power suggests that the OEM exhaust manifolds and exhaust system were highly restrictive in the ZL1 application, as was also the case with the similar L88. The $4,718 cost of the ZL1 option doubled the price of the 1969 Corvette, but resulted in a car with exceptional performance for its day. Just two production Corvettes (factory option at dealer) and 69 Camaros (non-dealer option from factory - COPO 9560) were built with the ZL1. Chevrolet capitalized on the versatility of the 427 design by producing a wide variety of high-performance, "over-the-counter" engine components as well as ready-to-race "replacement" engines in shipping crates. Some of the components were developed to enhance the engine's reliability during high RPM operation, possibly justifying the use of the description "heavy duty." However, most of these items were racing parts originally designed for Can-Am competition that found their way onto dealers' shelves, and were meant to boost the engine's power output. Beginning in 1969, the highest performance 427 models were fitted with the new open (vs. closed) chamber cylinder heads, along with design improvements in crankshafts, connecting rods, and pistons, adopted from the Can-Am development program. Chevrolet gave all 427 engines except the ZL1 a torque rating of . 427 performance specs Notes: 1966–1969 Chevrolet Biscayne 1966–1969 Chevrolet Caprice 1966–1969 Chevrolet Impala 1966–1969 Chevrolet Corvette 1968–1969 Chevrolet Camaro (most were dealer installed, but in 1969 both the L-72 and the ZL-1 were factory options) 427 Production codes LS-1: produced 1969, 10.25:1 compression, Q-jet carburetor, oval port closed chamber heads, hydraulic lifters, nodular iron crankshaft, and two-bolt main caps. It produced . L-36: produced 1966–9, had 10.25:1 compression, Holley or Q-jet carburetor, nodular iron crankshaft, hydraulic lifters, oval port closed chamber heads, and two-bolt main caps. It produced in 1967–68 full-size cars, in 1969 full-size cars and Corvettes (by exhaust system). L-68: produced 1967–9, had 10.25:1 compression, Tri-Power, nodular iron crankshaft, hydraulic lifters, aluminium oval port closed chamber heads, and two-bolt main caps. It produced , and was used in Corvettes. 454 For 1970, the Big-Block was expanded again, to , with a bore x stroke of . The 1970 Chevrolet Corvette LS-5 version of this engine was factory-rated at and , and the LS-6 engine equipped with a single 4-barrel Holley carburetor was upgraded to at 5600 rpm and at 3600 rpm of torque. It has been suggested that the LS-6 was nominally substantially underrated, which