09/10/05

Subject:                    Rob Stark / Chevrolet Cross Ram Manifold Assembly

Part Number(s):      Upper Half – PN# 3941130, Date code – None
                                    Lower Half – PN# 0-310510, Date code – 11-5-67 (Rev.12-11-67)
                                    Carburetors – LH & RH - List 4210, PN #3906631-DA/3941140
                                    Date code - 824

Historical Perspective/Overview - General

1967 was the first year for Camaro in the SCCA Trans-Am Series. It was a hard fought season with a single four-barrel induction system as per SCCA rule limits. For the 1968 Trans-Am racing season, the SCCA changed the rules from "Not more than one four barrel" to "Not more than two four barrels". This change was largely in response to unofficial factory prodding with the intention of creating a more exciting Trans-Am series.

As a result, Chevrolet developed and released the "2X4 Carburetor Conversion Unit”, PN #3940077 for use with the 302 Z/28 cuin engine. This special "Cross Ram Induction" system was made available over the counter through Chevrolet parts departments as a "Heavy Duty Service" option beginning December 1967.

 Cross Ram Components - General

•  Prototypes: Initially, there were four or five “working” prototype units built up inside Chevrolet Engineering slated for performance evaluation. Each of the units was O-Dashed Numbered (0-310510/0-310512) identifying them as pre-production or prototype components.  These prototypes were exceptional in the sense that they would be used in the SCCA Trans-Am Racing Series on various well-known racing chassis.

Performance evaluation on the first manifolds was conducted inside Chevrolet Engineering by the Product Performance Group and by Smokey Yunick at his shop in Daytona Beach.  It was on these first prototypes that specific upgrades and corrections were made in an effort to extract the full potential of the components. Some of these corrections are quite evident and appear distinct from the original casting having been accomplished by hand. These alterations include the critically significant fuel distribution fixes (runners and dams) inside the lower manifold and the addition and removal of material for flow correction.

• Production: The production manifold was assigned production part numbers for top and bottom halves and the final configuration of the distribution runners were now cast into the lower half. The oil splash shield that was part of the prototype was dropped as it was determined not to be necessary since there was no heat crossover to vaporize the oil that came in contact with the manifold. The water jacket was in its final configuration and the heater hose outlets redesigned with the 1969 water pump in mind.

Stark Cross Ram Assembly - Subject

General Description:

Chevrolet Cross Ram Manifold assembly consisting of lower and upper manifold halves, first design carburetors, carburetor linkage, fuel distribution hardware, stamped steel heat (oil splash) shield, and plenum style air cleaner. All components are original Chevrolet built or distributed. All appear in excellent condition.

• The manifold identified on the cover page and the subject of this document is consistent with the initial or first working group of prototypes produced by Chevrolet. It was among those four or five complete units built up inside Chevrolet Engineering and evaluated for performance by Engineering’s Product Performance Group and by Smokey Yunick at his shop in Daytona Beach. 

·        The distribution fixes (made to the manifold floor) were accomplished primarily by gas analyzation methods, that is, obtaining readings by taking samples at the exhaust ports.  Also utilized was the more gross method of reading the plugs after dyno runs following changes made based on educated guess work. 

 This particular area of development was complicated in that an intake port that is rich at low rpm can also demonstrate itself to be lean at high rpm. Needless to say, time spent in this area is crucial for the best obtainable performance results. It was through much educated guess work and extensive trial and error that the final runner configurations were made.

 Some flow work occurred simultaneously inside Chevrolet Engineering and it’s interesting to compare the manifolds developed by both camps to observe the variations (See Flow Distribution Anomaly below). The distribution fixes seen in this manifold were hand laid using aluminum wire and aluminized Devcon Epoxy.

IDENTIFYING FEATURES (Specific to the Stark Cross Ram)

 Several features related to this manifold make it significantly unique as follows:

§     Inversion error related the hand laid fuel distribution fixes (see Flow Distribution Anomaly section below).

§ The pairing of the lower O-dash numbered base and production numbered top without a casting date is consistent with the last units of first group of prototypes sent by Engineeing to key “team” consultants and racers. Recipients of those units included TRACO, Yunick, Penske and Chitwood etc. and were intended specifically for Trans-Am competition use. The carburetor date (824) also agrees with the last units prepared that were specifically targeted for the Sebring event.

