No other engine in muscle car history lost 25 hp simply by switching air cleaner labels, but that's the story of Chevrolet's 1966 427ci L72 big-block. When introduced for duty aboard the Sting Ray, the iconic, semicircular label affixed to the open-element air cleaner read "Turbo-Jet 450 HP." But then mysteriously, after several weeks of production, a new sticker reading "425 HP" replaced it. Minds boggled, gossip and speculation flowed.

Looking back, we now know the sticker switch was an effort to elude government safety zealots. No mechanical changes were made to the L72 427, and despite the "sticker shock," 5,258 Sting Rays were equipped with the snarling, solid-lifter rat motor in 1966, plus 1,856 installations in fullsize Chevys.

Vette owners weren't the only lucky ones. The L72 would find its way into a handful of 1967 Camaros built by Bill Thomas and others, kicking off a whole new wave of supertuned supercars.

Let's watch as Donnie Wood and the guys at R.A.D. Auto Machine take an L72 all the way out to 489 cubes, stuff in a hydraulic roller cam, and wind up with 589 hp and 641 lb-ft of torque. We are going to need a new air cleaner label!

See all 34 photos 1 The pen points to the external oil cooler attachment points seen on all solid-lifter 427 blocks. Ours was cast on February 14, 1966, and bears suffix code IP, telling us it was born to an L72 Sting Ray. L72s destined for Impala, Bel Air, and Biscayne applications bear suffix codes ID (manual transmission) or IO (automatic).

See all 34 photos 2 The pen indicates where the crankcase needs grinding to make room for the Eagle stroker assembly. The stock L72 bore and stroke are 4.251 by 3.760 inches. With a 0.030-inch overbore an enhanced stroke to 4.250 inches, final displacement grows to 489 cubes.

See all 34 photos 3 R.A.D. honcho Donnie Wood sets the forged steel Eagle stroker crank (PN 445442526385) in place. Bearings are fully grooved TRW items (PN MS 2976P). The oil clearance is 0.0025 on mains and rods.

See all 34 photos

See all 34 photos

See all 34 photos 6 With the L72-specific four-bolt main caps, windage tray, and new Melling pickup screen (PN 77S9) in place, Wood installs the pan. Note the Corvette pan's antisurge panel and spring-loaded trap door inside the sump. To preclude air leaks, Wood welds the suction tube to the pump body.

See all 34 photos 7 A Comp XR288HR hydraulic roller cam (PN 11-433-8) with 0.521/0.540-inch lift and 288/294 degrees of duration replaces the L72-spec 0.519/0.519-inch lift, solid flat-tappet cam. The pen points to the oil feed groove added by R.A.D. to the No. 5 cam journal. It works with a fully grooved Durabond No. 5 cam bearing (PN CH-9A) to ensure plenty of oil flow to the lifter galley, a critical detail with hydraulic roller lifters. Thanks to its cast steel core, the Comp roller cam is compatible with standard distributor drive gears. No bronze, plastic, or exotic distributor drive gear material is needed.

See all 34 photos 8 A double-row true roller timing set from Liberty Performance Components (PN LT98110T-9) has a narrow Torrington bearing between the top gear and block. The camshaft is installed 4 degrees advanced. The locking tabs secure the trio of cam bolts.

See all 34 photos

See all 34 photos

See all 34 photos 11 The pen points out minor chamber unshrouding work. The stock diameter 2.19-inch intake valves remain, but L88-sized 1.840-inch exhaust valves (up from 1.720) help feed the extra 62 cubes. Like the block, the heads are legitimate L72 items with casting number 3873858. When done, it is headed for a restored 1966 Sting Ray.

See all 34 photos 12 With the Fel-Pro gaskets' (PN 818OPT2) 0.042-inch crushed thickness, 0.015-inch block/0.010-inch head shave, 96cc chamber volume, and "zero" piston deck height, compression calculates to exactly 10.0:1. Despite the iron heads and 92-octane unleaded dyno gas, no preignition, detonation, or abnormal combustion was noted during our dyno tests.

