Subject:        Wetherall Chevrolet ZL1 Block (Prototype Raw Casting)

 Part Number(s):      Casting PN# 3946502, Date code - N/A

Historical Perspective/Overview – General

Chevrolet Aluminum 427 Cuin Engine – Can-Am and RPO ZL1 configurations

Chevrolets disappointment over the results of the 1966 Can Am series and the lack luster performance of the 327 prompted the “advanced thinking” which initiated Engineering’s development of the all aluminum 427.


Additional horsepower would be required to re-gain the competitive edge in the upcoming 1967 Can-Am season. The problem was that the 327 had reached the “end-development” stage and only more cubic inches would get the job done.  The Iron 427 was available and was demonstrated to be highly competitive in NASCAR - but the additional weight in the small Can Am chassis was a consideration that could hold it back. Since Can-Am had no restriction on production derivatives, materials, induction types etc., it was conceived by Engineering that experimentation using cast aluminum in limited production was a feasible study for the 427.


The success of the experimental program gave rise to the “Can-Am” Aluminum 427 which had its debut in the 1967 Daytona Continental Can-Am in the engine bay of the Chaparral 2F. The Chaparral immediately went on to set a record for the fastest lap and it held great promise for the series.  As it turned out, it was one of the most successful race engines of all time.


The Aluminum 427 engine became available (in theory) over Chevrolets Parts Counters, and carried an “O-Dash” Prototype (Interim) Casting/Part Number. Of course, availability to the general public was curtailed in accordance with Chevrolet’s policy of keeping “special parts” in the hands of a select few, and therefore Product Performance Groups “Back-Door” policy was in full effect.


            Above: Prototype CAN-AM ZL1 Block with O-Dash Number

Most of the Can-Am engines were built up inside engineering and sent off to Traco for Penske's and McLaren's use during for the  ‘68-‘69 seasons. The Production Engine Group took over assembly and the engine finally reached regular production for consumer consumption after much corporate debate in 1969. The ZL1 was made available for the 1969 Corvette and Camaro as Regular Production Option (RPO) ZL1 in extremely limited quantities.  These engines have regular GM production numbers as opposed to the prototypical O-Dash numbering system used on the experimental Can-Am engines.

Can-Am configuration shown assembled using an 0-Dash numbered block

Production Number located on the right rear corner of the block as shown on the Raw Casting (Subject)

Production ZL1 engine (block casting #3946502) as it sits in GM Reprographics for promotional “Glam” shots


General Description

                     Subject:  Wetherall Chevrolet ZL1 Block (Prototype Raw Casting) 

The Aluminum Engine block identified on the cover and subject of this report is a Chevrolet component produced by Winters Foundry using the production ZL1 molds complete with production casting number #3946502.  It is however, a deviation from the Regular Production run of ZL1 Blocks.


The subject ZL1 block was cast with a taller deck height, thicker webs and crank saddle. The intent was to create an experimental “Tall Deck” version of the ZL1 for evaluation. This block is considered experimental or “interim” for Engineering’s use in a prototype build.


Shown above is a comparison between a fully machined Production ZL1 block on the left and the raw cast “Tall Deck” ZL1 with casting PN #3946502 on the right.

 Subject History

 During the limited production run of RPO ZL1 engines, consideration was given toward increasing power output specifically for the Can Am effort. Several combinations of bore - stroke were considered and tried. In effort to gain additional stroke, the deck height of the block was increased.  The “Tall Deck” block was easily accomplished at Winters Foundry, by altering the original ZL1 molds to add additional material.  Special pistons and rods were then developed to match the deck height. Pistons under PN# 3963642 (un-released) had an altered pin to deck height and the finished ends of the special rods were bored with the center-to-center dimension changed from 6.405 to 6.385.


Several engine bore – stroke combinations (see below) were tried with the 430 CuIn 680 hp version winning out for Can-Am use. 


430 power band clearly shown to be an advantage for Can-Am
(Chevrolet Archives)

Examples of aluminum big block engines with different deck height, bore and stroke configurations

            The program was dropped for production and/or heavy duty service consideration along with many other high performance programs running at the time due Corporate priorities changing from performance to SMOG and economy. As a result, the aftermarket carried on with McLaren taking the lead in aluminum big block development. Minimal support by Engineering was given and only where it was advantageous in some way and a substantive return to Chevrolet could be quantified. 

Historical Importance 


            One of the most remarkable aspects of this special component is that it survived in its current unfinished state. It remains a testament to the progression of Aluminum big block and representative of end-development of Chevrolet power production. In addition, it marks the end of one of the most exciting eras in Chevrolet Racing History. Moreover, its discovery brings new interest and life back to the legendary dominance of Chevrolet Engineering.

Photographic Documentation 

 Subject Block:  Chevrolet Prototype “Tall-Deck” raw casting produced using ZL1 block with casting number #3946502

Face of block shows a larger than production Winters “Snowflake” on an  octagon pad with the ® symbol in the lower right hand corner

Production casting number 3946502 is shown on the top, right hand bell housing flange of the subject block.

Comparison between machined production ZL1 on top and the subject casting on bottom. Note the amount of additional material added to accomplish an increased deck and manifold height. Also note the increased size and higher location of the Winters Snowflake –An identifier symbolic of the larger block.

Comparison showing subject block (top) with machined ZL1 (bottom). The increased pre-machined deck height is in excess of a quarter inch, more than adequate for experimentation with taller finished deck heights.

Circles depict the area between the freeze-out plugs and deck (production ZL1 on left, subject on right). Again we can appreciate the degree of additional material added to increase the deck height for the “Tall Deck” block.

Copyright Wayne D. Guinn, 2005
All Rights reserved, No portion of this report
may be reproduced without the express permission of Guinns Engineering