Examining The Legendary Chevrolet Big-Block Engine

The ticking timeline to the ultimate Corvette performance engine family

Dave Emanuel Sep 22, 2005 0 Comment(s)
Corp_0509_big_1_z 2/8

The big-block Chevrolet engine had a 10-year run from 1965 to 1974,quite a feat by anyone's standards.

In 1963, a new Chevrolet big-block set heads spinning and lips flappingwhen Junior Johnson blasted around Daytona Speedway at the unprecedentedspeed of 166 mph. The powerplant that propelled Johnson's '63 Chevy tosuch an incredible speed had a displacement of 427 cubic inches and waspossessed of unique cylinder heads with canted valves. Such cylinderheads had never been seen before, and caused the new big-block Chevy tobe tagged a "Mystery Engine."

Officially known as a Chevrolet Mark II engine, the new big-blockquickly faded from sight when GM pulled its support of racing. But twoyears later, the Mystery Engine reappeared. It had gone through severaliterations of change, as denoted by the switch from a Mark II to a MarkIV designation, and it was released in 396ci rather than 427ci form, butthe lineage was unmistakable.

L78: The First Corvette Big-Block

Corp_0509_big_2_z 3/8

The '65 396/425hp engine was built only one year, but was the testbed ofthe incredible 427 powerplant to come in 1966.

Introduced in the middle of the '65 model year, RPO L78 established anew milestone for Corvette power--425 hp (at 6,400 rpm) and 415 lb-ft oftorque (at 4,000 rpm). It was the first big-block ever factory installedin a Corvette and the first Corvette engine with a rating of more than400 hp. Its installation required another first: a special hood withfunctional air vents and a unique bulge in the center to clear the aircleaner. Optional side-mounted exhaust pipes were another Corvette firstfor that model year.

Weighing 150 pounds more than the 327, RPO L78 achieved its displacementof 396 cubic inches by way of a 4.094-inch bore and 3.760-inch stroke.(A virtually identical version of the engine was rated at 375 hp,detuned with a hydraulic lift cam, and incorporated in the limitedproduction Z16 Chevelle package, of which only 201 were produced.) Thisengine featured a mechanical-lifter cam, an 11:1 compression ratio,large-port heads (with 2.19-inch intake and 1.72-inch exhaust valves),an aluminum intake manifold, and a Holley four-barrel carb.

L72: The First 425hp 427

For the '66 model year, a 425hp 427 replaced the 425hp 396. Apparently,although the increase in displacement was permanent, its effect onhorsepower was temporary. The engine's initial 450hp rating wassubsequently reduced to 425, although the original 460-lb-ft torquerating was maintained. Except for its 4.251-inch-diameter bore and theincrease in displacement that resulted from it, the L72 was virtuallyidentical to the L78. Had it not been for GM's 400ci displacement limit,the L72 would have appeared in 1965.

L71 :: The First (and only) 3x2-Barrel Corvette Engine

Corp_0509_big_3_z 4/8

In 1967, the L71 engine option was released as a 435hp monster, capableof killing just about anything on the street or strip.

With three high-capacity two-barrel Holleys serving as the inductionsystem's focal point, RPO L71 was rated at 435 hp at 5,800 rpm and 460lb-ft of torque at 4,000 rpm. With an 11:1 compression ratio, amechanical lifter cam, and an aluminum intake manifold, the L71 was atractable street engine with enough power to rivet driver and passengerto the backs of their respective seats. Like the L88, the L71 wasproduced from 1967 to 1969. It was the first and last Corvette enginewith three two-barrel carburetors and, although overshadowed by the L88,the L71 left its own performance legacy.

Production volumes were 3,754, 2,898, and 2,722, respectively, during1967, 1968, and 1969, so quite a few L71s have prowled the nation'sstreets and dragstrips. During these years, Corvettes could also bepurchased with a hydraulic-lifter version rated at 400 hp. Althoughbig-block Corvettes were produced through 1974, the passing of the L71marked the end of an era.

L88: The First Aluminum-Headed Big-Block

Corp_0509_big_6_z 5/8

This is an early L88 (1966) test engine on a pallet.

