Unlike the Z11, the CHEVY 427 MARK II MYSTERY MOTOR was not available in a car, or to the public. You had to have serious NASCAR cred to get one of the 20 built.
In the 1960s, Semon “Bunkie” Knudsen always seemed to be one step ahead of GM Chairman Fred Donner’s anti-racing missives. While running Pontiac, he had supported the Super-Duty Group that later, after he left, managed to get 421 Catalina Lightweights to drag racers before the axe fell. Then he moved on to Chevrolet in 1961 and supported the RPO Z11 drag racing and clandestine NASCAR 427 Mystery Motor projects. Both pure racing programs survived even though GM was officially out of racing. In the case of the Mystery Motor, everything was conducted through Chevrolet’s backdoor.
Except for cooperating with Ray Brock for a story in the May 1963 HOT ROD, Chevrolet PR did not issue any photos, information nor discuss the Mark II 427 with media. As far as Chevrolet was concerned, the engine did not exist. The Mystery Motor rattled the troops at Daytona, generated reams of misinformation and became a cult engine. Its real function was that of a “bridge” between the old school W-Series 348-409 and the next-gen 1965 Mark IV big-block.
The CHEVY 427 MARK II MYSTERY MOTOR makes use of the same bore/stroke block – 4.31-inch bore and 3.65-inch stroke – as the Z11 engine. But that’s where the similarity ends. Unlike W-Series engines with combustion chambers in the cylinders, the Mark II NASCAR engine utilizes canted and staggered-valve (Porcupine) heads with conventional chambers. This style head debuted in production 396-427 Mark IV big-block engines, affectionately dubbed “Rat Motors” by enthusiasts!
Although developed primarily as a NASCAR race engine, Chevrolet did produce a singular variant for street applications. Most likely the Mark II in street trim, displayed for many years at GM’s Tonawanda, NY engine plant, left, with GM engineer, Ken Kayser, was built to justify the expenses of building a racing-only engine. It is possible that at some point the project was referenced internally as RPO Code Z33. That would have been done only to disguise the 427 as an optional production engine so as not to attract unwanted attention. Interestingly, the Mark II engine was not produced at Tonawanda, the facility best known for Mark IV 396-427-454 engines.
Richard “Dick” Keinath, one of the industry’s top engine engineers, had the lead role in designing and developing the Mark II engine. The program started in 1960, before Bunkie Knudsen arrived and wrapped up in November 1962. To keep it off the radar, Mark II development, and testing by pro drivers in NASCAR racecars, was conducted at GM’s Winter Proving Ground in Mesa, AZ rather than in Milford, MI.
Part of the mystery surrounding the Mark II engine can be attributed to its planned public debut on February 24, 1963 at the Daytona 500. The first couple of engines were shipped to Smokey Yunick for use in Chevys being prepared for the 500. Junior Johnson and Johnny Rutherford were two high-profile racers originally slated to run this engine.
However, Mark II 427s were in two Z06 Corvettes competing in the 250-mile American Challenge Cup, at Daytona on February 16! This was a race for sports cars and one-offs, not NASCAR stockers. Few people at the time realized that two of the split-window Sting Rays in the Challenge Cup had Mark IIs under their hoods. They were actually the first big-block Corvettes.
For the complete story on the CHEVY 427 MARK II MYSTERY MOTOR, check out DAY ONE, https://www.amazon.com/Day-One-Automotive-Journalists-Muscle-Car/dp/0760352364/ref=sr_1_1?s=books&ie=UTF8&qid=1493561421&sr=1-1&keywords=Day+One+by+Martyn+L.+Schorr
The CHEVY 427 MARK II MYSTERY MOTOR is exposed in OVER-DRIVE magazine @ https://over-drive-magazine.com/2022/08/19/1963-chevrolet-mystery-motor/