Chevrolet 427 Mystery Motor

Inside the Mark II Mystery Motor

427 mystery motorControversy surrounds the Chevrolet Mystery Motor that Junior Johnson used to stomp the field in the first 100 mile qualifying race at the 1963 Daytona 500. Driving Ray Fox’s 1963 Chevrolet Impala Johnson displayed blistering speed in qualifying and in the race; proving the combination fast and formidable while exceeded 168 mph on some practice laps. When Johnny Rutherford dominated the second 100 mile qualifier with another mystery motor in Smokey Yunick’s car competitors nearly lost their minds. Three other mystery powered Chevrolets qualified for the race and all five cars exceeded the previous years qualifying speed by seven to ten mph. Chevrolets swept both of the100 mile  qualifying races, but ultimately failed to win the 500 due to various mechanical difficulties. Still, their speed was staggering and everyone wanted to know how they did it.

Junior Johnson 1963 Daytona 500

Junior Johnson 1963 Daytona 500

Johnny Rutherford 1963 Daytona 500

Johnny Rutherford 1963 Daytona 500

Chevy Mystery Motor

2-inch cast iron exhaust manifolds

Present day fascination with the nostalgic glory of the muscle car years has clouded our memory of events, cars and engines of that era to near epic proportions; sometimes too much. The Mystery Motor is a case in point. Legend and lore abound, particularly on the internet making it difficult to clarify the Mystery Motor’s content and performance as it relates to subsequent Mark IV production engines. So called internet forum “experts” claim that the engine was just a bored out 409 with a Z11 lower end and the newly designed “porcupine” cylinder heads. This theory completely ignores the obvious problem of deck angle and combustion chamber mismatch. Someone is always surfacing with a story about a long lost uncle who actually worked in the dyno room or the design shop and he just happened to tell his pharmacist who then told his pastor who told a friend who passed along that there were only eighteen motors and they were a special mix of W-motor specs and secret new technology. Well, maybe not.

The Mark II Mystery Motor was designed by Dick Keinath, a brilliant engineer who at one time held every job in the Chevrolet engineering group from draftsman to Chief Engineer. He worked on the original small block with Ed Cole and Ed Kelly developing an all aluminum small block V8. He did the original development work on the 348 and 409 cylinder heads and had primary responsibility for the 409 engine. As such he is factually the father of all Chevrolet big blocks. Truth be told, Dick Keinath had a hand in every modern Chevrolet engine from four cylinders to early 348 NASCAR engines to 302 Z28 engines and of course the now legendary big blocks.

With Ford sporting 427ci big blocks and Chrysler fielding 426 cubic inches Chevrolet’s  427 Mystery Motor was the logical choice to compete with them. The big mystery of course was the cylinder head design which incorporated large canted valves that moved away from the chamber walls and cylinder bores the farther they opened. This improved flow and subsequent cylinder filling by unshrouding the valves. The block was all new, adopting nothing more than the basic location of the W-motor’s main oil gallery, the same 4.84-inch bore centers and the same size main saddles.

Few people know that there were three versions of the Mystery Motor. It was originally designed, built and developed as an “all new” 409ci engine to capitalize on the existing 409’s robust reputation. Keinath explains that the only thing even closely resembling the W-motor design was the crankshaft. The mystery engine began as a 409ci engine based on the all new Mark II cylinder block and heads. It utilized a 409 crankshaft and bearings; nothing else. That means Mark II big block engines have smaller main bearings than the follow-on Mark IV versions. Keinath states that the Mark II also had a 4-5/16-inch bore (4.3125) and a 3.500-inch stroke (same as the 409). It was a NASCAR intended racing engine designed to replace the 409 and the displacement was retained to extend the benefits of the 409’s notoriety.  

CID Main
Bore Stroke Main
II* 409 2-bolt 4.3125 3.500 2.4990 2.2000 6.135 2.19/1.72
IIS 427 2-bolt 4.3125 3.650 2.4990 2.2000 6.135 2.19/1.72
IIS** 396 2-bolt 4.3125 3.390 2.4990 2.2000 6.135 2.19/1.72

S = Stroked
* Development engines, pre-November 1962
** Development engines, mid-1963, tested but never raced
Note: No adequate records exist to indicate exactly how many of each displacement were built. The 427ci version was the only one ever raced.

Most of the testing and development was done with 409ci versions. Development engineers were able to generate the required power at that displacement, but in October of ‘62 negotiations with NASCAR resulted in approval to enlarge it to 427ci to gain parity with Ford and Chrysler. A mad rush ensued to design longer 3.65-inch stroke crankshafts that were similar, but subtly different to the previous 409 based cranks. This development package became known as the Mark IIS (for stroked) and the new cranks eliminated the 409 crank from the package. The engines that ran at Daytona were 427ci Mark IIS versions. Later on, 396ci versions were also developed but never raced.

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