By virtue of the famed, past powerplants, the mystique of classic, high-performance Chevrolet cars has grown to legendary status. Looking back, perhaps no engine combination was more responsible for the legendary status of Chevrolet muscle than the Mark IV big-block. The Chevy big-block was introduced to the public via the Corvette model line, initially as a 396-cid (3859962, casting) powerplant in 1965, growing to 427 (390 and 425 hp) cubes in 1966. The 427 set the performance high-water mark for a generation, and that storied past is relived today in the cult status of collectability these original vehicles retain.
There were many variations on the 427 big-block theme, with the designation of the engine’s RPO option codes making up the lexicon. Two versions of the 427 debuted in the Corvette line up for 1966. The “mild” hydraulic-cammed 10.25:1-compression L36 rated at 390 horsepower, breathing through oval port head. The more serious powerplant in that year was the 11:1-compression, Holley four-barrel-equipped, 425-horse L72. This engine featured Chevrolet’s massive, high-flowing rectangular port heads, a solid lifter camshaft, and a bulletproof bottom end containing a forged crankshaft via a four-bolt main bottom end. The raw performance of these big-blocks made a dramatic impact in the automotive world, and the Chevrolet big-block legend was born.
Choices in 427 big-blocks were expanded in 1967 with three new Tri-power engines, adorned with an induction consisting of a trio of Holley two-barrel carbs. The milder 400hp L68 was based on the L36 engine, while the 435hp L71 otherwise shared specs with the L72 of the previous year. Closing out the ranks of Tri-power 427s was the L89, which was essentially an L71 with aluminum versions of the large port rectangular heads. The top dog 427 was the legendary, underrated, 430hp L88. The L88 was designed as a racing powerplant, with a serious 12.5:1 compression ratio, an 850-cfm Holley carb, dramatically beefed internals, and aluminum heads. For 1968, big-block options were unchanged, but in 1969, an addition was made to the lineup, which constitutes the holy grail of factory big-blocks, the all-aluminum ZL1. Exotic it may be, but don’t expect to find one sitting under a tarp, as factory production was little more than one-off. For 1970, the 454 replaced the 427 as Chevrolet’s premiere big-block, putting an end to the period recognized by the mighty 427’s dominance.
Our subject is an original 1966 vintage 425hp L72 427 Corvette unit, the property of Corvette collector Rick Stoner, who values the historical significance of these special machines. Rick is the proprietor of Westech Performance Group, a dyno facility with enormous expertise in building extremely powerful big-block Chevys. However, Rick approached this buildup with defined clarity of the objectives. The engine would be essentially stock to preserve the pedigree of this rare and classic Corvette.
Rick’s intent was to retain the original look, flavor, and feel of his classic big-block, and for him, this ruled out such ostentatious modifications as headers, aftermarket induction, or aftermarket high-flow aluminum heads. Rick relates, “If I put on headers, a giant cam, trick heads, it’s not anything like the cars were originally. 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, and at some point you have to then think about what’s the point of an original numbers big-block car.” It’s hard to find fault in that logic. Rick’s approach did, however, leave some flexibility in the selection of upgraded or modified components within the build, with the objectives of reliability, driveability, and yes, performance.
To meet these goals, some changes to the pure stock combination were deemed acceptable. As Rick puts it, “You’re always going to be changing parts in a rebuild, and if a modern Competition Cams’ version of the stock cam gives me a similar feel, sound, and vibe to the original, but with more power and rpm, I’ll take that upgrade. The cam isn’t making a permanent alteration to the engine, and it is pretty transparent when in there; it just works better. If a better aftermarket Comp valvetrain will add engine reliability and performance, deal me in.” Rick goes on, “I’ll blueprint the bottom end and have Steve Brulé, Westech’s engine builder and dyno operator, assemble it like a race engine, checking clearances, making sure everything is at the best specs for a balance of power and reliability. I’ll file fit and gap the rings for a better combustion seal than stock, I’ll use modern forged pistons with coated skirts. All this stuff was never done from the factory, but we’re just optimizing the assembly, and making sensible upgrades where the original parts are going to have to be replaced in a rebuild, like in the pistons, rings and cam. All of these changes add up to performance and reliability through higher quality in the build, instead of making big changes to the engine’s original combination.”
