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 and the little known fact that the Mystery Motor actually predates the Z11 according to the development engineers. 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.

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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.

Mark II Chevy Mystery motorCourtesy GM Media Archives

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 main 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 intended for NASCAR racing only and was designed to replace the existing 409ci W-motor. The same displacement was retained in the initial version to capitalize on the the benefits of the 409’s notoriety.

MarkCID in the
initial Series
Bore StrokeMain

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 initial testing and development was done with 409ci versions (NOT 409's, but 409ci canted valve big blocks). 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. Keinath also decided to address the stroke difference not just by altering the piston pin height, but partially in the rod length as well. 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.

According to Keinath, the W-motor was designed from the get go as a big truck engine that evolved to a racing engine because there was nothing else available. Designers saw what was wrong with it and switched strategies. Mark II series engines were designed from the outset as racing engines (the Mark II Mystery Motor) and were subsequently revised to accommodate truck and passenger car service. The difference was considerable as the new Mark series was designed for much higher efficiency than the W-motors which they easily out-powered. Keinath further remarks that the Mark II and later Mark IV series engine are externally almost identical, but that nothing really interchanges between them primarily because of revised port locations; head bolt locations, smaller mains and other revisions necessary to a production environment.

The cylinder heads carried 2.19/1.72-inch valves residing in closed style chambers and operated by 1.65:1 rockers. They have round exhaust ports with a bump on the upper side to provide additional material for adjacent head bolts. Unique cast iron pushrod guide plates are mounted with short bolts above the intake ports (see photos). All Mark II cylinder heads are asymmetrical in that there is a left and right hand design and they can not be interchanged like the later Mark IV heads. This was done to achieve the best possible port entry angles. Four different 180° hi-rise intake manifolds were designed each with subtle revisions to accommodate the different displacement versions of the Mark II engine. On the exhaust side, cast iron exhaust manifolds featured 2-inch semi-equal length primaries. These were essentially cast iron copies of tubular headers as NASCAR did not allow headers.

From his testing experience Smokey Yunick believed that the best one was casting number 0-233239, but it is unknown which engine this one worked best on. Speculation suggests the 427, but its power was subsequently matched by the 396ci version so it’s anybody’s guess until someone who has one of the remaining engines decides to pull a head to check stroke length and match up the casting numbers. Smokey was also dissatisfied with the 2-bolt mains that came on all Mark II engines. There were no 4-bolt versions, but Smokey devised his own main cap brace that bolted onto the 2-bolt caps and was secured to the main webs by two additional bolts to form sort of semi-4-bolt man caps. Mark II camshafts also had the grooved rear cam journal characteristic of early (1965-66) Mark IV engines and they still incorporated a canister type oil filter.

Retired Mystery Motor test and development engineer Bill Howell has stated that subsequent developments saw the 427ci engine destroked to 396 cubic inches because NASCAR planned to limit displacement. The 396 version was tested by Smokey Yunick in the fall of 1963 at the Ft. Stockton, Texas test track with initial plans to run the 1964 Daytona 500 under the table without corporate endorsement. According to Howell, this version was the best performing engine of the bunch even with the reduction in displacement.

According to Howell, the Mystery Motor predated the 427ci W-motor drag racing engine in that its design was well underway prior to Z11 development which occurred very late in 1962 after Mystery Motors were already running on the GM dynos. Zora Duntov's Corvette group groomed the Z11 along with the 327 small blocks while the base V8 group did the Mystery Motor. An internal rivalry developed and the Z11 ultimately lost because its architecture simply didn't lend itself easily to true high performance applications.

In his book, “Best Damn Garage in Town ”Smokey Yunick offers high praise for Dick Keinath and his engine design talents. According to Smokey, 42 sets of parts were produced for these engines, but Tonawanda engine plant records indicate that a total of 60 sets of engine components were manufactured for the Mark II program. All of them had a 4.3125-inch bore and 2.5-inch main journals. By the end of the program 3 distinct stroke lengths had been used to achieve 3 different displacements as shown in the accompanying chart.

Smokey distinctly recalls 42 set of parts to complete 42 engines coming through his shop, but development engineer Billy Howell says that Smokey came late to the Mystery Motor program and was not brought up to speed until after he attended the initial high speed testing secretly conducted at the Desert Proving Grounds in November 1962. Hence there were any number of 409ci development versions being tested prior to Smokey's engagement. At nearly the same time they received word that they could stroke the engine to 427ci to match the displacement of the Fords and Chryslers.

Production Mark IV 396 V8s (1965) required a reduction in bore size to accommodate foundry based casting issues attributed to relocated head bolts and coolant passages. And it required a stroke increase to 3.76-inches to match the revised 4.094-inch bore. Main journals were also enlarged to 2.75-inch to ensure the desired durability in truck applications. Keinath also confirms that the 396 displacement originally came about because of NASCAR limiting displacement to 6.5L about then and because there was unofficial corporate pressure to stay below 400ci. Despite this engineers eventually matched the 427’s power level with both the Mark II 396 and the production Mark IV 396.

See Part II of the Mystery Motor Development story with Billy Howell, lead development engineer and get the real facts from the horses mouth. Click Here: Mystery Motor Part II

The accompanying photos have circulated freely on the internet with no specific credit or indicated source. They were initially found on the Jalopy Journal under the H.A. M. B. section. Thanks to wrench409 for providing them. Anyone with confirming or contact information is urged to click here to contact us regarding source of these photos and contact information so proper credit can be assigned. We are seeking more information from all owners of mystery motors or parts for hi-resolution press quality photos for a new book. THANKS!