By 1970, the writing was on the wall. It was no longer a matter of if government regulation would restrain engine power, it was a question of when. Nevertheless Corvette’s big block 427 would have its stroke increased for the 1970 model-year, raising displacement to 454 cubic inches. Translating to 7.4 liters, the new big-block almost doubled the displacement of Corvette’s original blue-flame six. In production form it lacked the power of the 427, but to this day the 454 remains the largest Chevrolet factory engine installed in a production car; it stayed on the Corvette option list through 1974.[1]
Source: Corvette Museum Archives, GM, Motor Trend, Hot Rod Magazine, Super Chevy, and AnotherApex estimates
Despite the displacement bump for Corvette’s big-blocks, it was the A-body Chevelle that could boast the most in 1970. That year only, the Chevelle SS could be optioned with a 450 gross horsepower version of the 454 V8 – the “LS6” – while the Corvette menu topped out with a less potent 454 – the “LS5.” Indeed, the LS6 Chevelle’s Holley 4-barrel carburetor, aluminum intake, and performance iron heads ushered a high-octane mix through its cast iron block with a compression ratio of better than 11:1 [2]. While thousands of Chevelles harnessed this 450 horsepower engine, the regal Corvette would have to wait for a big-block 454 that that offered more than 400 horsepower.[3]
Pic 1 and 2: A 1970 Chevrolet Chevelle SS LS6 454 cubic inch big-block V8 rated at 450 gross horsepower. Pictures courtesy of Corvette Mike Midwest, in Burr Ridge, IL https://corvettemikemidwest.com/used-corvettes-for-sale/1970-chevrolet-chevelle-ss-rare-ls-coupe/
Pic 3: A 1970 Chevrolet Corvette’s LS5 454 cubic inch V8 rated at 390 gross horsepower Pic courtesy of Corvette Mike in Anaheim, CA; no longer for sale.
GMs decision not to release a 460 gross horsepower version of the 454 in 1970 – despite christening an “LS7” engine option in Corvette brochures [see table below] – discreetly gave the Chevelle SS the superlative rated horsepower limelight of the big block era.[4] While the Corvette’s LS5 punched a respectable 390 gross horsepower that year, its 454 lacked the race inspired solid lifters, aluminum heads, and Holley four barrel that often crowned Corvette’s top engines in that period. Thank goodness Chevrolet’s GM, John DeLorean, approved extending the virile 1969 Corvette production run – that offered three aluminum headed big-blocks – well into Chevy’s 1970 model-year.[5]
This table was included in Chevrolet’s 1970 Corvette sales brochure and summarizes the extra-cost engines, as well as some of the transmissions and differential combinations, that could be added. Although included in the brochure, the solid-lifter big-block LS7 was dropped before production. Table presented with permission from GM Heritage Center.
Things started to look grim after President Nixon signed the National Environmental Policy and Protection Act into Law in January of 1970. Soon after that, GM President Ed Cole reportedly mandated that 1971 model year cars require no more than 91 research octane fuel. [6] This “knocked” high compression engines – like the L88 and the lost LS7 – out of consideration for next year. But with Sticky Fingers in 1971, [7] Corvette buyers could snag a big bock with aluminum accoutrements one last time as the LS6 cameoed in its Stingray solo. Less than 200 LS6 Corvettes were produced in 1971, including a dozen with the hallowed “ZR2” package.
A 1971 Chevrolet Corvette’s LS6 454 cubic inch V8 rated at 425 gross horsepower Pic courtesy of Corvette Mike in Anaheim, CA; no longer for sale.
Unfortunately even with the Corvette LS6’s aluminum heads, Cole’s edict had caused compression to drop to 9:1 in ‘71, resulting in the loss of 25 gross horsepower. And although 425 horsepower was still very respectable, for the first time Corvette’s brochures also noted the LS6’s 325 “net . . . as installed” horsepower equivalent.[8] Although the change in rating method contributed only a semantic loss of power, it was a harbinger for the power plight on the horizon. In the coming years enthusiasts would pine for the days of 300+ horsepower. We’ll tackle that in the coming installments.
Written by Jez Stephens
This table was included in Chevrolet’s 1971 Corvette sales brochure and summarizes the extra-cost engines, as well as some of the transmissions and differential combinations, that could be added. This was the first year that SAE net horsepower was included alongside gross horsepower. By the end of the decade, net horsepower would be the only figure offered.
[1]10/1/2009 https://www.popularmechanics.com/cars/a913/4199067/ accessed 7/23/2019
[2] Patrick Hill 2/1/2011 http://www.superchevy.com/how-to/additional-tech/sucp-1102-chevrolet-muscle-car-dyno-wars/ accessed 7/23/2019
[3] Mike McNessor 12/2016 https://www.hemmings.com/blog/article/1970-chevrolet-chevelle-ss-454-ls6-convertible/ accessed 7/23/2019
[4] The 1966 L72 427 was also briefly rated at 450 gross horsepower before the published rating was reduced to 425 gross horsepower likely for political reasons. Collector’s Originality Guide
[5] Change-over for Corvette’s 1970 model-year occurred in December of 1969 adding an additional four months to 1969 production. The Complete Book of Corvette; Every Model since 1953, Mike Muller, Motorbooks 2006, page 152
[6] Corvette, America’s Star-Spangled Sports Car, The Complete History, Karl Ludvigsen, Automobile Quarterly Library Series, 1977 page 234
[7] Rolling Stones http://www.rollingstones.com/stickyfingers/ accessed 2/1/2019
[8] “As installed,” incorporates the addition of all intake and exhaust plumbing. https://www.gmheritagecenter.com/docs/gm-heritage-archive/historical-brochures/Corvette-Historical-Brochures/Corvette-Historical-Brochure-1971.pdf accessed 2/1/2019
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