While the subtle changes identified so far are aimed at performance and long-term reliability, there were other aspects of the build where some of the factory specification was backed out in favor of improved utility and driveability in today’s world. The primary factor here is compression ratio. The factory-rated compression ratio of the L72 was 11:1, which was just right when you could pull up to the pump and ask for 100+ octane fuel. These days, 91 octane is about the best you’ll get from pump unleaded premium. Rick’s take, “I want to just get in, fill it up on the road, and go, just like in the old days. I’m not going to want to toss in a bottle of octane booster, mix up special higher octane fuel, or worry about where to find gas to make this thing go. I’d rather just back some of the compression ratio out. That will cost some power, but with the other changes I should have that more than covered.”
Piston dome and chamber volume are the key contributors to compression ratio with a given engine combo, and here the obvious choice to dial-in the ratio was to select the appropriate piston. Rick explains, “These earlier 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 cut down on the compression ratio. For this build, I used a set of SpeedPro forged pistons, No. 2300, which have a dome volume of 16.8 cc. We found when building the engine that the valve-to-piston clearance on the intake side was not enough, and had the pistons’ valve relief notches fly cut 0.070-inch deeper to give a safe clearance. This reduced the dome volume another couple of cc, down to 14 cc. With the pistons fitting at 0.006-inch below the decks, and a 0.051-inch thick head gasket, the final ratio in my engine worked out to 9.86:1. That’s the true compression ratio, and it is still high enough to make good power, but is a lot safer with today’s gas and iron heads.”
The cylinder heads offered some opportunity for improved power, and also required a few mods for longevity. This began with a good, machined, multi-angle valve job. According to Rick, “The valve job was a place where I wanted the best workmanship possible, since machining the seats is a basic part of the rebuild. I didn’t skimp here. There is a power difference in how well the job is done.”
Although porting the stock heads would be a possibility, Rick decided that he wanted to keep these rare factory castings stock. As Rick told us, “I didn’t want anyone carving on these rare stock heads with custom porting, even though it would have made more power. It just doesn’t make sense to me to cut on something this rare and expensive. I did have hardened exhaust valve seats installed when the heads were rebuilt, since the seats were hammered and the no-lead gas means they’ll always be in line for a beating. The hardened seats just add durability, and I didn’t want problems down the road.” The valves were replaced with a new high-performance stainless steel set (2.19/1.88-inch) from Competition Cams. Rick explains, “I just went to Comp for the works to assemble the heads, from the valves to the springs, locks, retainers and guideplates. I know from experience that this stuff is bulletproof.”
The cam selected is Competition Cams’ CB Nostalgia LS-6+ cam. The specs for this solid flat tappet cam are fairly stout for a replacement-style solid flat tappet. Specifications measure 239/246-degrees duration at 0.050 and a base advertised duration of 276/283 degrees measured at 0.015-inch tappet rise. Gross valve lift measures a lofty 0.544/0.539-inch, while the valve lash is kept to a tight 0.012-inch. The lobe separation is ground at 112 degrees. Plenty of number there with portents of great power.
Comparing these specs to the stock L72 cam gives some insight into the additional performance potential, though some of the subtle advancements in cam technology and design cannot be read off a spec sheet. The factory cam came through with an advertised duration of 306 degrees, and measured 242 degrees duration at 0.050. Gross lift with the stocker was 0.520-inch, however, the lash was much greater at 0.020/0.024. While both these grinds seem similarly serious by the specs, the modern Comp grind reaches higher lifts faster by virtue of a higher-intensity lobe design, and therefore provides more area under the lift curve for better breathing and power.
Outside, the engine build would retain all of the major external cues that signify this as a stock early Corvette big-block. The factory high-rise aluminum intake manifold would sit between the heads, drawing air from the factory list No. 3247 Holley 780-cfm vacuum secondary carburetor. To ensure that the vintage carb functions as new, Rick enlisted the services of Sean Murphy Inductions of Huntington Beach, California, to fully rebuild and restore the piece. On the opposite end of the heads, the factory iron exhaust manifolds were retained, again to impart an appearance of originality in this installation.
