![]() A mad rush ensued to design longer 3.65-inch stroke crankshafts that were similar, but subtly different to the previous 409 based cranks. ![]() 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. Most of the initial testing and development was done with 409ci versions (NOT 409's, but 409ci canted valve big blocks). The 427ci version was the only one ever raced. Note: No adequate records exist to indicate exactly how many of each displacement were built. ** Development engines, mid-1963, tested but never raced The same displacement was retained in the initial version to capitalize on the the benefits of the 409’s notoriety. It was intended for NASCAR racing only and was designed to replace the existing 409ci W-motor. 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). That means Mark II big block engines have smaller main bearings than the follow-on Mark IV versions. It utilized a 409 crankshaft and main bearings nothing else. The mystery engine began as a 409ci engine based on the all new Mark II cylinder block and heads. Keinath explains that the only thing even closely resembling the W-motor design was the crankshaft. It was originally designed, built and developed as an “all new” 409ci engine to capitalize on the existing 409’s robust reputation. 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.įew people know that there were three versions of the Mystery Motor. This improved flow and subsequent cylinder filling by unshrouding the valves. 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. 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. 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. As such he is factually the father of all Chevrolet big blocks. He did the original development work on the 348 and 409 cylinder heads and had primary responsibility for the 409 engine. He worked on the original small block with Ed Cole and Ed Kelly developing an all aluminum small block V8. ![]() 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. 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. This theory completely ignores the obvious problem of deck angle and combustion chamber mismatch and the little known fact that the Mystery Motor actually predates the Z11 according to the development engineers. 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. 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. 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. ![]()
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