See all 34 photos

See all 34 photos 14 By most accounts, the factory-issued dual-plane L72 intake manifold is a very capable unit. Thanks to its aluminum composition, it only weighs 19.6 pounds. Observed markings include integrally cast part number "3855069" and a casting date of "9-26-65." The 1.7:1 roller rocker arms (PN 0845403) are from PRW Industries.

See all 34 photos 15 The PRW roller rockers fit inside the stock Corvette valve covers without contact. The "spires" direct streams of oil toward pivot points during operation. Big-block Corvettes didn't get chrome valve covers until the 1968 model year. Duntov's team elected to paint the covers orange to make the engine really fill the engine bay.

See all 34 photos 16 Lacking the original 855-cfm Holley 4150 carburetor, we tested with an 850-cfm Q-850 unit from Quick Fuel Technologies. The only major difference is its double accelerator pumps. The only Corvette with a factory-issued double-pumper Holley was the 1967-1969 L88. The rest were single pumpers. Ignition is handled by a Street Fire HEI (PN 8362) set at 36 degrees BTDC.

See all 34 photos 17 More streamlined than any Chevy small-block "ram's horn" exhaust manifold, the iron L72 castings weigh 18.8/17.0 pounds (driver/passenger) and were used on big-block Vettes with only minor variations from 1965 through 1974. Note the plumbing for the optional K19 Air Injection Reactor antismog system. The round pod is a one-way check valve that lets air in but blocks outward exhaust gas flow.

See all 34 photos 18 Ever run a marathon with a thermometer under your tongue? Typically found on California cars, the K19 system cost $44.75 and was installed on 2,380 of the 27,720 Corvettes built in 1966. The stainless tubes are fed by a belt-driven air pump. The air excites the superhot unburned hydrocarbons for one last shot at combustion before exiting the dual tail, or side, pipes.

See all 34 photos 19 The best of several baseline runs on R.A.D.'s Land and Sea dyno delivered a stout 508.2 hp at 5,700 rpm and 562.6 lb-ft at 3,500 rpm. Compared to a stock L72's 425 (or is that 450?) horses at 5,600 rpm and 460 lb-ft at 4,000 rpm, the stroke and big cam more than make up for the one-point drop in compression ratio. To recap, we have now got an extra 83.2 hp (or 58.2, depending on the air cleaner label) and 102.6 lb-ft.

See all 34 photos 20 Like so many L72 owners who added headers on day two of ownership, R.A.D.'s Bob Petersen swaps the iron manifolds for a set of 2-inch Hooker Chevelle tubes. Everything else is left alone.

See all 34 photos 21 Holy smokes! The free-breathing headers unlocked an extra 81 hp and 79.2 lb-ft of torque! Totals are 589.2 hp at 5,800 rpm and 641.8 lb-ft of torque at 3,400 rpm over the stroked L72 exhaling through iron manifolds. Compared to a bone-stock 450-horse L72, we are 139.2 hp and 181.8 lb-ft ahead! It's nice to see how well the Quick Fuel Q-850 carburetor adapted on its own with no adjustments needed or made.

See all 34 photos 22 To keep up with the high-revving solid cam, all L72s came standard with the Delco-Remy K66 transistorized ignition system. Instead of bounce-prone mechanical breaker points, the K66 distributor (left) uses a magnetic pickup to trigger the spark. With the iron manifolds back in place, the retro ignition delivered 504.6 hp at 5,700 rpm and 560 lb-ft of torque at 3,600 rpm, a mere 3.6 hp and 2.6 lb-ft behind the HEI unit (right).

See all 34 photos 23 Just unpacked, this N.O.S. K66 transistorized ignition system was the go-to sparker for hot rats in the pre-H.E.I. days. Note the Corvette-specific tach-drive distributor body. The finned black box is the amplifier unit. Failed boxes can be remedied with repair kits from Lectric Limited.

See all 34 photos 24 A quick way to measure the compression ratio of any assembled engine, the Katech Whistler taps into the number-one cylinder and calculates the squeeze as you slowly rotate the engine by hand. R.A.D.'s Steve Chmura quickly and easily verifies the mathematical calculation of 10.0:1 on this stroked L72 427. These machines are popular with busy engine shops and race organizations with compression ratio limits.