Legendary Vintage Horsepower – Chevrolet Big-Blocks

We recently highlighted The First Fuelies, a story which focused on the original fuel-injected small blocks. The “fuelies” forever cemented Chevrolet performance. But once the fuelies started to lose their grip on performance domination, Chevy had an ace up its sleeve, known as the big block (aka the “rat motor”).

There’s no question the small block was legendary, but on the streets of Smallville, USA, big-block grunt was the order of the day. The big-block horsepower race was on, and few cars could touch a well-tuned and well-driven, big-block Corvette. In fact, the big blocks were pretty much unstoppable. The romping rat motor quickly ascended to the top of the Corvette food chain, and like their fuel-injected, small-block brothers, the rat motor Vettes became legendary.

L78 396/425 & 396/375 – 1965-1970

The big difference in the 396 L78 power ratings (aside from the decal) was the RPM at which the horsepower was rated: 6,400 rpm showed 425 horsepower and 5,600 presented only 375 ponies.

In the early-60s, Bow Tie performance was in trouble. The GM corporate hammer came down on the in-house Chevy (and Pontiac) racing programs, which effectively killed the 427 “mystery motor” program along with the 409-based Z11. The standard 409 cubic inch “W” motor was getting long in the tooth, and for racetrack railbirds, there seemed to be precious little engine development.

Corvette was “stuck” with a small block while Cobras were starting to run rampant. But, Chevy was hard at work on what could easily be called “The Ultimate Big Block” — an engine so well designed it would continue to win and dominate races five decades later. In fact, the big-block Chevy is the most common engine in drag racing today.

Chevy’s initial high-performance big block came out with a bang. Displacing 396 cubic inches, the solid lifter L78 produced a whopping 425 horsepower when it was first introduced in Corvette for only 1965. Later sticker-changes relegated the engine to only 375 horsepower.

With the dawn of the 1965 model year, a brand-spanking-new engine appeared on the option charts, completely replacing the 409. Chevy offered it in several different variations, but most hardcore enthusiasts were fascinated by the L78; a powerhouse that displaced 396 cubic inches and could develop as much as 425 horsepower. That was the same advertised power as Ford’s top of the line, dual-four-barrel 427.

The L78 would go on to become the backbone of Chevrolet high-performance over the next five years. The 375 HP 396 was replaced by the 427 in Corvette and passenger-car applications after one short year, however it continued in service until the end of the 1970 model year for other applications. In the single year of Corvette use, Chevrolet produced 2,157 examples. By the time 1970 rolled around, the L78 396 grew slightly in displacement (to 402 cubic inches), but for all intents and purposes, all of the variants remained true to the original 375/425 horsepower blueprint.

L78 Specifications

Note:  All specifications listed apply to the first-design L78 engine released during 1965.

Displacement:  396 cubic inches

Cylinder Bore: 4.094 inches

Crankshaft Stroke: 3.760 inches

Advertised Compression Ratio:  11.0:1

Brake Horsepower, Advertised:  425 @ 6,400 rpm

Maximum Torque:  415 lb-ft @3,600 rpm

Valve Lifters:  Solid

Carburetor:  Holley

Fuel:  Super Premium

Cylinder Block Material: Cast Iron

Cylinder Head Material: Cast Iron

Crankshaft:  Steel Forging

Piston Type:  Forged Aluminum, cam ground skirt

Camshaft Material: Precision-Molded, Special Alloy

Valve lift, Theoretical Intake:  0.520-inch

Valve Lift, Theoretical Exhaust:  .0.520-inch

Operating Tappet Clearance: 0.024-inch Intake — 0.028-inch Exhaust

Theoretical Duration: 316-degrees intake; 302-degrees exhaust

Overall Valve Head Diameter, Intake:  2.185 to 2.195-inches

Overall Valve Head Diameter, Exhaust: 1.715 to 1.725-inch

Initial Ignition Timing: 4-degrees BTDC

Maximum Centrifugal Advance: 32 degrees

Maximum Vacuum Advance: 15-degrees @ 12 in. HG

L72, L71 & L89 427

The most prolific big-block powerhouse had to be the L72 engine option, which spawned several different variants as well. Introduced in the fall of 1965 as a 1966 engine RPO, the 425 horsepower 427 not only saw service in the Corvette; it was also used (optionally, of course) in full-size Chevys and in later years, Camaros and Chevelles. The L72 saw service over four consecutive model years and was replaced by the 1970 LS6 454.

