From Mystery Motor to Main Street

Corvette’s first big-block traces its lineage back to a NASCAR racing engine

March 22, 2018
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All the way back in 1965, anyone with about $4,500 to spare could buy an L78-optioned Corvette and, with it, rule the streets. The formula was simple: a Mark IV big-block Chevrolet engine displacing 396 cubic inches and delivering 425 horsepower in a car that weighed about 3,200 pounds. This engine was a direct descendant of the fabled 427 Mark IIS “Mystery Motor” that scorched a path to NASCAR victory circles in 1963 in the hands of Johnny Rutherford, Junior Johnson, Rex White and others. Like the small-block Chevrolet introduced in 1955, it set the racing world on fire and upended the auto industry with an unbeatable combination of high power and torque, low mass, excellent reliability and low manufacturing cost.

The Corvette’s 396 big-block lineage can be traced back to Chevrolet’s first large-displacement V-8. Called the W-series or Mark I engine, it was introduced in 1958 with a displacement of 348 cubic inches, and later enlarged to 409 cubes. The W engines were commercially successful, most notably in Impala Super Sports, as immortalized by the Beach Boys in their 1962 hit song “409.” They were also quite successful in competition, beginning with Junior Johnson’s 1960 Daytona 500 victory in a 348-powered ’59 Chevy.

Though they were winners on and off the track, the W-series powerplants did have some weaknesses and limitations that ultimately led to their demise. They were relatively heavy, tended to run somewhat hot and couldn’t be enlarged beyond about seven liters (roughly 427 cubic inches) without extensive changes. Moreover, their exterior dimensions made it impractical to install them into Corvettes or the small and intermediate cars coming down the pipeline, such as Chevy IIs and Camaros. But perhaps most concerning of all, the Chevrolet engineering team eventually hit a wall regarding the W engine’s power output because of design limitations. The combustion chamber was partially in the block, and the top of the block was not square to the centerline of the bore or the top of the piston. This convoluted arrangement restricted important considerations, such as combustion characteristics, valve size and placement, and bore diameter.

The way around these inherent shortcomings was a new, clean-sheet design. In mid-1962, shortly after Bunkie Knudsen moved from Pontiac to take the helm as general manager of Chevrolet, he gave Chevy chief engineer Harry Barr the go-ahead to design a replacement for the 409 race engine. The team working under Barr, led by brilliant engineer Dick Keinath, went to work in July 1962 to design an all-new engine, which would be called the Mark II.

Aside from a common bore spacing of 4.84-inches, the Mark II engine shared nothing with its predecessor. The new cylinder case was completely square, with the block’s deck surface at 90 degrees to the crank centerline, and the pistons square to the deck surface. The cylinder heads were completely new as well, with integral combustion chambers and canted valves, which gave rise to the nickname “porcupine” heads. The machining for the valves, as well as all of the related components, was particularly challenging in the era before computer-aided design and tools, but the new head’s superior airflow characteristics were well worth the investment of time and expertise.

The Mark IIS (“S” because it was a 427-ci stroked version of the 409 cid Mark II) made its debut at Daytona in 1963. Unknown to most people, however, the engine’s maiden appearance was not in the Chevy stock cars contesting the Daytona 500. Instead, their very first race was in Daytona’s American Challenge Cup, a 250-mile event that included GT cars. Two of Mickey Thompson’s Z06 Corvettes entered in that race had been retrofitted with Mark IIS 427 engines by Smokey Yunick. They were the fastest cars in the field, with Junior Johnson driving one to first and Rex White driving the other to second in the event’s qualifying race. But poor handling and issues resulting from heavy rain relegated them to Third and 13th in the final standings.

Six days after the American Challenge Cup, Mark IIS–powered Impalas swept the Daytona 500 qualifying races. Johnson won the first 100-mile qualifier in a Ray Fox–entered car at a record-setting average speed of 164.083 mph, while Johnny Rutherford won the second qualifier with an average speed of 162.969 mph. The Mark IIS Impalas were the fastest cars in the Daytona 500, with three of the four leading at some point, but all suffered mechanical problems and bad luck that put them well back at the end.

Also from Issue 121

  • ’72 C3 "Wagon"
  • $8K Buyer's Guide
  • '54 "Blue Flame" Roadster
  • LS7-Powered C5 Z06
  • '72 LT-1 Vintage Racer
  • Inside the Corvette Engine Lab
  • Restored ’60 Fuelie
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