presence of Posi-Traction, but with a twist: The badges were applied to the dashboard, inside the vehicle, where the only people impressed were the passengers. Also, these badges were only installed on regular passenger cars, not on Corvettes. Used only on 1958–1959 passenger cars, the dash-mounted Posi-Traction emblems were gone by 1960 and, surprisingly, never reappeared, not even aboard such muscular performers as the Chevelle SS396, Camaro Z/28, or Impala SS427. The Beach Boys immortalized GM’s clutch-type limited slip differential in a 1962 song with the lyrics, “She’s my 4-speed, dual-quad, Posi-Traction 409.” The only other Detroit automaker to establish metal emblems for its limited-slip-differential-equipped vehicles was Studebaker. From 1958 to 1963, cars and pickups with Twin Traction limited-slip equipment each bore a one-piece chrome emblem affixed to the tail panel. The emblem consisted of twin, interlocked circles with a bold letter T at the center of each. The TT stood for Twin Traction. To my knowledge, Detroit automakers did not otherwise tout limited-slip differentials in the Corvette years.
Found inside a triple-carbureted 348-powered 1958 Biscayne, this Posi-Traction dash badge hinted that twin strips of burnt rubber were just a foot stomp away. But did Corvette passengers ever see this emblem? See Fact 199 for the answer.
200 Although no Corvette ever wore blatant Posi-Traction badging, Posi-equipped models contained warning stickers with bright red letters on a white background affixed to the underside of the deck. These stickers warned owners not to change flat rear tires with the engine running and the transmission in gear because the limited-slip unit was capable of energizing the tire in contact with the ground and driving the car off the jack. The only other externally visible markings found on 1957–1962 Corvettes equipped with Posi-Traction were a circular metal tag fastened to the lube-filler plug alerting the need for a friction-modifying oil additive and an integrally cast letter P (for Posi-Traction) featured on the driver’s side of (most) 1957–1962 center sections. The lettering of the P was machined differently and also appeared on Chevrolet passenger cars equipped with Posi-Traction. Many say Chevrolet squandered the opportunity to bring even greater recognition and fame to Corvette by ignoring the Posi-Traction advancement.
201 The open-face/open-tail rear axle housing used on 1953–1955 models was replaced in 1956 by a new housing that eliminated the bolt-on inspection cover. The new housing made its debut a year earlier on 1955 Chevy passenger cars and featured a welded-on rear cover, also known as a removable carrier, for extra strength in the brave new world of V-8s.
202 A detail that Arkus-Duntov and the Corvette development team struggled with through 1962 was General Motors’ decision not to share the 1955 passenger car’s modern ball-joint front suspension with the Corvette. Budget constraints mandated the continued use of the kingpin front suspension used since 1946 on passenger cars. Lacking the anti-dive geometry designed into the 1955 configuration, Corvette road