Just because people like Bunkie Knudsen and John DeLorean were driving innovation at Pontiac, that didn’t mean the rest of GM’s divisions were snoozing. No sir. Over at Chevrolet, Chief Engineer, Ed Cole, saw what the buying public wanted and formulated an approach that would result in a Chevrolet product becoming the best-selling car in America for over a decade.
The Chevrolet Impala was introduced in 1958 as the top option level in the company’s Bel Air line. The Impala option added special trim, a deluxe interior and resulted in the highest selling price in the Chevrolet lineup. Performance was a priority from the beginning, as the first Impala could be ordered with a 315 hp, 348 V8 engine.
Cole had defined the Impala as a “prestige car within the reach of the average American citizen,” and those citizens responded – in droves. In its first year of production, Chevrolet sold 125,480 Bel Air Impala coupes, along with 55,989 convertibles. It was a great start that would get better yet.
The Chevrolet Impala SS spawned an automotive phenomenon through the 50s, 60s and into the 70s. More than 13 million were built in the early years.
A major restyling across the entire Chevrolet line in 1959 did nothing to slow down the Impala, which became a distinct model that year. Sales hit 473,000 units, making it Chevy’s top-selling car nameplate. Changes as the decade of the 60’s arrived saw the Impala’s styling become more conservative. Harley Earl’s influence was waning and Bill Mitchell was the rising star.
By 1961, Earl’s tail fins had disappeared throughout the Chevrolet line and later that same year dawned the “Super Sport” model. The Impala SS was to be a true performance car, starting from the fact that only the high-performance 348 engines or the new 409 could be ordered. The 348 could be ordered with up to 350hp, but it was the newly-introduced 409 V8 that set the industry in a pursuit of horsepower that defined the following decade.
Responding to Ford’s 390cid V8, the 409 was a bored and stroked version of the 348, equipped with an aluminum intake manifold, a Carter four-barrel carburetor, a solid lifter camshaft and an 11.25:1 compression ratio. Impalas ordered with this engine would only be built with a four-speed manual transmission. No choice of axle ratios was given, but owners or dealers could install alternates which delivered quarter mile times in the high 15-second range.
When equipped with the RPO Z11 option, Impala SS models were built with a special 427cid engine that used a first-ever, for Chevrolet, cowl induction intake.
In 1962, the SS option could be ordered on any two-door coupe or convertible, and with any engine in the GM lineup. Production of the 409 engine was increased and it became available in all full size Chevrolet’s – Biscaynes, Bel Airs and Impalas. Some might say that the Impala SS had been mongrelized, but that was more than offset by a new version of the 409 V8.
Sporting a new, lightweight valvetrain and dual Carter AFB four-barrel carburetors, Chevrolet astonished the world by announcing that the engine produced 409 horsepower. The one-hp-per-cubic-inch displacement barrier had been breached before but not for an engine this large. With that accomplishment, the 409 motor carved out a permanent place in the history of performance.
The following year, the 409 would continue to grow in both esteem and output. For serious enthusiasts, Chevrolet offered a solid lifter, four-barrel carb version that delivered 400hp. Pumping the solid lifter 409 with a pair of four barrel carbs tipped the scales at 425hp. Ongoing engine development, in search of domination in the country’s drag racing tracks as well as oval tracks, led to the release of a 427cid engine in mid-year.
Fewer than sixty Z11 Impalas were built. Perhaps as few as seven remain today. When Junior Johnson arrived with one at the Daytona 500 in 1963, he immediately set a new lap record.
Earlier, Junior Johnson had arrived for the 1963 Daytona 500 with a white Impala and a “mystery motor.” Based on the existing 409 V8, the motor had a reduced bore diameter and increased stroke. Special cylinder heads featured angled valves and a new intake manifold held a pair of Carter AFB 4-barrel carburetors. Johnson would dominate the weekend and set a new stock car record at the track. Derived from what would be the publicly available 427cid engine, it had proved its performance ahead of time.
Based on the standard 409 block, the Z11 plant was fitted with a stroker crank. The Z11 heads also differed from the standard W-head 409 designs, featuring large, oval intake ports that were significantly taller than the HiPo 409’s. Atop the engine was a two-piece aluminum intake capped with the same dual Carters used on the 409. Over those was a completely unique air cleaner which featured an “arm” that plugged into the firewall (sealed by a rubber boot) that drew air from the cowl.
The engine became available as RPO (Regular Production Order) Z11, but there was far more to a Z11 optioned Impala than just an engine swap. It was all-business and designed for drag racing. The radio and front sway bar were history. Luxuries such as sound deadener, radio and a heater were tossed. Only the “dog leg” shifter and Delco electric tachometer were standard.
An extensive list of aluminum front end components substituted standard production parts, including the hood, fenders, front and rear bumper, as well as the brackets and braces, grille brackets, a two-piece fan shroud, the grille filler panel and hood support catch. With these, the car weighed in at about three hundred pounds less than an equivalent 409-powered Impala off the production line.
This Z-11 example was owned by Jack May, driven by Larry Wilson and race prepared by Ronnie Sox of the Sox & Martin racing team. Ronnie owned and raced a Z11 Impala of his own.
The Z11 also featured a deep sump oil pan – which looked somewhat “homemade” to the untrained eye, but touted a deeper sum welded into the pan from the factory. The Z11 heads also used push rod guide plates, while the Impala rolled with sintered metallic brakes, special air scoops in the backing plates for cooling, a Borg-Warner T-10 4-speed transmission and strengthened Posi-Traction rear axle built with 4.11 gears. The result was a hugely capable car that, in the hands of Ronnie Sox, was regularly turning in low 11.20-second ET’s.
The GM-corporate racing ban that came out of the blue in 1963 put an end to development of the 427, forcing racing teams to seek their fortune with other manufacturers. In total, it is believed that around 50 to 57 Z11 427 Impalas were built. However, once support from the factory was lost, the cars became unimportant and few survived. By some reports, there are only seven known Z11 Impalas remaining today.
In other forms, the Z11’s history lived on. The derivative “mystery motors” used at the 1963 Daytona 500 were, in fact, prototypes for the 1965 Chevrolet Turbo-Jet 396cid V-8 engine. Breaking ground in the 1965 Corvette, as the L78 option, and in the Z16 Chevelle, it would go on to create an entirely new chapter of its own in the book of Chevrolet performance.
Extensive use of aluminum replacement parts reduced the weight of the Z-11 Impala by three hundred pounds, compared to a similar production version.
Continuing for a decade as the best-selling automobile in the USA, Impala broke the record for single nameplate sales, with over 13 million units built. Impala held this record until 1977, setting an all-time industry annual sales record of more than 1 million units along the way. For some, the Impala SS was the defining model credited for the beginning of the muscle car wars.
It would take a war of another kind to temper the American public’s love affair with big, fast cars, but along the way, there were some glorious years.
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