GM's Mystery Motors - Part 1


This is one of those articles that took weeks to compile as there were 100's of articles to read, check for accuracy and separate fact from fiction. In 1963, Chevrolet delivered two completely new engines to race teams around the country. They were called "The Mystery Motor". That's not a mistake. In 1963, many people believed there was only one Mystery Motor. Factually there were two.


Manufactures had introduced powerful new OHV (overhead valve) V8's in the mid to late 50's. Their
small-block configuration, roughly 260 to 327 CID, was great for everyday use in the average car, but the technology of the day was inadequate to extract the kind of brute-force power necessary for heavy duty or performance applications. 

In the case of Chevrolet, their answer was what has become known as the "big block". True to the name, these engines were significantly larger in both physical size and displacement. 


Chevrolet, like all automobile manufacturers, was caught up in the design of larger cars and the growing demand for performance based vehicles. The largest available engine was the 409 CID "W" motor, aptly named due to the unique shape of the valve covers. The 409 had an amazingly powerful and flat torque curve, but despite it's legendary capabilities, the 409's performance suffered greatly above the 6,000 RPM range.


The two 427 CID "Mystery Motors" were entirely different and went to different types of race teams. The MK 1 version was given to drag racers, who were already enjoying the winning benefits of the 409. The other was given to NASCAR race teams. This was the MK II big block. They were very easy to tell apart visually, as they new MK II had a rectangular valve cover similar to the ones seen on Muscle Cars of the 70's, while the MK 1 version retained the unique "W" valve cover.
The "W" valve cover (L) vs the MK II valve cover (R)
Chevrolet's goal was to get some real world experience using newly designed engine configurations in preparation for future models. The quickest way to determine which of the two designs would emerge as the leader, was to place both versions into service at the same time.

As mentioned, the "W" suffered from lackluster performance as the engine rev'd past 6,000 RPM, but had an amazingly flat torque curve. This limitation was primarily due to the combustion chamber designed within the block versus in the head and the vertical arrangement of the valves. 
The "W" head (L) vs. MK II style heads (R) Closed Chamber (Upper) Open Chamber (Lower)
This engine was a part of a package known as the RPO Z-11. This package had one purpose; racing. It was only available on the 2 DR Impala (non SS), consisting of a stripped down version of the car, aluminum front fenders, hood, bumper and brackets (see below for the complete list of items included).

But the star of the option was the 427 cubic inch motor. While there was no available information to corroborate our theory, the deduction appears rather simple. A 427 CID version might still suffer from the same high RPM issue, but the torque and horsepower would be significantly greater within the usable RPM range.

THE "W" 427

The engine was little more than a revised version of the 409 including additional high performance parts. While these components may have been unknown regarding their specifications, the biggest difference was the the increased displacement. Not much mystery, just bigger. But just the mention of a "super-secret" engine would have caused quite the stir. 

The factory produced "W" 427's life was short lived. None were made after the first few left the factory and there were no records available to us indicating the potential to order individual components through the Chevrolet parts system.

Again, with no documentation to prove or dispel facts, the "W" 427's announcement as a "Mystery Motor" may have been used more as a diversionary tactic to hide the second "Mystery Motor" by GM's brass.


The ending of a factory produced version of a "W" 427 has not stopped the legendary engine. With literally 100's of 1,000's of 348 CID and 409 CID engines built, the aftermarket system can now supply a wide variety of high performance parts to take these once "boat anchors" tonew performance heights.


There were varying "facts" regarding availability of the RPO Z-11 package and the W-427. We have not been able to definitively confirm all of them...yet, but we'll try. They include:
  • There were between 50 - 57 actual units produced.
  • The only units with the W-427 went to select drag race teams, the others were equipped with 409's but included many of the special W-427 performance and lightweight parts adaptable to the 409 configuration and body.
  • The high performance and lightweight items in the RPO Z-11 package (excluding the W-427) were available through Chevrolet parts departments in small quantities.
  • The W-427 underwent continuous refinement during the year with several variations released to race teams.
  • Late production 1962 Impalas were made available to select drag race teams with the Z-11 package including the W-427 disguised as their 409 CID counterparts.
  • Additional 1962 Z-11 lightweight components were sold to race teams as a parts counter order

Although GM banned factory sponsored race teams in January of 1963, the information reflects there may have been a few "loopholes" which allowed the distribution of these engines via the RPO (regular production option) system and over the counter parts orders. 

But with only about 50 total Z-11 units produced, it is far more likely all of them were produced prior to the ban. Of course, as the ban would have been in discussion well in advance of the 1963 model year, so it does add some credibility to the potential production of Z-11's as 1962 models.

As you can tell, there are numerous items which remain "plausible", but with little more than recollections and hearsay, our research will need to continue. If and as we uncover more, we will update this article.

Next in this series is the more appropriately named "Mystery Motor", the MK II.

Z-11 Components and Part Numbers:

  • Produced only in 1963 
  • The R.P.O. Z-11 option was an additional $1240.00 
  • Shipping weight-3245 lbs. 
  • Curb weight-3405 lbs. 
  • 427 CID engine factory rated at 430hp@6000 RPM
  • Block casting #3830814. Code "QM" 
  • Block part number #3838253 
  • Cylinder Heads #3837731 2.19 intake/1.72 exhaust 
  • Valve Covers #3837739 (left), #3837740 (right) 
  • Oil pan (deep sump) #3816322 
  • 2 piece Intake manifold #3830623 (top)-#3837733 (bottom) 
  • Water pump #3837691 (aluminum) 
  • Cam #3837736 .556 intake/.556 exhaust lift with 325 duration 
  • Crankshaft #3837472 
  • Distributor #1111023 
  • Coil #1115107 
  • Generator #1100628 
  • Starter #1107274 
  • 3.65" piston stroke 
  • 4.3125" bore (same as 409) 
  • 13.5:1 compression ratio 
  • Special cowl induction air cleaner #6418986, #3839764 (duct) 
  • Tach (redlined at 6200 rpm's) 
  • Borg Warner T-10 4 speed transmission (close ratio) 
  • Special posi rear with 4.11:1 rear gear ratio 
  • Sintered metallic brake shoes 
  • Screened rear brake backing plates with air scoops 
  • 15 X 5.5 wheels with 6.70 X 15 bias ply tires 
  • Front coil springs #3824693 
  • Rear coil springs #3824694 
  • Deleted front sway bar 
  • Optional sound proofing and insulation delete on most 
  • Optional heater delete on most 
  • Hood #3837710 (aluminum) 
  • Hood catch #3837718 (aluminum) 
  • Grill filler panel #3837750 (aluminum) 
  • Left fender #3837765 (aluminum) 
  • Right fender #3837766 (aluminum) 
  • Front bumper center #3837751 (aluminum) 
  • Front bumper LH #3837755 (aluminum) 
  • Front bumper RH #3837756 (aluminum) 
  • Rear bumper center #3837752 (aluminum) 
  • Rear bumper LH #3837753 (aluminum) 
  • Rear bumper RH #3837754 (aluminum) 
  • Front bumper bracket LH #3837763 (aluminum) 
  • Front bumper bracket RH #3837764 (aluminum) 
  • Rear bumper bracket LH #3837757 (aluminum) 
  • Rear bumper bracket RH #3837758 (aluminum) 
  • Rear frame to bumper brace LH #3837759 (aluminum) 
  • Rear frame to bumper brace RH #3837760 (aluminum)