Back in 1966, the stock L72 big-block was rated at 425 gross horsepower. We had essentially a mildly revised version of this engine, using all stock major components. The balance was tipped with about one point less compression, but with a more modern cam profile; an upgraded valve job; and top-notch machining, assembly techniques and replacement parts. How would the various changes factor in terms of the power at the crank? Naturally, the crew at Westech had a dyno test in mind for this in-house project, and we were eager for the results. The engine was loaded onto Westech’s SuperFlow engine dyno for the numbers. To closely simulate the as-installed arrangement, the engine was installed with a belt driven water pump and the head pipes were bolted to the manifolds. The one compromise to originality was the installation of a modern MSD distributor in place of the factory ignition. This substitution was required since the original distributor was out for restoration and was not available in time for the scheduled test day. A set of MSD wires were installed to direct the spark to the fresh spark plugs.
Since this was a new engine combination, there was more to do than simply fire it up, pull the handle, and record the power curve. The engine was first filled with conventional 10w40 motor oil and the lubrication system thoroughly primed using a priming shaft driven by a drill motor through the distributor hole. Next, the engine was statically timed with the engine off and the fuel system was checked, baselining the mixture screws at 1 1/2 turns out from lightly seated, and the float levels checked and adjusted, with the fuel supplied by the dyno’s electric pump. With the preliminaries out of the way, the ignition was hit and Rick’s 427 fired instantly. With a flat tappet camshaft, break-in is critical to avoid cam failure. The engine was immediately brought up to 2,300 rpm and the oil pressure and fuel mixture were verified on the dyno’s instruments. With everything looking good, the timing was adjusted to establish 34 degrees of total ignition advance and the engine was run for 20 minutes to complete the SuperFlow’s automated break-in cycle. Dyno operator Steve Brule examined the running engine with a mechanic’s stethoscope to listen for any unusual internal sounds or valvetrain maladies.
Finally, we were ready for the testing. The engine was brought up for a short sweep test, running from 3,500-4,500 rpm to get a quick gauge of the wide-open-throttle mixture. The dyno instruments showed that the Holley carb’s jetting was a little outside the zone, recording a lean mixture. The dyno pull also showed that this 427 was a truly torquey beast, powering over 450 lb-ft of torque right from 3,500 rpm and holding nearly flat right to the top at 4,500. With minor re-jetting, everything proved dialed in, so we opened up for a sweep test over a broader range, extending the dyno controls to pull to just over 6,000 rpm. 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, the engine was recording higher output than the factory gross rating of 425 hp. Credit the Comp cam and valvetrain, as well as the detailed prep and assembly. The mild Rat really liked to rev, holding its torque production high enough in the rpm range to register a nice, lofty power peak. The 427 was known to rev, and this one seemed to confirm that reputation, making horsepower right past the magic 6,000-rpm range.
With the recorded data making us feel secure that the air/fuel ratio was right in the optimal range, additional tuning would be limited to making several pulls in an ignition timing loop to determine the optimal total spark timing setting. We proceeded and found the big-block to favor 36 degrees of total timing, which is not at all unusual for a Chevy Rat. The final best power figure came in at a surprisingly solid 455 hp at 5,900 rpm, very stout for our 9.86:1 427. The unquestionably raucous power of this “stock” Rat makes it a worthy testimony to the 427’s legacy.
|Superflow Engine Dyno -- Tested At Westech Performance Group|
|Bold Values are Peak|
01. Inside, the engine’s greatest deviation from stock were a set of lower-compression pistons designed to provide about 10:1 compression with early, small-chamber heads. Stock L72 engines featured 11:1 compression, so here we were giving up a full point of compression to facilitate running on today’s pump gas. SpeedPro’s forged No. 2300 slugs handled the task. The rods are heavy-duty factory forgings, fully reconditioned and fitted with ARP bolts.
02. The rods and pistons will be acting upon the factory forged steel crankshaft, which was likewise fully reconditioned and balanced. Chevy did not skimp on the internals on these factory heavy hitters.
03. To work with our selected combination of camshaft, the pistons required custom machining of the intake valve relief to an additional 0.070-inch depth before installation. Here the piston is fitted to the bored, decked, line-honed, and indexed factory 427 block. Dougan’s Machine, in Riverside, CA, handled all the machining and balancing. Rings are SpeedPro’s excellent plasma-moly units, file-fit to the bores.