§ First design carburetors - hand built inside Engineering (See Carburetors section below). The carburetors on this manifold assembly are key in helping to narrow down the changeover date from Prototype Carbutretor to Production.

§ Production part number 3941134 is hand written (using a paint pen) on the oil heat shield (see image below). The shield was assigned the aforementioned production part number and cataloged but never used outside of the prototype units. The part number was written on the shield while inside Engineering for Identification purposes since the part number was not stamped into the steel part. This marks the first time (outside of engineering) the number has been seen physically associated with the part, giving confirmation to the assigned number.

§ In addition to the above, the following are less significant observations related to condition: The upper and lower manifold castings are exceptionally clean and free of defects in comparison to others from the same batch release. Manifold and Carburetors appear to have very few hours use.

Flow Distribution Anomaly 

• A most interesting and distinctive anomaly appears on the lower half of the Stark manifold.

 The fuel distribution runners were inadvertently laid into the manifold inverse to the accepted configuration that Engineering so painstakingly developed. It’s a certainty that this anomaly had an adverse effect on fuel distribution and ultimately engine performance, since in the final configuration, the runners were designed to lean out number 1, 5, and 7on the right bank and only 2 and 4 on the left bank. Theoretically, the error causes 6, 8, 5, and 7, to be lean.

It was not uncommon for an error of this type to occur since much of the HD race development was done “after-hours” inside Engineering (See “Camaro Untold Secrets” for a similar example of an “inversion error” i.e. pg. 59). This “extra effort” was driven by the commitment of a handful of Engineers who truly cared about the product, understood Chevrolets the race-oriented goals, and appreciated the magnitude of what was what was at stake. In fact, many of the Engineers were racers and enthusiasts themselves, and ultimately the products they developed carried them to many personal victories on the track.

Along with the strong commitment and deep desire to outrival the competition, came long hours and inevitably fatigue, which may have been a contributing factor in the formation of this unusual anomaly.

 It should be understood that the process for laying-in the runners was completely manual and that no blueprints were used. A completed working prototype was used as a model to finalize the next. The technician would position the manifolds side by side (or one above the other) and using the finished unit as a guide, he would proceed to copy the patterns to the next manifold. The aluminum wire was formed and affixed to the runners using aluminized Devcon epoxy.

 It’s quite apparent the technician did not notice the manifolds were positioned opposite on the workspace during the set-up and all throughout the operation. Something that’s not hard to do considering the tub area of the manifold itself is symmetrical and appears alike either way you view it.  And, since the tub was the central focus of the operation, the technician overlooked the obvious indicators on the periphery. Those being the thermostat housing and distributor boss that help define manifold position.

One of the most interesting aspects about this manifold is that it left Engineering without the error being recognized. Although, it’s explainable since the production of the first batch was rushed well into the “eleventh hour”. The pressure to get the units completed and out to the racers was enormous. As a result, the last couple units were not run on the test cell Dyno before leaving Engineering. The Dyno results would certainly have detected the error as part of a basic performance quality check. What also supports the fact that this manifold was among the last of the group, is that it bears the later style top. A paired lower O-dash numbered base with the later production numbered top without a casting date is consistent with the last few units of first batch that left Engineering.

KEY COMPONENTS 

Carburetors:

The carburetors provided with the subject cross ram package were of the first design, dual feed, single pump, PN# 3941140, identified by List # 4210. They were hand built inside Engineering using production carburetor PN# 3906631-DA. The following gives a detailed explanation related to the Part Number alterations appearing on the air horn:

• Initially, PN # 3906631-DA (List 3810) carburetors were used as the basis to build PN # 3941140 carburetors due to the urgent need for a workable prototype for competition use. PN # 3906631-DA (List # 3810) carburetors were re-built and hand scribed in Engineering to produce the specified carburetor configuration that was later assigned Production PN# 3941140.

