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
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