04. The block was painted the requisite Chevy Orange on the outside, while the inside was painted to aid in sealing the cast surface, and promote oil drain back. The crank spins in new Federal Mogul SpeedPro bearings, which were used throughout the build.
05. Early big-blocks require a grooved rear camshaft journal for proper oiling. We ordered our Competition Cams’ part No. 11-671-4 Nostalgia LS-6+ camshaft with the provision. This cam is designed to sound like the original, but with added punch in power production. Comp was the source for all the camshaft related equipment, including the solid lifters and timing set.
06. Early 427 engines featured compact closed-chamber heads, which are great for combustion efficiency and contribute to the high factory compression ratio. We dropped that ratio some, but we’re sure the Comp stainless steel valves and the detailed high-performance machined valve job would help gain some of that performance sacrifices back. For durability, hardened exhaust seats were also installed.
07. The valvesprings are Competition Cams’ No. 924 dual spring assemblies, the recommended pieces for our choice of camshafts. Additionally, the retainers, locks, studs, and guideplates are all high-performance Competition Cams parts.
08. We followed through the assembly of the valvetrain with Comp’s Pro Magnum rockers and pushrods. All of the valvetrain upgrades would serve a dual purpose of enhancing both power and long-term reliability.
09. When installing a variety of custom cam and valvetrain components, it is important to make a check of the valvetrain geometry. Pushrod length will affect the contact point and range of motion of the roller rocker tip on the valve stem tip. The valve tips were given a coat of dye and the pattern checked. This is just about perfect.
10. As was the original, the Comp Nostalgia cam we selected is a solid lifter design, requiring valve adjustment to provide the specified lash clearances, 0.012-inch for this particular cam. We always reset the lash hot after the engine is run-in.
11. In keeping with the objective of a stock appearance, the factory intake manifold was retained. This original two-plane aluminum high-rise unit is a fairly good piece for an engine of this size and specification. The aftermarket water neck is for dyno hook-up only, and a stock unit will be substituted before the engine is installed.
12. The beautifully restored factory Holley 780-cfm vacuum secondary carb is the handiwork of Sean Murphy Inductions of Huntington Beach, CA. Again, the factory’s choice in components here lends itself well toward the goal of performance.
13. Compared to a set of tubular exhaust headers, there was no doubt we’d give up power potential with the stock manifolds, but they certainly are in the character of the resto theme of the build. We can say definitively that there are far worse designs in iron manifolds than the factory Chevrolet Corvette units.
14. Compared to a set of tubular exhaust headers, there was no doubt we’d give up power potential with the stock manifolds, but they certainly are in the character of the resto theme of the build. We can say definitively that there are far worse designs in iron manifolds than the factory Chevrolet Corvette units.
15. For testing purposes, MSD ignition components were fitted in the dyno installation, including the distributor and wires. The factory ignition and distributor were in the process of restoration, however we expect those pieces would provide comparable output in a combination such as this.
16. Similarly, the factory pulley set was unavailable so we substituted a set of unknown origin found at Westech to turn the mechanical water pump. The crankshaft vibration damper is a rebuilt original unit.
17. Fully assembled and rigged on the dyno, the 427 fired instantly. It was given a 20-minute break-in cycle. Break-in is especially critical with flat tappet performance cams, so it is vital that the initial setting are correctly made to ensure the engine is ready to run when fired. This 427 sounded healthy.
18. Preliminary testing showed a somewhat lean mixture at wide-open throttle, which was quickly remedied with a jet change to richen the mixture.
19. Optimal timing was found to be 36-degrees total; a measure of the full timing with the centrifugal advance fully engaged, but with the vacuum advance disconnected. This simulates the operating conditions at wide-open throttle.
20. When it was all said and done, the mildly altered L72 427 cranked out 455 hp at 5,900 rpm, substantially above its 425 hp rating, even at the lower compression ratio. The engine willingly revved to the 6,300-rpm range in our testing, and made use of the rpm by hanging on with the power production. Torque was stout throughout the curve, reaching a peak of 460 lb-ft, and showing muscular twist throughout the rpm range.