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."
While the subtle changes identified so far are aimed at performance andlong-term reliability, there were other aspects of the build: Some ofthe factory specifications were backed out in favor of improved utilityand driveability in today's world. The primary factor is compressionratio. The factory-rated ratio of the L72 squeezed to a proportion of11:1, which was just right when you could pull up to the pump and askthe attendant to fill it up with ethyl, with an octane rating of100-plus waiting to splash out of the nozzle. These days, you'll grabfor the hose yourself, and here in California it's pumping from a poolof 91-octane unleaded premium. Rick says, "I want to just get in, fillit up on the road, and go--just like in the old days. I don't want totoss in a jigger of booster, mix up a concoction of higher-octane swill,or worry about where to find gas to make this thing go. I'd rather justback some of the compression ratio out, and that will cost some power,but with the other changes I should have that more than covered."
Piston dome and chamber volumes are the key contributors to compressionratio with a given engine combo, and here the obvious choice to dial inthe ratio was to select the appropriate piston. Rick explains, "Theseearlier 427s used closed-chamber heads that measure around 100 cc stock,and I wasn't going to consider anything but the numbers-correct heads.With the small early chamber, the trick is to use a smaller dome to cutdown on the compression ratio. For this build, I used a set of Speed-Proforged pistons (PN 2300), which have a dome volume of 16.8 cc. We foundwhen building the engine that the valve-to-piston clearance on theintake side was not enough, and had the piston's valve relief notchesfly cut 0.080-inch deeper to give a safe clearance. This reduced thedome volume another couple of cc's, down to 14. With the pistons fittingat 0.005 inch below the decks, and a 0.051-inch-thick head gasket, thefinal ratio in my engine worked out to 9.86:1. That's the truecompression ratio, and it's still high enough to make good power, but isa lot safer with today's gas and iron heads."
The cylinder heads offered some opportunity for improved power, and alsorequired a few mods for longevity. Although porting the stock headswould be a possibility, Rick decided he wanted to keep these rarefactory castings stock and limit the flow enhancement to a good,machined, multi-angle valve job. According to Rick, "The valve job was aplace where I wanted the best workmanship possible, since machining theseats is a basic part of the rebuild. I didn't skimp here, because thereis a power difference in how well the job is done. I didn't want anyonecarving on these rare stock heads with custom porting, even though itwould have made more power. It just doesn't make sense to me to cut onsomething this rare and expensive. I did have hardened exhaust-valveseats installed when the heads were rebuilt, since the seats werehammered and the no-lead gas meant they'd always be in line for abeating. The hardened seats just add durability, and I didn't wantproblems down the road. The valves were replaced with a newhigh-performance stainless steel set (2.190/1.88 inch) from CompetitionCams. The new valves improve flow and durability compared to theoriginals. I just went to Comp Cams for the works to assemble the heads,from the valves to the springs, locks, retainers and guideplates. I knowfrom experience that this stuff is bulletproof."
The engine build wouldretain all of the major external cues that signify this as a stock earlyCorvette big-block. The factory high-rise aluminum intake manifold wouldsit between the heads, drawing air from the factory list No. 3247 Holley780-cfm vacuum secondary carburetor. To ensure that the vintage carbfunctioned like a fine artifact, Sean Murphy Induction of HuntingtonBeach, California, was called upon to fully rebuild and restore thepiece. On the opposite end of the heads, the factory iron exhaustmanifolds were retained, again to impart an appearance of originality inthis installation.
In 1966, the stock L72 big-block was rated at 425 hp. We hada mildly revised version of this engine, essentially using all stockmajor components with about one point less compression, but with a moremodern cam profile, an upgraded valve job, and top-notch machining,assembly techniques, and replacement parts. How would the variouschanges balance out in terms of the power at the crank? Naturally, thecrew at Westech had a dyno test in mind for this in-house project, andwe were eager for the results. The engine was loaded onto Westech'sSuperFlow 901 engine dyno to determine the answer. The engine wasinstalled with a belt-driven water-pump setup, and the head pipes thatwill be installed in the car were bolted to the manifold exits. The onecompromise to originality was the installation of a modern MSDdistributor in place of the factory ignition. This substitution wasrequired since the original distributor was out for restoration andwasn't available in time for the scheduled test day. Experience hasshown there is little difference in output between these ignitions iffunctioning properly. A set of MSD plug wires was installed to directthe spark to the fresh spark plugs.
Since this was a new engine combination, there was more to do thansimply fire it up, pull the handle, and record the power curve. Theengine was first filled with conventional 10W-40 motor oil, and thelubrication system was thoroughly primed using a priming shaft driven bya drill motor through the distributor hole. Next, the engine wasstatically timed with the engine off, and the fuel system was checked,baselining the mixture screws at 11/2 turns out from lightly seated. Thefloat levels were checked and adjusted, with the fuel supplied by thedyno's electric pump. With the preliminaries out of the way, theignition was hit, and Rick's 427 fired instantly. With a flat-tappetcamshaft, break-in was critical to avoid cam failure. The engine wasimmediately brought up to 2,300 rpm, and the oil pressure and fuelmixture were verified on the dyno's instruments. With everything lookinggood, the timing was adjusted to establish 34 degrees of total ignitionadvance, and the engine was run for 20 minutes to complete theSuperFlow's automated break-in cycle. Dyno operator Steve Brule examinedthe running engine with a mechanic's stethoscope to listen for anyunusual internal sounds or valvetrain maladies. With everything lookingand sounding great, the engine was shut down, the valve adjustment waschecked and reset, and the oil was changed to Lucas synthetic.
Finally, we were ready for the testing. The engine was brought up for ashort sweep test, running from 3,500 to 4,500 rpm to get a quick gaugeof the wide-open-throttle mixture. The dyno instruments showed that theHolley carb's jetting was a little outside the zone, recording a leanmixture. The dyno pull also showed that this 427 was a truly torqueybeast, powering over 450 lb-ft of torque right from 3,500 rpm andholding nearly flat to the top at 4,500. With minor re-jetting,everything proved dialed in, so we opened up for a sweep test over abroader range, extending the dyno controls to pull to just over 6,000rpm. This time we recorded a peak power output of 451 hp at 5,900 rpm.Even with the lower compression in deference to today's pump gas, theengine was recording higher output than the factory gross rating of 425hp. The real credit goes to the Comp Cams camshaft and valvetrain, aswell as the detailed prep and assembly. The mild Rat really liked torev, holding its torque production high enough in the rpm range toregister a nice, lofty power peak. The 427s were known to rev, and thisone seemed to confirm that reputation, making horsepower right past themagic 6,000-rpm range.
With the recorded data making us feel secure that the air/fuel ratio wasright in the optimal range, additional tuning would be limited to makingseveral pulls in an ignition timing loop to determine the optimal totalspark timing setting. We proceeded and found the big-block to favor 36degrees of total timing, which isn't unusual for a Chevy Rat. The finalbest power figure came in at a surprisingly solid 455 hp at 5,900rpm--very stout for our 10:1 427. The unquestionably raucous power ofthis "stock" Rat makes it a worthy testimony to the 427's legacy.
Superflow 901 Engine Dyno
Tested At Westech
STP Correction Factor
*Bold values are peak