GM's Mystery Motors - Part 2

The "Mystery Motor" on display at the GM Heritage Center
The MK II was a huge step forward for Chevrolet and opened up development responsible for bringing the MK IV to the market. For those unfamiliar with the various MK (Mark) versions, the MK IV is the variant we know as the 396, 427, and 454. There was a MK III, but it never made production. Based on limited available information, here are the details as reportedly detailed by Francis Preve, the Chevrolet engine historian.

"By the time it was in production, it was obvious the 348"- 409"- 427" W-series head/block interface was a technological dead-end and Chevrolet Engineering studied several options for their next big block. Chevrolet needed a better big block engine for the bigger, heavier cars and trucks to come. From late 1957-63, GM had four study teams working":
  • The Mark I study was to take the already in-production 348" out to 427" and beyond. They knew the heads were not the way to go and the bottom end proved to be too weak for the 500" they figured to need twenty years down the road.
  • The Mark II option was the new Mystery Motor porcupine head design introduced as a proof of concept on the modified 427" W-short block at Daytona in 1963.
  • The Mark III study was to buy the Packard V8 tooling for pennies. It could easily go to 500", but Marketing didn't want to be associated with a loser. Engineering knew they could do better than a first-iteration Kettering knockoff and wanted the chance to try some new cylinder head ideas. That, plus the Packard V8 has 5.00" bore centers and was "not invented here" killed it. 
  • The Mark IV study continued the Mark II proposal, further developed the cylinder heads and bottom end which ultimately became the 396", 402", 427", 454", 502" and 572" big block Chevrolet, introduced in mid-1965 and still in production today. As previously mentioned, the Mark IV was both evolutionary and designed in-house; it combined the best of several ideas, but also kept the same 4.84" bore centers and 9.800" deck height as the W-engine.
Lets look at the basic configuration of the MK II. After numerous refinements reviewed in Part 1, the 409 block proved to be a reliable starting point, so other than the revision to accept the new heads, the majority of the previously engineered block remained intact. 

The Mk II 427's 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 Z-11 version of the 409.  

The lower end shared similar dimensions with the Z-11 and the cranks were interchangeable. Smaller main bearing diameters (2.5 inch) were retained to reduce friction as they were determined to be sufficiently strong to support racing requirements.

The lifter bores were all aligned instead of offset as in Mark IV engines, but there was no information if the changed occurred during the development of the 396 cubic inch MK II version or incorporated later for the Mark IV series. 

Combustion chamber area and notch for valve clearance
MK II angled valve (L) vs MK I (R)
Let's cut to the chase on the importance of this new head design. The "W-427" was rated at 430 HP. According to available documents, in pre-race dyno testing, the MK II 427 made about 540 HP with a single Holley on a 180 degree high rise aluminum manifold. 

There were two main issues overcome with the new head design. First was the combustion chamber. In the "W-427", it was located at the top of the cylinder bore. It also required the block to be notched for valve clearance.

Second was the head. With the valves located vertically and straight, the intake and exhaust runners were inefficient. Both issues were eliminated in the MK II head design.
"Porcupine" valve set up (L) and in head combustion chamber (R)
We were able to uncover an interview with Bill Howell, an engineer working on the MK II project. There is no better way to document the facts than speaking with the person who actually did it.

“The design was conceived in 1960 or 1961, as they already had running prototypes in May or June of 1962. I took over the test engineer job in July of 1962. The mystery motor development program was running two shifts and moving rapidly to get engines ready for Daytona.” Bill Howell

Howell described the development pace as “breakneck speed.” The development engineer would spec a series of tests for the test engineer to run overnight so results could be evaluated the next morning and revisions planned for the next testing sequence.  He particularly likes the tuned length headers which began development with 40-plus inch straight primary pipes that were progressively shortened two inches at a time to ballpark the length before the shape and collectors were developed.

Howell fondly recalled the Mark II program as one of the most intense R&D efforts of his lengthy career. After the GM racing ban fouled up the program, under the table development continued with 396 cubic inch versions in hopes that they could be run for the 1964 series. 

These engines were track tested in a Smokey Yunick Chevelle at Fort Stockton, Texas and were found to be equally fast. Howell described the 396 cubic inch Mark II as a real humdinger of an engine before it was abandoned. He said that 50 sets of parts were run for the Mark II 396's, but they were never completed and ninety percent were ultimately scrapped.  "Smokey Yunick had most of what was left so if you got a motor from Smokey’s auction it’s possible that you own a 396ci Mark II and not a 427".

