Chevy L-88 427 Engine - Horsepower!

524HP L-88 427 Big-Block Chevy
Jim Grubbs Motorsports, Valencia, CA
Today, a 1,000hp power-adder street engine is almost commonplace. At this escalated power level, it's easy to forget that not all that long ago, a 500hp big-block Chevy was considered the holy grail of street runners. When Chevy replaced the aging W-engines with the MKIV big-block in 1965, it was the equivalent of Chevy landing D-Day troops on the shores protected by Ford and Mopar. The initial 425hp 427 engines in the Corvette were heavy hitters. By 1967, the Bow Tie brigade launched the L88 427 with then-unheard-of factory aluminum heads. According to Al Colvin's book Chevrolet by the Numbers, Chevrolet never officially released '67 427 L88 horsepower numbers, waiting until 1968 to claim 430 hp with conventional wisdom placing the actual number far greater. L88 was the RPO code for the engine that became famous overnight, only to be eclipsed by the all-aluminum 427 ZL1 in 1969. While the all-alloy ZL1 was Can-Am exotic, the L88 was nearly affordable, and you could purchase a complete engine or short-block right over the counter. The first-design L88 427 included a four-bolt main iron block, a forged 1053 steel crank, 7/16-inch bolt rods, 12.5:1 compression, a solid lifter cam with 0.562/0.584-inch lift, aluminum heads with 106cc closed chambers, 2.19/1.84-inch valves, and a dual-plane intake with an 850-cfm Holley carburetor. This was top-drawer stuff all the way through the mid-'70s because at that time there was no such thing as aftermarket street heads for a big-block Chevy. This engine was built by Jim Grubbs Motorsports (JGM) for a customer who wanted this original L88 rebuilt with a few subtle additions. Tested with open headers and the original dual-plane intake manifold, its 524 hp is no doubt close to the actual power these engines made 40-odd years ago.

Horsepower Curve
The factory L88 was rated at a conservative 430 hp with plenty of speculation that a stone-stock L88 made well above 500 hp-we've even seen fanciful claims of up to 600 hp. JGM ran this particular engine on its dyno with 1 7/8-inch open headers, creating the following power curve. This curve is probably not far removed from what an original L88 is capable of producing with headers. While this motor is down on compression, the engine's 10:1 compression is probably down 6 to 7 percent power from an engine-squeezing 12.5:1 compression.

A. Induction The factory L88 engines all came with aluminum, dual-plane intake manifolds that also wore the Winters casting mark. According to the Colvin book, the plenum divider was partially removed to help top-end breathing. The L88 also came with the largest Holley carburetor ever used on a factory GM engine as a Holley 850 vacuum-secondary. All L88 Corvette engines came with a distinctive wire cage over the air cleaner base. This engine runs a new Holley 850 mechanical-secondary carb (PN 0-4781C).

B. Ignition The early L88s used a cast-iron magnetic pulse distributor that was the forerunner to electronic ignition. Since these original magnetic pulse distributors are rare and not always reliable, this PerTronix aluminum black cap distributor suffices. Of course, the ultimate deception would be a PerTronix kit that converts a factory cast-iron point distributor to electronic.

C. Rocker Covers These are aftermarket cast-aluminum valve covers. Most factory L88 engines came with chromed, stamped-steel valve covers.

D. Aluminum Heads This is the heart of the L88 option. If you look closely, you can see the Winters snowflake on the top portion of the head just above the No. 1 or 7 exhaust port that identifies these heads as original factory aluminum castings. These early-version L88s were closed-chamber heads with 106cc chambers. These have been modified slightly at 108 cc after milling. Stock valve sizes were 2.19/1.84, although these heads use lighter 11/32-inch stem valves. One big advantage to these alloy castings was the solid 80 pounds shaved off the heft of a pair of iron Rat heads. Lift an iron big-block head over the fender of a car and you'll swear the difference is more than 40 pounds.

E. Short-Block In 1967, the 427 was Chevy's king-of-the-hill engine. Expanded from the 396, the 427 retained the smaller engine's 3.76-inch stroke, but it pushed the bore out to a much larger 4.25 inch. Since brute power was the L88's sole purpose in life, it also came with a four-bolt main block, a 1053 hardened crank, forged 4340 steel rods with 7/16-inch bolts, and full-floating pins supporting TRW forged 12.5:1-compression pistons. This engine retains the crank and rods but replaced the pistons with a set of Diamond forgings. JGM milled off a portion of the domes to reduce the compression to 10:1 so it can run on pump gas.

F. Mechanical Cams There were several versions of the L88 cam, but all were mechanical flat-tappet cams. JGM's Jeff LaTimes reports this engine used a Crane version of an original L88 mechanical cam but with slightly different specs listed at 255/262 degrees at 0.050-inch tappet lift with 0.554-inch lift for intake and exhaust. It also enjoys a set of aluminum 1.7:1 roller rockers as opposed to the original stamped rockers.