Mystery Motor Development Engineer Bill Howell Tells All

Our first story on Chevrolet’s legendary Mystery Motor generated a lot of new questions and a few sour comments by desktop experts still insisting it was a revised 409 W-motor. So we decided to solicit more facts straight from the horses’ mouth. We consulted an old friend who supervised all the testing and development of the mystery motor program as a young Chevrolet engineer.  Having known Bill Howell for nearly four decades we felt sure we could get the straight scoop and clear up some misconceptions.  We were right. Now retired and still going strong at 80 years young Howell has fond memories of his work on the Mark II program and was pleased to share them.

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.

To begin he offered the correct pronunciation of engine designer Dick Keinath’s name. While many have interpreted it as “ken-nath” Howell informs us that the correct pronunciation is more like “Ki-neth” with the Ki rhyming with “sky.”  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 ’63 Daytona 500. For details, refer to the previous article “Inside the Chevrolet Mystery Motor.” Our friend Bill Howell confirmed pretty much everything we described in that article and further elaborated on one of his favorite projects during his tenure at Chevrolet.

Rosie came to Phoenix in November 1962 when we were testing the Mk II and rode around the track with Rex White at 160+ MPH sitting on the floor of the race car with a smile on his face from ear to ear.

Howell then reminded us of another instrumental person whose name is rarely mentioned in relation to the mystery motor. According to Bill that would be Maurice Rosenberger or Rosie as he was often called; 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.”

One thing Howell specifically cleared up is that there was never a factory drag racing program associated with the Mark II program. The Mark II engine was strictly targeted at stock car racing.  He is not aware of any dual four barrel intakes ever being cast for drag racing and if there were they would have been hand built and he never saw one. He did say that one of the engineers had a son who put a backdoor four barrel equipped mystery motor in a car that he was drag racing locally, but he didn’t know what happened to it and has no recollection of any factory built 620 HP dual four barrel mystery motors for drag racing. Zora Duntov’s group was still in full development on the W-based Z11 drag racing engine and had no real involvement with the mystery motors.

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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.
That’s likely true in our estimation. If a 427ci Z11 was good for 430 HP on paper and we give it a 50 HP fudge factor and call it 480, it’s still a long way to 620 HP with the cams and cylinder heads of the day. And a mystery motor of similar displacement good for an honest 530-540 horsepower with a four barrel still needs 80-90 horsepower to reach 620 HP. 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.

Bill recalls those days as exciting times for a young engineer. The mystery motor development program was running two shifts and moving rapidly to get engines ready for Daytona.  The ’61 409 W-motor was good for about 425 HP at 6200 rpm which was not enough to mount a serious reliable challenge to the other competitors.  People will undoubtedly point to the 427 Z11 engine and the 425 HP dual four-barrel 409s and disagree because drag racers got more power out of them and it is likely true; at least for 12-13 second bursts on the dragstrip. The difference is durability. A single four-barrel W-motor in stock car trim was maxed-out at 425 HP and barely able to hang together for 500 miles. Don’t forget it was originally a truck engine with massive heavy pistons and few design features suitable for racing.

We asked Bill about the Z33 RPO designation that so many people claim was attached to the mystery motors. He doesn't recall any such designation and further stated that the mystery motor was never a production engine and thus never had an RPO code. Where the Z33 came from is anybody's guess, but Bill feels that it is incorrect to attach it to the mystery motor. We suspect that the term evolved from some former reference in a magazine article suggesting that the Mark II series might eventually become a production engine and Chevy lore took it from there. The only reference to a Z33 in GM literature is a spare tire carrier. Hence, mystery motors are the real deal; Z33's are not.

Howell also confirmed that the Mark II engines used asymmetrical cylinder heads. These heads were Keinath's best shot at equalizing port efficiency, but objections from the foundry led to a redesign that was easier and cheaper to produce for the later Mark IV engines that had interchangeable heads. We found confirmation of this in an earlier Keinath interview in which he remarked that the individual left/right cylinder head configuration was more efficient because of the superior port entry angles. These were used on all Mark II engines.

Mark II intake manifold

 The intake manifold was a unique design, a 2 plane with rounded runners, and much development was concentrated on its air flow and mixture distribution.