He had a difference of opinion regarding the 620 HP - 630 HP reported by others.

“The highest numbers I remember from the Mk II program were about 530-540 BHP.  So I am sure the 620 BHP numbers are false.
If a dual four barrel setup even existed, it’s unlikely that one would provide that sort of increase in 1963. Not that you couldn’t approach it today with modern cam and valve train refinements and some judicious head porting. ” 

Howell also spoke of other people, involved in development of the MK II, but who are rarely mentioned in relation to the mystery motor. According to Bill they were engine designer Dick Keinath and Maurice Rosenberger.  Keinath was handed the assignment to design a new Chevrolet racing engine for stock car racing. 

Design work began in earnest in the spring of ’62 with the intent of building  an all new racing engine to set the competition on its ear at the 1963 Daytona 500. Rosenberger (AKA-Rosie) was the man who probably made the decision to go ahead with the design and spend the money. He was chief engineer for powertrain at Chevy during that period, a larger than life individual who started his own career as a test engineer for Cadillac about 1932. “He knew all aspects of engines, and I’m sure he had a hand in picking Keinath as the designer.”  

He further confirmed there were never any factory sponsored drag racers who received the MK II. They all received the Z-11 427's. Additionally and a point previously unknown, the W-427 was the product of the Zora Duntov team. The group was still in full development on the W-based Z11 drag racing engine and had no real involvement with the mystery motors.

Kind of ironic the Duntov team was working on a dead-ended motor as big block Corvettes, especially those equipped with the L-88 427 are considered some of the rarest and most coveted cars of all time. 

When asked about the Z-33 RPO designation many claim was attached to the mystery motors, Bill said he doesn’t recall any such designation. Further the mystery motor was never a production engine therefore could never have had an RPO code nor does he think it appropriate to refer to the engine as the Z-33.

Unlike the MK IV engines, the heads of the early MK II's were NOT interchangeable. The individual left/right cylinder head setup was discarded fairly early, only used on the early 409 Mark II development engines. The different heads were reportedly beneficial to the development of early intake manifold design, but proved to be too complex for production.
ONE of the numerous MK II Intakes showing the dual-plane configuration
The intake manifold of the new engine was a unique design, a two plane with rounded runners, and much development was concentrated on its air flow and mixture distribution. Howell said Keinath was pleased that the original intake and port designed worked so well and said that it wasn’t until twenty years later that racers discovered the potential of the short port configuration.


In early November of ’62 Howell and the engineering team traveled to the GM Proving Grounds in Mesa, Arizona (Phoenix) to conduct back to back tests with two brand new race prepped ’63 model Chevrolet stock cars. This was a run-off between a fully race prepped 409 Mark I powered car and a similarly prepped car with the all new mystery engine. It would have been a 409 Mark II engine since the switch to 427 cubic inches was not initiated until about the same time period per negotiation with NASCAR. 
The tests were run on the sweeping 5-mile high speed test track with noted stock car driver Rex White driving both cars. The Mark I 409 W-motor car achieved a maximum speed of 157 mph while the new Mark II powered car hit 164 mph right off the trailer and was further tuned to speeds exceeding 175 mph. Hot Rod Magazine published a story claiming speeds exceeding 178 mph, but Bill only recalled sustained 175-plus speeds after extensive tuning efforts.

In addition to comparison testing between the two racing motors, a primary goal was to develop the newly conceived cowl induction system that pulled intake air off the base of the windshield. This was the first such unit devised for stock car racing and it initially caused calibration problems due to uneven air entry into the carburetor. The carburetor was an 850 Holley that had been dyno tuned for max power at 6200-6400 rpm.  

"Initial runs were fast, but the carb was unbalanced due to the nose down attitude of the car and uneven airflow from the cowl induction unit. Severe distribution problems were resolved by re-jetting the carburetor and hanging tabs off the boosters toward the lean cylinders to enhance the low pressure area. A low restriction diffuser was added to the cowl induction unit to direct air around the carburetor entry so equal airflow was available to each venturi. Carburetor vent tube height and shaped were also tweaked to further refine the calibration."

When asked about problems during the Daytona 500 and after the engines had dominated both of the 100 mile qualifying races so dramatically, he recalled that the teams made their own revisions to the engines after they received them. He also said there were no major failure issues other than normal development problems.

We would like to acknowledge Hot Rod Engine Tech for the details of the Bill Howell interview and some of the above photos. We hear there may be another report regarding the details of the engine internals, but have not yet been able to locate it. If we do, we will update this post or add another.

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