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Chevy Big-Blocks—the Mark IV and a New 427

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Light-duty trucks and some larger vehicles used a 366-cubic-inch (6.0L) version of the Mark IV from the mid 1960s until the mid-’90s. A 396-cubic-inch (6.5L) version of this V8 was available for the Corvette in 1965 as the L78 option as well as for the Z16 Chevelle. The engine had four-bolt main bearing caps, a bore of 4.095 inches (104.0 mm), and the stroke was 3.760 inches (95.5 mm). The 396 version delivered 375 hp (280 kW), developing 560 lbs.-ft. of torque and performs well running in the upper 6000 rpm range. The 402-cubic-inch (6.6L) is a variation of the 396, bored 30/1000 of an inch. This engine sold as a 396 in the smaller vehicles, then marketed as the Turbo-Jet 400 in the full-size automobiles, although light-duty pick-up trucks had the identical engine as the other two but was referred to as the 402.

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For the 1966 model year, the Mark IV engine showed up on the sales floor as the 427-cubic-inch (7.0L), but it was available as a power option on the full-size Chevys and also available under the hood of the Corvette. The bore increased to 4.25 inches (108 mm) ,but depending on where the engine was, the horsepower rating varied.

If the engine was in a family sedan, the car would have a smooth, even idle with hydraulic lifters, but a rough-idling high rpm version with solid lifters was also a choice and often put into a basically equipped unit such as a Biscayne two-door. In the Biscayne with RPO L72, the solid lifter 427 delivered 425 hp (317 kW). The plain Jane, lightweight Biscayne was a sleeper, looked much like a taxicab with two doors, and got the moniker Bisquick.

Beneath the hood of the Corvette from 1967 to 1969, the quintessential version of the 427 Tri-Power was available from the factory as RPO L71. This was three two-barrel carburetors that fed the fuel into the combustion chambers and developed 435 hp (324 kW) although it’s the same engine as the RPO L72 introduced in ’66. However, that version had a single four-barrel bolted to the intake.

Both of the 427 variations featured a high-lift, long-duration, high overlap camshaft, and to increase airflow, the cast-iron heads had large ports delivering more power at high rpm. A review of the Corvette stated the car gave “the ultimate in sheer neck-snapping overkill.” Both the RPO L72 and L71 did a standing start to 60 mph (97 km/h) in less than six seconds with quarter-mile time around the 13.5 second mark, achieving a top speed of 105 mph or better in the process.

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Aluminum heads for the L71 were also available as RPO L89, but this option did very little to increase the performance in a straight line. What the option did do was reduce the weight of the vehicle by about 75 pounds, which improved the weight distribution, resulting in improved handling. Super Chevy magazine conducted a chassis dyno test on a factory-produced Copo L-72 equipped Camaro. Although this car was well tuned, it only registered a peak of 289 actual horsepower delivered to the wheels. This test shows just how inaccurate the old style “gross” horsepower ratings actually were.

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