1963 Chevrolet Canted Valve V-8
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1963 Chevrolet NASCAR Canted Valve Engine Exposed

The “Mystery Motor” or “Porcupine”, as it was called back in 1963, was NOT an early production Mark IV 396-427 called the “Rat” by many. It was a 1963 Chevrolet W-Series 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 in 1965 as Chevrolet’s new Matk IV big block.

But first we need to understand that GM’s 1963 racing ban hammered Chevrolet’s plans for the mystery Motor’s competition in NASCAR and NHRA – not to mention SCCA, FIAA and USAC. Unfortunately, the Mystery Motor had one shot to prove itself, and then it was withdrawn into oblivion . . . until the new Mark IV appeared in 1965 – back then seemingly from nowhere – but know the truth – it was an outgrowth of the 1963 program.

As early as 1961 the brass knew that the 409 cylinder head design was at a dead end, and that a typical “wedge” engine had its limitations, They also knew that Chrysler’s Hemi needed complex valve train and the heads were heavy and huge – precluding their anticipated fitment in the Corvette as some point. Fitment in the Corvette became paramount  because they knew that larger cubic inch engines would be needed in the future to keep the heavier Corvette competitive in sports car racing. Thus a new approach was needed. They went back to the drawing board and came up with a compromise cylinder head that emulated the breathing of a hemispherical head with the lighter weight and size of a typical wedge head. While we take for granted canted valve engines today, the concept was revolutionary in 1963.

What they had learned by the valve placement in the W-Series engine helped with the development. There the intake and exhaust valves were not in line, but off-line from one another. Therefore they recognized that by using their lightweight valve train and “pedestal” rocker arms they could cant the intake and exhaust valves toward the port like a Chrysler Hemi AND even twist it so that it lined up with where the port had to be – allowing for ultra high-flow numbers. Since the stamped steel rockers sat on an individual pedestal and did not use a typical single rocker shaft to hold the rockers in place this was much easier to do. The new engine did not look that different until the valve covers were removed. Even then it took a discriminating eye to see the genius in the design.

This Hemi-like airflow was now achieved with a relatively narrow head and this assured that the motor would fit in the C2 Corvette, then due out in 1963. That the later Mark IV appeared in the Corvette first in 1965 was no accident – it was designed for that car. The Mystery Motor was always considered merely a test bed for the cylinder head, and it was not to be the new motor itself which would be completely a new design.  The 1963 NASCAR Mystery Motor was just a way to prove the process would result in huge HP gains.

Mystery Motor Specs

The Mystery Motor block was a special casting W-Series casting designed to allow the fitment of the heads, remembering that the W-Series block had the head surfaces machined at 74 degrees to perpendicular rather than 90 as was normal. The W-series block castings for the 427 Mystery Motor would have standard 90 degree surfaces, requiring special cores. But all else, the MM was similar in specification to the Z11 427 designed for drag racing. However, due to the unique castings, the Mystery Motor had its own designation, it was called “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 abbreviated form – which is the date the blocks were cast.

In a pre-Daytona dynometer test, the 427 Mystery Motor made 620 HP with a single Holley carburetor 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 four 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″ intake and 1.72″ exhaust. Chambers were very similar to the 1965 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 any milling. The rocker arm ratio was 1.75:1. The Exhaust ports were round. They were delivered with cast iron exhaust 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 also said to be GM’s test bed for screw in rather than pressed in rocker studs, as the canted arrangement did cause the pressed in studs to pull out under load..

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


The Mystery Motor’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 427 CID Z11 version of the W-series engine. 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.


This copy resides at the GM Heritage Center. Few exist otherwise.

This shot from the May 1963 issue of HR magazine shows the familiar layout of Chevrolet canted heads, and the oval exhaust ports used in this version. The heads were smaller and lighter than those of the 1965 and later Mark IV.

The combustion chamber is what would later be called “closed” and is similar to what is found in the early 396 CID Mark IV. The smaller size of the heads as compared to the later Mark IV is apparent here.

The bottom end was standard W-Series 409/427 fare.
Note that only 2-bolt mains were used.

Here is the engine installed in a
1963 Daytona 500 race Chevrolet

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