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Home/Technical Info/Chevrolet/01. Engines/ 1958-1965
Gen I Big Block W Series
1963 Chevrolet NASCAR Canted Valve Engine Exposed

This is the display engine located at the GM Heritage Center
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Hot Rod Magazine Report on the Mystery Engine 1963-05_HR_Chevy_427_Mystery_Engine_1-9.pdf

Information about this item:

The "Mystery Motor" or "Porcupine", as it was called back then, was NOT an early production Mark IV 396-427. It was a 409-427 with special heads designed for "high speed" (rpm) operation on the NASCAR speedways such as Daytona. Of course, the lessons learned with the Mystery Motor convinced Chevrolet, that it could be revamped and delivered as Chevrolet's new big block.

But first, we need to understand that GM's 1963 racing ban hammered Chevrolet's plans for competition in NASCAR and NHRA - not to mention SCCA, FIAA and USAC. The Mystery Motor had one shot to prove itself, and then it was withdrawn into oblivion . . . until the Mark IV appeared in 1965 - back then seemingly from nowhere.

As early as 1961 the brass knew that the 409 cylinder head design was at a dead end, and that a "wedge" engine had its limitations, but Hemis needed complex valve train and the heads were huge. precluding fitment in the Corvette. They also knew that larger cubic inch engines would be needed to keep the heavier Corvette competitive in sports car racing.

Chevy had an ace in the hole and they knew it. The stamped steel rockers sat on an individual pedestal and did not use a single rocker shaft to hold the rockers in place. In a huge inspiration, they recognized that they could cant the intake and exhaust valves toward the port  like a hemi AND even twist it so that it lined up with where the port had to be - allowing for ultra high-flow numbers.

What this also allowed was hemi-like flow with a relatively narrow head - assuring that the motor would fit in the C2 Corvette, then due out in 1963. That the Mark IV appeared in the Corvette first was no accident.

Mystery Motor Specs

The Mystery Motor was a special casting designed to allow the fitment of the heads, but was similar in specification to the Z11 427 designed for drag racing. It had its own designation, "Mark II".

Mark II engines carried the casting number of  "0-217199". The "0" denotes a pre-production casting. Also cast in the block is 9-13-62 - the date of Sept 13, 1962 in standard short abreviated form - which is the date the blocks were cast.

In a pre Daytona dyno test, the 427 Myster Motor made 620 hp with a single Holley on a 180 degree high rise aluminum manifold. There were 4 different intake manifolds developed for the engine. All were 180 degree design. Smokey Yunick stated the best of the 4 designs was that with casting number 0-232166.

Each head is 25 lbs lighter than it's later Big Block Mark IV brother. The valves on the Mystery Motor heads were 2.19" Int and 1.72" Ex. Chambers were very similar to the 65 to 70 Mark IV closed wedge type heads. The heads were dubbed by early writers as "Porcupine" heads but were officially called "Canted Valve" heads by GM.

Pop up pistons gave the Mystery Motor a 12.4:1 compression ratio before milling. The rocker ratio was 1.75:1. The Exhaust ports were round. They were delivered with cast iron exahaust manifolds as was the practice for NASCAR high speed tracks and they had 2" primary tubes that were 33" long which dumped into a 4" diameter 26" long collector.

The Mystery Motor was said to be GM's test bed for screw in rocker  studs.

Here are the head flow cfm numbers provided by Valley Head Service tested a 28 inches.


The Mk II’s 427 short-block used the same 4.84" bore spacing as the 348/409, and its 4.31" bore and 3.65" stroke were identical to those of the 427ci Z11 version of the 409. 12.4:1 pistons were used and listed as part #0-233239. It is believed by Mystery Motor experts that 18 engines were produced by GM.

For more information and additional pictures, click HERE to read the article in the May 1963 issue of Hot Rod Magazine authored by Ray Brock.

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This engine is one that Smokey Yunick did not return to GM when required and later sold at auction.

This shot from the May issue of HR magazine shows the familiar layout of the canted heads, and the oval exhaust ports used. The heads were smaller and lighter than the Mark IV.

The combustion chamber is what would later be called "closed" and is similar to what is found in the early 396. The smaller size compared to the Mark IV is apparent here.

The Bottom end was standard 409/427 fare - note only 2-bolt mains.

Here is the engine installed in a Daytona Chevrolet

A redo of the famous Junior Johnson Daytona Mystery Engined Race car - with the Smokey Yunick engine installed.
















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