• The origin and uniqueness of these pieces is immediately recognizable since they bear Engineering’s telltale “fingerprint” - that being the hand scribed 4210 list number and the obliterated original list number 3810. At the time these prototypes were being developed, the production Part Number (PN #394110) had not yet been assigned, consequently it was not scribed onto the early 4210 carburetors, only the list number was altered and the original PN # 3906631-DA remained.

Therefore, in the case of the early PN# 3941140 carburetors, the list number should always be used as the correct identifier since the early 4210’s (PN# 3941140) were hand built using Part Number 3906631-DA carburetors  (See illustrations on the following page)

The initial or production build date (824) stamped on the subject carburetor(s) indicates that they were produced during the fourth week of February ’68. The carburetor build/re-build date sequences are consistent with the build dates of Engineering’s prototype Cross Ram units released for the Sebring event. 

 The conversion from 3810 to 4210 carburetors occurred inside Engineering beginning November of 1967 Carburetor. The subject carburetor having been part of the conversion helps us narrow down the time frame in which the conversion of prototype Carburetors ended and regular production began.  PN# 3941140 (List # 4210) having the proper factory stampings went into regular production somewhere between the fourth week of February 1968 (subject carburetor date) and the fourth week of April 1968 (reference carburetor date above).

KEY COMPONENTS -

Peripheral Hardware:

 Production “hard” fuel lines, and distribution block were provided with this Engineering release. The fuel distribution block was fitted by the recipient of this unit with an aftermarket Aeroquip fitting for racing use. Hardware also included with this release was as follows: Carburetor Linkage Rod with Heim ends/special step screws and spacers, Throttle Spring Return Bracket and Spring and the Throttle Cable Bracket and Cable Clamp as illustrated below.

 Good over view above illustrates the components that constitute the Cross Ram package. The cleanliness of the unit indicates low hours of use.

Air Cleaner

Chevrolet Cross Ram Manifold Air Cleaner – Second Design Cold Air Box- Consisting of upper and lower Air Cleaner Plates (chrome plated, stamped steel construction), and Phenolic Plastic Plenum Ring. Components are original Chevrolet built and distributed and appear in good condition.

•  The Air Cleaner provided with the subject cross ram package is consistent with prototypical parts. The plenum ring is the second design Cold Air Box. This version has a slightly larger cross sectional area to the plenum to facilitate increased airflow to the carburetors. The release of this version is consistent with the dating of the carburetors in this package and again with the release of the last couple units of the first or initial prototype units produced. It is also noteworthy that it is the second design plenum ring that appears on the infamous “Carburetor Installation Instructions” released as part of the Cross Ram Package, dated March 20, 1968. 

•  The second design Cold Air Box plenum ring was developed after engineering determined airflow to the carburetors was too restrictive with the first design plenum ring. The cross-sectional area of the ring was enlarged for the second design enabling more flow to the carburetors. However, it was later determined that the poor performance characteristics related to the air cleaner were actually due to a combination of the carburetors air horns being too close to the air cleaner lid, which created turbulence by forcing the air flow to turn too acutely, and the inadequate surface area of the air filter (dictated by the need for hood clearance) that were the real problems. Penske resolved the issues with the flimsy and restrictive plenum rings in 1968 by having Ron Fournier build an all aluminum cowl air cleaner and used a larger Ford Element. Engineering later resolved with the development of the plenum hood induction system (RPO ZL2) for 1969.

• The first and second design plenum rings are easily distinguishable (see Illustrations below). The increased cross sectional area of the second design necessitated the back portion of the ring to be altered for additional clearance. Both first and second design rings use the same Purolator filter element (AFP-22).

• Most apparent and noteworthy are the imperfections in the chrome plating on the prototype air cleaner top and base plates. The blemishes were left by steel rods, which were silver soldered to the plates (see photographic documentation section) in order to suspend them in the plating tanks during the plating process. The marks appear on both ends of the top and bottom plates on the inside surfaces.

