Chevrolet Big-Block Engine - Generation 2: Mark IV Series - 427

427

The highly successful and versatile 427 cubic inch (426.7cuin)(7.0 L) version of the Mark IV engine was introduced in 1966 as a production engine option for full sized Chevrolets and Corvettes. The bore was increased to 4.25 inches (108 mm), with power ratings varying widely depending on the application. There were smooth running versions with hydraulic lifters suitable for powering the family station wagon, as well as rough idling, high-revving solid lifter models that resembled racing powerplants.

Chevrolet produced the Big Block 427 until the mid-1990s. The only 427 available from 1972 to 1995 was a truck version that was used in GM medium duty trucks such as the Chevrolet Kodiak and GMC Top Kick until 1995. Throttle body electronic fuel injection was used on the 427 from 1988 to 1995.

Not every version of the 427 was available in every car, and ordering the highest performance versions often required that other options be added to or deleted from the car (for example, power steering wasn't available with the high performance models). This relationship between engine configuration and vehicle options often resulted in what was jokingly referred to as a "racing taxicab," the description usually applied to a minimally equipped, plain looking, two door Biscayne sedan fitted with the 425 horsepower (317 kW) version of the 427— (RPO L72), resulting in a vehicle whose performance was the polar opposite of a taxi. This lightweight, big-block Biscayne was also commonly referred to as "Bisquick."

Perhaps the ultimate 427 for street applications was the 435 horsepower (324 kW) L71 version available in 1967 to 1969 Corvettes, and in the Italian Iso Grifo. This engine was identical to the 425 hp (317 kW) L72 427 (first introduced in 1966) but was fitted with three two barrel carburetors (known as "Tri-Power") in lieu of the L72's single 4 barrel. Both engines used the same high lift, long duration, high overlap camshaft and large port, cast iron heads in order to maximize cylinder head airflow (and, hence, engine power) at elevated engine operating speeds. Consequently, the engines offered very similar performance and resulted in a car whose performance was described by one automotive journalist as "the ultimate in sheer neck-snapping overkill." Typical magazine road tests of the day yielded sub-6 second zero to 60 miles per hour (97 km/h) times and quarter miles in the mid 13 second/106 MPH range for both the L72 and L71.

In 2011, SUPER CHEVY MAGAZINE conducted a chassis dyno test of a well documented, production-line stock but well-tuned L-72 "COPO" Camaro and recorded 287 peak HP at the wheels, demonstrating how distorted the old "Gross" HP ratings actually were.

RPO L89 was an L71 fitted with aluminum heads While this option produced no power advantage, it did reduce engine (and hence, vehicle) weight by roughly 75 pounds (34 kg). This resulted in superior vehicle weight distribution for improved handling, although any difference in straight line performance was essentially negligible.

The 1969 ZL1 version of the 427 engine was developed primarily for Can-Am racing, where it was very successful in cars like the Chaparral 2F and McLaren M8B. The ZL1 had specifications nearly identical to the production L88 version of the 427, but featured an all-aluminum cylinder block in addition to aluminum cylinder heads, which dropped total engine weight into small block Chevy territory (approx. 575 lb/261 kg dressed). The engine was also fitted with the new open combustion chamber cylinder heads, a light weight aluminum water pump, a camshaft that was slightly "hotter" than the L88's and a specially tuned aluminum intake manifold. Like the L88, the ZL1 required 103 octane RON (minimum) fuel, used an unshrouded radiator and had poor low speed idle qualities - all of which made the two engines largely unsuitable for street use.

As impressive as the ZL1 was in its day and despite the "larger than life legends" surrounding it, actual engine dyno tests of a certified production line stock ZL1 revealed 376 SAE net HP, with output swelling to 524 Gross HP with the help of optimal carb. and ignition tuning, open long tube racing headers and with no power sapping engine accessories or air cleaner in place. ZL1 DYNO TEST - COPO CAMARO WEBSITE A second engine dyno test conducted on a second production line stock (but recently rebuilt and partially blueprinted) ZL1 revealed nearly identical figures for the various "Gross" conditions.

Period magazine tests of the ZL1 were quite rare due to the rarity of the engine itself. "High Performance Cars" tested a production line stock but well tuned example and recorded a 13.1 second/110 MPH quarter mile, which correlates quite well with the previously referenced 376 Net HP figure. "Super Stock and Drag Racing Magazine" recorded an 11.62 second/122.15 MPH quarter mile in a ZL1 Camaro that was professionally tuned and driven by drag racing legend Dick Harrell, although that car was also equipped with open long tube S&S equal length headers, drag slicks and minor suspension modifications. Using Patrick Hale's Power/Speed formula, the 122.15 MPH trap speed indicated low 11 second ET potential (e.g. with larger drag slicks) and suggested something on the order of 495 "as installed" HP in that modified configuration. This large difference in 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 4718 dollar 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 on to 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, which, 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 460 lb·ft (620 N·m).

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)