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Chevy Mystery Motor - Horsepower!

Ever seen the mystery 427? You didn't see it here. Or did you?

Photography by

427CI Mystery Big-Block Chevy Engine 1963
Tom McIntyre, Burbank, CA
Sometimes the most innocent-appearing engines hide a rich, secretive past if you bother to look a little deeper. The prototype for the now-famous Mark IV big-block Chevy was the Mark II that debuted in February 1963 when GM bolted its revolutionary powerplant into Junior Johnson's '63 NASCAR stock car (among several others) for the Daytona 500. The story goes that 50 engines were built to take on the 421 Pontiacs, 427 Fords, and 426ci Max Wedge Mopars. Johnson won the pole and was leading the race when the engine expired. Eventually, all the other Mark II-powered Chevrolets retired as well for various reasons. As the tale goes, GM rounded up all the engines, but Smokey Yunick kept the engine seen here and later sold it at auction to vintage racer and entrepreneur Tom McIntyre. The Daytona effort served as solid development work that contributed to the eventual debut of the now-famous 396ci Mark IV engines that appeared in '65-and-later fullsize cars, the Chevelle Z-16, and some Corvettes.

A. The cowl-induction air cleaner was also relatively new to NASCAR stock-car racing, ducting cold air directly to the carburetor from high-pressure air fed from the base of the windshield. It is similar in design to the cowl-induction air cleaners used on the '67 and '68 Z/28 Camaros.

B. This is one of the original Z06-equipped Vettes, and it was once owned by Mickey Thompson.

C. The deep-groove crank and water-pump pulleys and the large-diameter alternator pulley are all intended to keep the single belt from jumping its track.

D. Even though this 427 was intended as a race engine, it was equipped with a very stock-appearing aluminum dual-plane intake manifold and a Holley 780 vacuum-secondary carburetor with side-hung float bowls.

E. There's lots of misinformation out there around this engine. Often, the Z-11 427ci "W" motor is mistakenly referred to as the Mystery engine. Both the Mark II and the Z-11 displace 427 ci and both were introduced in 1963. However, the Z-11 was a stroked version of the 409 and has few connections to the Mystery engine other than displacement. The 348/409 engines are referred to as W engines because of their odd-shaped valve covers that look like an upside down W.

F. Since we were not able to uncover specifics about internal components, we can only estimate the bore and stroke as the same as a later-production 427 with a 4.250-inch bore and a 3.76-inch stroke. The H.P. book photos show a two-bolt main block and a steel crank.

G. While detailed information on the Mystery 427 is scarce, H.P. Books' original How to Hotrod Big-Block Chevys, written by Bill Fisher and Bob Waar, offered a few interesting photos by Alex Walordy that detail a few of its internal components. The iron cylinder heads featured large, rectangular intake ports and the canted valves that would eventually make the big-block famous. The closed-chamber heads also had exhaust heat crossover passages that indicated these engines were intended for production.

H. The exhaust manifolds are clearly the most rare and identifiable pieces on this engine. The right-side casting offers individual runners down to a flat, four-bolt flange that connect to tubular steel runner extensions that eventually form a curved collector. The left-side casting is shorter and also connects to longer-tube steel runners that also feed into a large single collector.

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