•  The imperfections left by the rods required abrasive action to smooth the surface after the rods were removed. In addition, the solder interfered with the plating process causing a slight dis-colorization in a “halo” pattern around the affected areas. Production pieces that followed used a different suspension process (magnets) and do not have, to the same extent, the equivalent degree of discoloration and plating flaws

• Hard shellac appears on the inside of the Air Cleaner lid around the carburetor stud holes. This material was used as a locking compound and was applied to the stud, wing nut and air cleaner surfaces. All other fasteners (nuts/bolts/pins) used on the engine and chassis were safety wired. This exception was necessary due to the inevitable need to quickly disassemble the Air Cleaner assembly with out too much difficulty but at the same time assure the wing nut would not loosen under the extreme vibrations of racing. This practice was most commonly associated with Penske Racing but may have been adopted by others.

•  The lack of provision for a crankcase breather connector is consistent with the first assemblies. All of these pieces were built up and pushed out the “back door” of Engineering to racers to help complete the Cross Ram package. The Cross Ram “Carburetor Installation Instructions” were issued on 3-20-68, only a few weeks after the release of the subject cross ram. The sheets show the final configuration of the plate with a provision for Air Cleaner Connector to satisfy National AIR Regulation mandates for production. The state of configuration, distinct anomalies and the time of build are consistent with units pushed out for use in Sebring.

• Remnant adhesive from duct tape is apparent on the plenum ring. Vibration would often cause the ring to separate from the lid and base plates so the “racers friend” was called upon to hold the ring to the cover throughout the rigors of racing.

Headers: 

• The subject Headers (see photo below) use 1-3/4 inch pipes, 37-inch length, with a 3-1/2 inch collector. The configuration for the Headers and the head flanges themselves are consistent with the Second Design headers sanctioned by Chevrolet to a handful of consultant manufacturers such as Jere Stahl and Kustom Headers, and the few dealer/performance combines such as Motion, Nickey, Bill Thomas and DANA who distributed them. (Ref. First Design PN #3942501-52, pieced tubes; Second Design PN #3916383-84, Mandrill bent)

• The subject headers are consistent with those marketed by Bill Thomas Race Cars in Anaheim CA. These headers are tuned for the Cross Ram using the “140” cam and fully ported “TRACO” style heads. These pipes were designed for Trans-Am competition and would have been used with a 50-inch collector/tailpipe turnout for each side that would exit directly in front of the rear wheel (See Stahl Headers below)

Engineering Usage:

•  Evaluation of the Cross Ram unit components within Engineering consisted of a visual inspection as well as a functional assessment for form, fit and function and was gauged against performance expectations and pre-specifications set forth by Chevrolet Engineering. Therefore, the components would have been fitted and run on an engine dynamometer within the Engineering’s Product Development engine test cell and tweaked accordingly. However, as previously mentioned this Cross Ram system was passed out the door without Engineering’s evaluation due to the following circumstances:

1. The need to get the product out to the racers overrode the protocols involving collection of data and quality checks related to this specific unit.

2. The manifold was at end development for the initial phase and the confidence level was high due to previous successful manifold testing. Unfortunately, no one could have predicted the human error associated with this unit. We can only speculate that the error was learned of after it left Engineering during actual use (hence the apparent low hours use) and rediscovered during research 38 years later as documented in this report.

• It was not unusual for a pre-production prototype or evaluation piece to be pressed into service due to the high demand and short supply of these components.  Often times Chevrolet Engineering and Product Performance would send out what they had on hand to racers/combines who were considered “insiders”. Many of the parts processed through the ‘back door’ were pre-production pieces or prototypes that were clearly marked as such. Due to the specific design state, estimated build time, it is evident that these components were among those pushed to one of the “insider” race teams.

Historical Importance:

•  This research attempt concludes with the successful and comprehensive documentation of a significant prototypical race bred Chevrolet development. A works in-progress touched by the greatest engineering and racing talents in the world. An effort that changed the course of racing history and facilitated the most successful automotive marketing strategy for Chevrolet to date by helping capture the prestigious SCCA Trans-Am Series Manufacturers Cup for two consecutive years.

•  In particular, the subject Cross Ram holds a most significant place in Chevrolet/Camaro racing history as it is consistent with the dating and the final configuration of the Cross Ram units used during the '68 Sebring and later events on any one of the insiders Race Chassis including the Penske Sunoco Racing, Takondo Racing, Joie Chitwood Racing, Smokey Yunick etc

Photographic Documentation – Stark Cross Ram

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