The L72 was truly Chevy’s frontline muscle motor. It wasn’t as radical as an L88 or a ZL1, but it was powerful none-the-less (more livable too). Over time, the L72 passed through several developmental changes — which mandated a different option code. The L71 was a six-barreled carburetor version of the L72 and was only used in Corvettes. The L89 option added aluminum, closed-chamber cylinder heads to the L71. In this case, it was only available in Corvette applications (although the L89 option was also available on select L78 396 engines).

Thousands of L72’s were produced. This was Chevy’s high-performance workhorse. While other examples received more “flash”, the first Corvette engines made do with painted valve covers.

It’s difficult to determine exactly how many L72 engines were actually produced.  In Corvette applications, 5,258 were constructed (early production Corvette L72 engines were rated at 450 HP, later models say a rating of 425, but there were no differences aside from the air cleaner decal). The 1967 tri-power L71 Corvette production numbers work out to 3,754 units, while  2,898 L71 Corvette engines were constructed in 1968. Another 2,722 were constructed in the 1969 model year. Much lower L89 Corvette production totaled 16 in 1967, 624 in 1968, and 390 in 1971.

In the 1969 model year, both COPO Camaros and COPO Chevelles made use of the L72. In addition, the L72 was available in full-size Chevrolet models from 1966 to 1969 inclusively. The numbers of over-the-counter crate engines are impossible to document, but we suspect many were produced and sold this way. Bottom line? This was Chevrolet’s most prolific, special high-performance V8.

L72 Specifications

Note:  All specifications listed apply to the L72 single four-barrel, iron-cylinder-head engine.

Displacement:  427 cubic inches

Cylinder Bore:  4.251 inches

Crankshaft Stroke:  3.760 inches

Advertised Compression Ratio:  11.0:1

Brake Horsepower, Advertised:  425 @ 5,600 rpm (450hp rating in early 1966 Corvettes)

Maximum Torque:  460 lb-ft @4,000 rpm

Valve Lifters:  Solid

Carburetor:  Single 4BBL Holley

Cylinder Block Material:  Cast Iron

Cylinder Head Material:  Cast Iron

Crankshaft:  Steel Forging

Piston Type:  Forged Aluminum, cam ground skirt

Valve lift, Theoretical Intake:  0.520-inch

Valve Lift, Theoretical Exhaust:  0.520-inch

Operating Tappet Clearance:  0.024-inch Intake — 0.028-inch Exhaust

Theoretical Duration:  316-degrees intake; 302-degrees exhaust

Overall Head Diameter, Intake:  2.185 to 2.195-inches

Overall Head Diameter, Exhaust:  1.715 to 1.725-inch

Initial Ignition Timing:  4-degrees BTDC

Maximum Centrifugal Advance:  32 degrees

Maximum Vacuum Advance:  15 degrees @ 12 in HG

The L88 – 1967-68-69

Photo: GM

Big-block Chevy-powered cars won all sorts of races in the ‘60s and ‘70s — legal and clandestine — but the leader of the pack had to be the L88. When it comes to the L88, it all began in 1966. Some important folks at Chevrolet were not happy with the publicity of the Cobra.

Not only had the small block snake run rings around the Corvette, the 427 models were really rubbing salt in the wound. GM brass had recently (and quickly) pulled the plug on the Grand Sport Corvette effort and another flyweight Corvette was out of the question. The solution was simple, build a thumping, big-power engine and install it in the Corvette.

Several L88 Corvette prototypes were up and running well before the end of the 1966 model year. Roger Penske obtained a very early 1966 factory-L88 prototype (development car #9). With the help of Dick Guldstrand, he immediately prepped it for the 24 hour Endurance Race at Daytona. Insiders were in awe. Even after suffering a major race accident, the L88 smashed the GT Class record. In the end, it finished First in GT Class and 11thoverall — a legend was in the making.

