were fabricated in Flint, Michigan at the facility now known as GM Flint Metal Center. Unlike the later, second-generation 427, it was based on the W-series 409 engine, but with a longer stroke. A high-rise, two-piece aluminium intake manifold and dual Carter AFB carburetors fed a 13.5:1 compression ratio to produce an under-rated and . Fifty RPO Z11 cars were produced at the Flint GM plant. Extant GM Documents show 50 Z11 engines were built at the GM Tonawanda Engine plant for auto production, and 20 partial engines were made for replacement/over-the-counter use. Unfortunately, there is no evidence from GM that shows 57 cars were built. Mark II The so-called Mystery Motor, known internally as the Mark II or Mark IIS, is a race-only engine produced for the 1963 season. Development began with a version (Mark II) and ended with a variant; however only the engine (Mark IIS) was ever raced. It gained its nickname due to the incredible speeds cars equipped with it attained during its debut, being considerably faster than the well known W-series powered cars. The engine was first used in Mickey Thompson's Z-06 Corvettes at Daytona in the 1963 Daytona 250 Miles – American Challenge Cup, and then in 1963 Daytona 500 where the number 13 car, driven by Johnny Rutherford, finished four laps down (in ninth place), with the top five cars being the heavier 1963 Ford Galaxie 500's. This "secret" engine was a unique design incorporating aspects of both the W-series and the mid-1965 introduced Mark IV, referred to in sales literature as the "Turbo-Jet V8". Richard Keinath, the original Mark II and IV design engineer stated that the MK III was a regular MK II design with a larger bore, but the Tonowanda plant didn't want to cast a block with a bore that large. The Packard rumor was just that, a rumor. Mark III Packard's V8-engine tooling and production rights were considered for purchase by Chevrolet. Project did not proceed. Mark IV The Mark IV differed from the W-series engine in the placement of the valves and the shape of the combustion chambers. Gone was the chamber-in-block design of the W-series engine (which caused the power curve to drastically dip above 6500 rpm), and in its place was a more conventional wedge chamber in the cylinder head, which was now attached to a conventional 90 degree deck. The valves continued to use