This configuration had more equal flowing ports, but in an earlier interview Keinath alluded to cylinder head and intake manifold revisions required to more effectively accommodate the long port/short port configuration required by a centrally mounted carburetor. He described the short port tuning as ideal for the predicted engine speeds, but said it had to be compromised due to manufacturing requirements. If you look at Mark II intakes from above you’ll see a notable difference from later Mark IV intakes. 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. The intense development program devoted to the 2-plane 4-barrel intake explains the different casting numbers seen on different Mark II intake manifolds.

mystery motormystery motor

Mark II cylinder heads are identified by their slightly taller, more rectangular intake ports that are completely different from Mark IV square ports. The top of some runners have threaded holes to accept the cast iron pushrod guide bolts and it is also important to note how the manifold runners are configured to provide more favorable entry angles to the ports. Compare the layout of the cylinder head port runners and note the positioning of the manifold runners that match up to them. Also note that all component casting numbers are preceded by a zero indicating pre-production development pieces.

mystery motormystery motor
Once we found out how much performance there was in a tuned 4 pipe header, we had to duplicate it with a 4-hole cast iron manifold, and a matching 4-pipe collector that attached to it.

Exhaust system development was defined ultimately, by NASCAR’s rule requiring cast iron exhaust manifolds.  “Once we found out how much performance there was in a tuned 4 pipe header, we had to duplicate it with a 4-hole cast iron manifold, and a matching 4-pipe collector that attached to it. The other high horsepower revelation was how much cam overlap worked with exhaust tuning. “

A second unique feature at that time was the use of a Holley carburetor.  “Center pivot float bowls had not been invented yet, nor mechanical secondaries.  All development through the summer of 1962 was done with Carter AFBs.  Holley claimed to have a carb that would give more air flow in their flow boxes.  We were willing to try anything in the name of HP, though Holley had never been used by GM (we thought of it as a FORD part). The first sample they brought us for test had the secondary venturis bored out and low restriction booster nozzles, vacuum operated secondaries, and side pivot float bowls.  The balance between the primary and secondary fuel flow was so bad that we had to re-jet the carb 3 times to make a complete power run from bottom to top.  But, it was about 15 BHP better that the Carters.  After sending it back and forth a couple of times, we finally got proper venturis and booster nozzles in it, and it would run throughout the whole RPM range.  Because this carb was so far out to lunch when Holley first brought it to us, I don’t think Ford or Chrysler were using them yet for racing.”

mystery motormystery motor
mystery motormystery motor

The lower end shared similar dimensions with the Z11 and the cranks were interchangeable. Smaller main bearing diameters (2.5_inch) were retained to reduce friction as Keinath determined that they were sufficiently strong to support racing requirements. Sharp eyes will note (see photos) that the oil pump and pickup are similar to 4-bolt side inlet small block/W-motor pumps, but the pump cover is configured with a bottom inlet and pickup that provides a more direct path to the pump gears. So the pump cover and pickup are unique to the Mark II series, but the pump housing is standard small block.  And the pickup is angled to take advantage of the unique deep sump pan.

The lifter bores were not aligned, but instead offset as in Mark IV engines.  The difference is only 5 degrees but it is very clear when you see it. Numerous revisions were made to later Mark IV blocks including larger 2.75-inch mains for severe duty truck and high performance service and the later VIN pad extending ahead of the passenger side cylinder head.  Mark II heads are also more rounded on each end with no machined pads to accommodate accessory drives.  If we had one right in front of us we could pick out numerous other differences that characterize the transformation to production engines, but the Mark II itself was a pure racing engine specifically suited to the rules package of sixties stock car racing.

mystery motor

Rex White and chief mechanic Louis Clements at the November 1962 factory test session at the GM Proving Grounds in Mesa, Arizona.

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 409ci Mark I powered car and a similarly prepped car with the all new mystery engine. This would have been a 409ci 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 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.

“At the Phoenix tests, we discovered how sensitive the float bowl vents were to air flow, as you mentioned distributing the airflow around the carb 360 deg in the cowl induction.  We tested several variations of the exhaust system at Phoenix, and verified the basic 4 tube design as the best.  We ended up frying all the test engines after a couple of weeks in Phoenix (we weren’t using racing fuel, and the carb problems gave us difficult mixture distribution problems), but we ended up at 175 MPH.  We went back to Michigan and worked on the carb, mixture distribution in the manifold, upped the CR to 12:1, and put in a better dynamics cam.”

We attempted to run a 500 mile durability test at Phoenix in January and immediately ran 178, averaging over 175 for 400+ miles.

“We attempted to run a 500 mile durability test at Phoenix in January and immediately ran 178, averaging over 175 for 400+ miles when we stuck an intake valve (peeled chrome) and terminated the test.  Everything else was working fine, and Rex could run flat out all the way around the 5 mile circle.  By the way, Rex was the 1960 NASCAR points champion, with a 409 W-motor.

In addition to comparison testing between the two racing motors, a primary goal was to further 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.