Talk about breaking new ground, the L88 did so literally andfiguratively. Although it shared the same dimensions (4.251-inch bore,3.76-inch stroke) as its other 427 stablemates, the L88 has becomelegendary for its combination of stump-pulling torque and high-flyinghorsepower. The engine wasn't given a horsepower rating when it wasintroduced in mid-year 1967, but, ultimately, engineers at Chevroletplanted their tongues firmly in their collective cheeks and proclaimedits output at 430 hp at 5,200 rpm. The issue wasn't that the rating wasfalse, it was merely skewed. L88s undoubtedly produced something on theorder of 430 hp at 5,200 rpm. But left unsaid was that it also producedapproximately 500 hp at 6,400 rpm (the engine speed at which most othersolid-lifter-equipped big-blocks were rated). It's also been claimedthat when equipped with headers and an open exhaust, actual output wasin the vicinity of 550-575 hp.

The L88 was set apart from other 427 engines by its large-port aluminumcylinder heads and 12.5:1 compression ratio. Other special equipmentincluded a specially modified aluminum intake manifold, an 850-cfmHolley four-barrel, a mechanical lifter cam with 264 degrees of intakeand 269 degrees of exhaust duration (measured at .050-inch lift),.560-inch intake and .580-inch exhaust lift, special valvesprings, a2.19-inch intake, 1.84-inch exhaust valves, and 7/16-inch-diameterpushrods.

The L88 was a race engine. Period. However, race engines typically findtheir way into street-driven cars, and the L88 was no exception. But asCorvette News noted in an exercise in understatement, "The L88 is not anengine for ordinary everyday driving. It gives a rough idle, is not theeasiest engine to start, and was not designed with high fuel economy inmind."

Available from 1967 through 1969, L88 Corvettes were limited-editionautomobiles. (The L88 option was priced at $947.90 in 1967 and 1968, andat $1,032.15 in 1969.) Only 20 were produced the first year, 80 in 1968,and 116 in 1969. However, quite a few L88 engines were purchased overthe parts counter, so there are probably now more "L88 Corvettes" inexistence than were ever produced by General Motors.

ZL1: The First Aluminum Big-Block

Corp_0509_big_5_z 6/8

The infamous '69 ZL1 was the ultimate big-block . Unfortunately, onlytwo documented production-line cars were reputed to have been built,primarily due the extreme cost ($4,718) of the option.

It borders on blasphemy to talk about infamous Corvette big-blockswithout including the ZL1, but this engine's impact on Corvette enginehistory is minimal since only two documented vehicles were everproduced. The ZL1 was simply an L88 with an aluminum block and a $4,718price tag. Although it's considerably lighter than its cast-ironcounterpart, the ZL1 didn't produce any more power. (See L88 above.)

LS6: The Last Corvette Big-Block

Corp_0509_big_8_z 7/8

1970 was the first year for the 454. Available in LS-5 form (390 horse) or LS-6 form (450 horse) it would be the last big block in a Corvette.

Although 1971 was the first year of reduced compression ratios, it wasalso the first year--and last--for a solid-lifter version of the 454ci(4.251-inch bore, 4-inch stroke) big-block. Wearing option RPO LS6, itwas rated at 425 hp at 5,600 rpm and sported aluminum cylinder heads, a9:1 compression ratio, an aluminum intake manifold, and an 800-cfmHolley four-barrel. Other niceties included a cross-drilled, nitridedcrankshaft, domed pistons, and a $1,220.70 price tag.

In essence, the LS6 was a streetable version of the L88. The increase instroke somewhat offset the loss of compression ratio, so all in all theLS6 was a better engine for non-racing use. And in spite of emissionsregulations and all the other baggage that Detroit had to carry in theearly '70s, the LS6's 425 hp (325 net) was underrated.

Some would argue that the LS7 should occupy this space, but although theengine was announced and specifications printed, the LS7 was neverinstalled in a production Corvette.

Legendary Big-Blocks

Corp_0509_big_4_z 8/8


How To
All you have to do is say big block and Corvette enthusisasts start to drool. Here's the brief history of the biggest, baddest ...
Dave Emanuel Sep 22, 2005


Connect With Us

Get Latest News and Articles. Newsletter Sign Up

subscribe to the magazine

get digital get print