Speaking of legends, the late Dick Guldstrand reinforced his personal status by driving a lightly-prepped 1967 L88 Corvette from the Orly airport in Paris to LeMans, and then competed in the race.

The L88’s air-filter element was simply mounted right in the hood scoop as shown here. This setup remained the same for all three model years, even though the body style changed in ’68.

Over its three-year life span, the L88 was rated as 430 horsepower at 5,200 rpm. The rated torque figure was pegged at 450 lb-ft at 4,500 rpm. Though these figures were lower than the L71 tri-power Corvette engine, the power and torque figures were misleading. The L88 power curve continued past 5,200 rpm and it didn’t peak until after 7,000 rpm. Real-world horsepower figures put the closed-chamber L88’s power output in the 540 range with open headers, while the open-chamber setup easily contributed to another 40 to 50 horsepower. It was certainly sufficient to get anyone’s attention.

Note that the L88 aluminum heads (left) were painted orange. That’s not a mistake. All 1967 L88’s came from the factory this way. Heater delete was also a mandatory “option” with the ’67 L88, but it wasn’t available (as a delete) in later years. Radios and radiator shrouds weren’t included with any L88s. These were race engines, pure and simple. 1968 and 1969 L88 engines were fitted with smog pumps in place of the road draft tube and a Positive Crankcase Vent was fitted.

The total L88 Corvette production for 1967 measured a paltry 20 units. The 1968 examples accounted for 80 vehicles, while 1969 production increased to 116 vehicles. Numbers of actual Corvettes built with the option weren’t exactly high, however the publicity gained was considerable. Keep in mind the L88 engine was available over-the-counter for decades after the end of 1969 Corvette production. We’re sure hundreds of these engines were constructed.

L88 Specifications

Note:  All specifications listed apply to the second-design L88 released during mid-1969.

Displacement:  427 cubic inches

Cylinder Bore:  4.2510 inches

Crankshaft Stroke:  3.760 inches

Advertised Compression Ratio:  12.25:1

Brake Horsepower, Advertised:  430 @ 5,600 rpm

Maximum Torque:  450 lb-ft @4,400 rpm

Valve Lifters:  Solid

Carburetor: Holley (double pumper in open-chamber-head models)

Fuel:  Super Premium

Cylinder Block Material:  Cast Iron

Cylinder Head Material:  Cast Aluminum

Crankshaft:  Steel Forging

Piston Type:  Forged Aluminum, cam ground skirt

Valve lift, Theoretical Intake:  0.560-inch

Valve Lift, Theoretical Exhaust:  0.600-inch

Operating Tappet Clearance:  0.024-inch Intake — 0.026-inch Exhaust

Theoretical Duration:  347-degrees intake; 359-degrees exhaust

Overall Head Diameter, Intake:  2.185 to 2.195-inches

Overall Head Diameter, Exhaust:  1.875 to 1.885-inch

Initial Ignition Timing:  12-degrees BTDC

Maximum Centrifugal Advance:  30 degrees

Maximum Vacuum Advance:  None

The Lightweight Alternative

The new ZL1 consisted of an all-aluminum cylinder block along with the L88’s aluminum heads. The internals were all race quality for the time and as it turns out, all ZL1 engines went through a special “audit” at the Tonawanda engine plant (obviously adding more cost to the bottom line). Photo: GM

One of the most expensive — if not “the” most expensive vintage Chevrolet engine option — had to be the ZL1. It cost a whopping $3,000 extra, over and above the cost of the L88 option in a Corvette. In the case of the Camaro, the engine package was even more costly, adding $4,160 to the price of the vehicle. That was colossal money in 1969, especially when you consider the base price for a Camaro sport coupe was only $2,727. By the way, $3,000 in 1969 works out to $21,230 today!

What was the reason for the enormous option cost? Aluminum. The ZL1 foundation consisted of a unique aluminum block casting that included press-in iron cylinder liners. If you compare this engine’s specs to the second design, open chamber L88 above, you’ll find they’re identical. The reality is, the ZL1 was almost identical to the last L88 incarnation, save for the block construction.