In some of the online forums we’ve seen a lot of desktop criticism of the original cylinder heads for what some have called the worst combustion chamber ever designed. Well, perhaps by today’s standards maybe, but it was 1962 and just consider what they were trying to improve upon (W-motors). The 409 pistons and chambers had a very high surface to volume ratio that was not conducive to good power. The Mark II bathtub style chambers were an attempt to gain compression ratio, minimize surface to volume ratio and of course support the canted valve design. More piston surface area for combustion pressure to push on is desirable, but excessive chamber and wall surface area as in the 409 simply cooled the burn and reduced cylinder pressure. And as everyone knows, further development to shallower open chambers and revised piston crown shape ultimately resulted in some pretty stout rat motors in later years.

There were no major failure issues other than normal development problems.

We asked Bill about mystery motor problems during the Daytona 500 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. Bill says there were no major failure issues other than normal development problems. The wet sump oiling system was found to be up to the task with the small block based oil pumps although Smokey Yunick later pushed for pump revisions to resist cavitation and eliminate the spark scatter problem.

“At Daytona, Ray Fox had two cars.  Junior's car had a valve train failure.  The second car (GC Spencer’s) was the one with Mickey Thompson pistons that failed (we made the rest of them into ash trays).   Rex White finished fourteenth.  Bubba Farr was the 5th car and I believe he had a fuel pump failure.  Smokey’s car finished, but Rutherford looped it coming onto the backstretch and got a lap or more down.” He believes that Johnson and Ray Fox ran the entire season with mystery motors, but Rex White ran out of equipment half way through.  
While the development program was fast paced and rapidly changing, valve train testing was also conducted with the goal of raising valvetrain durability up to 7200-7400 rpm with the stamped rockers and screw-in studs. As discovered by drag racers later on, mass produced stamped rockers often suffered from ratio drift so you never knew if you were getting the design spec ratio from any given rocker arm. It often required several sets to put together a full set of equal ratio rockers. This likely contributed some early valve train issues that had to be worked out.

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 recalls 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 the 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 in 1963 and were found to be equally fast. Howell describes the 396 cubic inch Mark II as a real humdinger of an engine before it was abandoned. He told us that 50 sets of parts were run for the Mark II 396s, 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.

Smokey Yunick did development work on the MK II and the MK IV for several years, and contributed several useful improvements.

“Smokey Yunick came late to the mystery motor program, but he was a quick study and did development work on the MK II and the MK IV for several years, contributing  useful improvements.  He came up with the 3 angle valve job, isolating the intake manifold from engine heat and pinpointed a problem that was causing spark scatter and destroying our distributors early.  The spark scatter was caused by oil pump cavitation from too small an inlet pickup tube.  It was cured by going to a 5/8-inch tube.  And of course Smokey also identified the problem of main cap walking for which he developed his own girdle that fit over the factory cap to further secure it and prevent movement. Smokey also developed the angled spark plug change in our small block cylinder heads.  By the way, I am referred to in Smokey’s book as that pain in the ass from Detroit." 

By the way, I am referred to in Smokey’s book as that pain in the ass from Detroit.

That’s okay Bill, you’re probably not the only Detroit engineer Smokey referred to in that manner. One has to wonder if some of the engineers may have similarly referred to Smokey as “that pain in the ass from Florida.”

Mystery motor lore abounds and we’re pleased to have the opportunity to have Bill Howell enlighten us about them. As previously noted, he refers to it as a very intense and exciting period for a young engineer, but one that he thoroughly enjoyed. Mystery motors and parts are still floating around out there in various personal collections, but no cohesive program exists to catalog them and determine which ones are which and who has them. Some parts are no doubt lying around collecting dust on shelves in various shops. Interested parties who own motors or parts are invited to contact us at to help establish a mystery motor registry to preserve their existence and help catalog the subtle differences between them. Many thanks go out to Bill Howell for taking the time to clear up some of the misconceptions and discrepancies that exist about these remarkable engines and as we learn more we’ll revisit them from time to time. 

Bill Howell of Howell EFI talks about his involvement as a GM ...

Interview with our very own Bill Howell at the The SEMA Show TEN: The Enthusiast Network booth which had the Mickey Thompson owned, Junior Johnson driven, Smokey Yunick tuned Corvette that is on the cover of Hot Rod Magazine. This Corvette is the original car that featured the "Mystery Motor" Big Block Chevy prototype which Bill was test engineer on in his early days at General Motors & Chevrolet. Very cool piece of history and Bill was happy to see it in person again :-) #SEMA2015

Posted by Howell EFI on Tuesday, November 17, 2015


Thanks Bill.

Editor’s note:  If you have any valid information about mystery motors, original photos, parts, etc. we invite you to post in the forum under the General Tech Section so everyone can enjoy it.

Update: Heres a link to the Super Chevy story about a mystery motor drag car. It was pieced together from mystery motor parts and built as a Hayden Proffitt tribute car. Looks legit from the heads and exhaust manifolds. Not sure about the hybrid intake, but a pretty fun effort even though there were never any real mystery motor drag cars. We'll try to contact them to see if they have any build pics.