Corvettes have always been known for performance and, by virtue of someof the famed powerplants of the past, the classic Corvette mystique hasbecome legendary. Looking back to its history, perhaps no enginecombination was more responsible for that legendary status than the MarkIV big-block. The Chevy big-block, as it's known today, was introducedto the public via the Corvette model line, initially as a 396-cidpowerplant in 1965, and growing to 427 cubes in 1966. The 427-inchCorvettes set the performance high-water mark for a generation, and thatstoried past is relived today in the cult status of collectability theseoriginal vehicles retain.
There were many variations on the 427 big-block Corvette theme, with thedesignation of the engine's RPO option codes filling out the lexicon inCorvette circles. Two versions of the 427 debuted in the Corvette lineupfor 1966, with the "mild" hydraulic-cammed 10.25:1 compression L-36rated at 390, breathing through oval port heads. The more seriouspowerplant that year was the 11:1 compression, Holleyfour-barrel-equipped, 425-horse L72. This engine featured
Chevrolet's massive, high-flowing rectangular-port heads, a solid-liftercamshaft, and a bulletproof bottom end containing a forged crankshaftvia a four-bolt main-cap design. The raw performance of these big-blockCorvettes made a dramatic impact in the automotive world, and the 427Corvette legend was born.
Choices in 427 big-blocks were expanded in 1967, with three newTri-power engines adorned with an induction consisting of a trio ofHolley two-barrel carburetors. The milder 400hp L-68 was based on theL-36 engine, while the 435hp L71 otherwise shared specs with the L72 ofthe previous year. Closing out the ranks of the Tri-power 427s was theL89, which was essentially an L71 with an aluminum version of thelarge-port rectangular heads. The top-dog 427 was the legendary,underrated, 430hp L88. It was designed as a racing powerplant, with aserious 12.5:1 compression ratio, an 850-cfm Holley carb, dramaticallybeefed internals, and aluminum heads. For 1968, the big-block optionswere unchanged, but in 1969, an addition was made to the lineup, whichconstitutes the Holy Grail of 427-cube Corvettes: the exotic ZL1all-aluminum big-block. Exotic it may be, but don't expect to find onesitting under a tarp, as factory production was little more thanone-off.
For 1970, the 454 replaced the 427 as Corvette's big-block offering,putting an end to the period recognized by the mystique of thiscelebrated powerplant.
For many in our hobby, building up an original 427 powerplantfor an original big-block Corvette comes with a duty to the historicalsignificance of these machines. Rebuilding any engine necessitatesdeviations from originality to some extent, as by definition an engineis original only once, and that was when it was assembled on the GMline. New replacement parts such as rings, bearings, and gaskets have tobe included, and once the factory seal is broken, where to draw the lineon originality becomes an individual's choice. From a power productionstandpoint, nearly four decades of development on the popular big-blockChevy has advanced the potential of these engines tremendously, however,the price of this allure is paid in the cost to originality.
Our subject is an original '66 vintage 425hp L72 427, the property ofCorvette collector Rick Stoner, who values the historical significanceof these special machines. Rick is the proprietor of Westech PerformanceGroup, a dyno facility with expertise in building extremely powerfulbig-block Chevys. However, he approached this buildup with definedobjectives. The engine would be essentially stock to preserve thepedigree of his rare and classic Corvette.
Rick's intent was to retain the original look, flavor, and feel of hisclassic big-block Corvette and, for him, this ruled out suchostentatious modifications as tube headers, aftermarket induction, oraftermarket high-flow aluminum heads. Rick relates, "If I put onheaders, a giant cam, trick heads, it's not anything like the cars wereoriginally. If I did all that, why not just stroke and bore it . . .then I might as well build an 800hp monster with an aftermarket block."Rick continues, "At some point, all of the engine's originality is lost,then you have to think about the point of having an originalnumbers-matching big-block car." We find it hard to discern fault inthat logic. Rick's approach did, however, leave some flexibility in theselection of upgraded or modified components within the build, with theobjectives of reliability, driveability, and performance.
To meet these goals, some changes to the pure stock combination weredeemed acceptable. As Rick puts it, "You're always going to be changingparts in a rebuild, and if a modern Competition Cams version of thestock cam gives me a similar feel, sound, and vibe to the original, butwith more power and rpm, I'll take that upgrade. The cam isn't making apermanent alteration to the engine, and it's pretty transparent when inthere; it just works better. If a better aftermarket Comp valvetrainwill add engine reliability and performance, I'll take a seat at thetable for that--deal me in.
There is little doubt that our unexpectedly strong power numbers were adirect result of the special Competition Cams CB Nostalgia LS-6+ cam.The specs for this solid flat-tappet cam are enough to suggest the powerpotential of the hot solid-lifter grind. Specifications measure out withduration at 0.050 inch of 239/246 degrees, and a base advertisedduration of 276/283 degrees measured at 0.015-inch tappet rise. Grossvalve lift measures a lofty 0.544/0.539 inch on the intake and exhaustlobes, respectively, while the valve lash is kept to a tight 0.012 inch.The lobe separation is ground at 112 degrees.Comparing these specs to the stock L72 cam gives some insight into theadditional performance potential, though some of the subtle advancementsin cam technology and design cannot be read off a spec sheet. Thefactory cam came through with an advertised duration of 306 degrees, andmeasured 242 degrees at 0.050-inch tappet rise, with a gross lift of0.520 inch; however, the lash was much greater at 0.020/0.024 inch.While both grinds seem similarly serious by the specs, the modern Compgrind reaches higher lifts faster by virtue of a higher-intensity lobedesign, and therefore provides more area under the lift curve for betterbreathing and power.
"I'll blueprint the bottom end and have Steve Brule (Westech's enginebuilder and dyno operator) assemble it like a race engine, checkingclearances, making sure everything is at the best specs for a balance ofpower and reliability. I'll file-fit and gap the rings for a bettercombustion seal than stock and I'll use modern forged pistons withcoated skirts. All this stuff was never done from the factory, but we'rejust optimizing the assembly and making sensible upgrades where theoriginal parts are going to have to be replaced in a rebuild, like inthe pistons, rings, and cam. All of these changes add up to performanceand reliability through higher quality in the build, instead of makingbig changes to the engine's original combination."