Just like the L88, the aluminum ZL1 carried a deceptively-low horsepower rating. Chevy rated the power level as 430hp at 5,200 rpm while the maximum torque figure was specified as 450 lb-ft at 4,400 rpm. Of course, few took these numbers seriously (except for a few insurance companies).

Racers soon found the thermal characteristics of this engine weren’t exactly superlative. It was possible to make more power with less cost using a later version of the cast-iron-block L88. Keep in mind, the mechanics of an aluminum block with iron liners was in its infancy. As a result, most knowledgeable racers simply opted for lower-cost, second-design L88 hardware. When all was said and done, two ZL1 Corvettes were built (although a third car is said to exist, but it’s not documented), along with 69 COPO Camaros during the 1969 model year.

The Last Hurrah – The 1971 LS6

Photo: Mecum

When the 1970 model year rolled around, Corvette was delayed. It was introduced late in the model year due to several factors, not the least of which was a UAW strike. Given the circumstances, some of the planned engine combinations never saw the light of day. Case-in-point were two distinct DOA examples: the fabled LS7 454, along with something called an “LJ2”, which appeared to be a tri-power version of the 1970 Chevelle LS6 engine. The biggest big-block Corvette which saw the light of day was the 390hp LS5. It was the first time since the debut of the rat motor that a passenger car combination (1970 Chevelle) could be specified with a more powerful engine than the Corvette.

Fast-forward a few months and everything changed. Despite a corporate edict to build all engines to operate on unleaded fuel, Chevy did something few expected. They released a brand-spanking-new engine combination for the Corvette — a low-compression LS6. This was essentially a remake of the 1970 Chevelle LS6, but the compression ratio was clipped to an even 9:1. In order to offset the loss of squeeze, Chevy swapped the cast-iron, closed-chamber heads (found on the 1970 LS6) for a set of more efficient aluminum, open-chamber jobs from the second-design L88. The balance of the engine was almost identical to the big Chevelle engines, aside from some tuning differences. Despite the lowered compression ratio, the ’71 Corvette LS6 still punched out 425hp gross at 5,600 rpm along with 475 lb-ft of torque at 4,000 rpm.

There were 188 Corvettes built with the LS6 during the 1971 model year, and 12 LS6-powered ZR2 factory race cars were built. While the LS6 could be had with either a four-speed manual gearbox or a TH400 automatic, ZR2 examples were only available with a stick. The LS6 marked Chevy’s last big-block hurrah in the Corvette. The legendary rat motor king was dead. Long live the King!

LS6 Specifications

Note:  All specifications listed apply to the 1971 Corvette LS6.

Displacement:  454 cubic inches

Cylinder Bore:  4.251 inches

Crankshaft Stroke:  4.00 inches

Advertised Compression Ratio:  9.00:1

Brake Horsepower, Advertised:  425 @ 5,600 rpm (325hp Net)

Maximum Torque:  475 lb-ft @ 4,000 rpm

Valve Lifters:  Solid

Carburetor:  Single 4BBL Holley

Fuel:  Unleaded

Cylinder Block Material:  Cast Iron

Cylinder Head Material:  Cast Aluminum

Crankshaft:  Steel Forging

Piston Type:  Forged Aluminum, cam ground skirt

Valve lift, Theoretical Intake:  0.520-inch

Valve Lift, Theoretical Exhaust:  0.520-inch

Operating Tappet Clearance:  0.024-inch Intake — 0.028-inch Exhaust

Theoretical Duration:  316-degrees intake; 302-degrees exhaust

Overall Head Diameter, Intake:  2.185 to 2.195-inches

Overall Head Diameter, Exhaust:  1.875 to 1.885-inch

Initial Ignition Timing:  8-degrees manual; 12-degrees auto (BTDC)

Maximum Centrifugal Advance:  31-degrees manual; 28-degrees auto

Maximum Vacuum Advance:  12 degrees @ 12 in HG

About the author

wayne scraba

Wayne’s byline began with Super Stock & Drag Illustrated and since then, he has toiled as the Editor of many years within 33-years in the automotive journalism field. Wayne has written 6 automotive books.
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