xramania (cross-ram-ma-ne-a) [Gr madness] n. An extreme

degree of enthusiasm toward a special small block intake.

by Wayne D. Guinn, Author of "CAMARO, UNTOLD SECRETS".

Yes!... Amazing but true, once you've seen a Cross Ram intake manifold set up in an early Camaro engine compartment, you can't help from being affected. Sometimes to the extent that it initiates the bizarre (but extremely enjoyable) obsession defined above.

Most Camaro enthusiasts will readily admit that one of their favorite topics of discussion at Camaro gatherings is the Cross Ram induction system and how those cars equipped always get the draw! And why not? Aesthetically, the Cross Ram is awesome, the symmetry and proportion of the design is strikingly beautiful, especially with the massive chrome-topped air cleaner gleaming in place. And function?... In terms of performance, the special induction system has demonstrated itself to be capable of helping produce an amazing 458 HP from the 302 @ 7200 rpm..!

In fact... its sole purpose was to coax huge amounts of horsepower from the relatively small 302 c.i.d. engine in the upper rpm range without severely compromising low end power. And... if that wasn't a tall enough order, it had to do so under the severe restriction of "keeping it in its pants", or more accurately, under the hood due a safety rule imposed by the SCCA race sanctioning committee.

As it turns out, not only was Chevrolet able to accomplish all of the above, but they far exceeded all expectations and realized tremendous success doing so. Today, we recognize the legendary Cross Ram Intake system as being one of the largest contributing factors responsible for helping establish the Camaro as the 1968-1969 Trans-Am champion.

IT ALL BEGAN WITH... THE NEED FOR SPEED!

Coming onto the scene a few years behind the Mustang, the Camaro needed to play a hard and fast game of catch up.

Chevrolet would have to compete hard to capture some sales from the "Pony Car" market that the Ford Mustang created. So, when it came time to get competitive, Chevrolet got serious - both at the consumer level and at the race track.

Chevrolet's plan was simple: Their competitive formula was based on the theory that a winning image sells -- and it was through racing that Chevrolet successfully promoted the Camaro and tapped into the fast-growing youth market. To create that image, Chevrolet put together the Camaro Z/28 package to compete against their biggest rival, the Ford Mustang, in the Sports Car Club of America's (SCCA) Trans-Am road racing series, better known as "the battle ground".

The Camaro's first year in competition was hard fought with a single four barrel induction system. This was the limit established by the SCCA sanctioning body in order to keep speed at a reasonable rate for safety considerations. However, as the popularity of the series increased, all caution went with the wind and 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 came largely in response to unofficial factory prodding with the intention of creating a "more exciting Trans-Am series".

The other manufacturers were ready, and so was Chevrolet. They released for use with the 302 engine the "2x4 CARBURETOR CONVERSION UNIT" PN #3940077. The special "Cross Ram Induction" system was made available over the counter through Chevrolet parts departments as a "Heavy Duty Service" option beginning December 1967. Although somewhat ambiguous, the term "Heavy Duty Service", is Chevrolet's frequently used catch-all phrase meaning "intended for racing".

The release of this special racing equipment marked the begining of an incredible winning streak for the Camaro on the track and for Chevrolet in terms of sales. And now, the nuts and bolts of it...

SYSTEM DESIGN

The system consists of a special two-piece manifold, carburetors and related hardware needed for installation. The inlet manifold assembly is a cast-aluminum single plenum "Cross Ram" type with provisions for two four-barrel carburetors.

By 1967, the Cross Ram concept was by no means new, Chrysler engineers had developed the "ram" idea in the early '60's. Equal length tuned runners create a ram effect needed to "pack in" the fuel mixture when using long duration cams on engines with high-rpm capabilities.

The Chevrolet Cross Ram design was intentionally kept low by placing the carburetors "outboard" specifically for hood clearance. The vertical tunnel ram is more efficient but it obstructs vision in road racing. For this reason the SCCA and NASCAR rules required the manifold and carburetors to fit under a stock hood.

Cold air induction drawn through the cowl area was then adapted to further enhance the 2x4 system. Once again this was nothing new, it was done before in '63 on the Chevrolet Z11 stock cars in the NASCAR circuit. The high-pressure area at the base of the windshield is utilized to enhance the ram effect by allowing the carburetors to draw fresh cool air which creates a denser fuel charge thereby increasing volumetric efficiency and horsepower. Names synonymous with the system are: COLD AIR INDUCTION, FRESH AIR INDUCTION and COWL INDUCTION.

The offset carburetor configuration makes it possible to use Holley carburetors with full metering bodies, allowing easy jet changes for fine tuning, and center pivot float bowls needed to prevent flooding during hard acceleration and cornering.

Although the manifold plenum chamber below the carburetors is large, its size was kept minimal by its designers to retain good low end response. The runner length was tuned specifically for use with the 302 c.i.d. engine to produce peak horsepower at 7,200 rpm using the optional "140" cam and tuned headers. The full racing version of the 302 Cross Ram engine has an actual power curve that is somewhat narrow and requires optimum use of gearing to maintain rpm in the power range (4200-7200 rpm).

Chevrolet initially specified carburetor #3941140, Holley list #3810S, which featured a 585 cfm rating, dual feed and sliding cam actuated secondaries. Response problems led them to change to carburetor #3942595, Holley list #4210A which featured a 600 cfm rating, dual feed, sliding cam actuated secondaries with a different opening rate. Once again, response problems led to the third and final change which cured the problem. Carburetor #3957859, Holley list #4295, rated at 600 cfm, dual feed, sliding cam actuated secondaries and dual accelerator pumps did the trick. Because of the relatively large plenum area below the carburetors, accelerator pumps on primaries and secondaries are necessary to adequately richen the mixture during acceleration to prevent an excessively lean fuel mixture condition which causes bogging, power loss and ultimately burnt valves. For optimum tuning the front carburetors primaries face forward and the rear carburetors primaries face rearward. There are no provisions for manifold heat or chokes on the carburetors, which is typical of a race-only induction system. Winters Foundry, who has done almost all of Chevrolet's aluminum casting, also produced the Cross Ram units. Development of the Cross Ram was a collective effort on the part of many talented individuals inside and outside of Chevrolet engineering including Smokey Yunick who performed test evaluations and some "hands-on" development.

After initial development was complete, the prototype castings were test evaluated on the engine dynamometer. Studies revealed "distribution fixes" were needed to prevent wet fuel travel. Chevrolet specified a few additional changes over the initial prototype units which included the relocation of the vacuum take off, from the bottom half of the manifold to the upper top plate, to eliminate the possibility of wet fuel entering the vacuum lines and to gain a more direct route for plumbing. Provisions for heater hoses were also added to facilitate the newer water pump design and hose routing scheduled for use on the 1969 models.

THE CONCEPT AND DEVELOPMENT

According to engineer Bill Howell, the concept for the small block Cross Ram evolved from an experimental tuned, long crossing runner, multi-carbureted manifold, that was developed in latter part of '66 for the big block Mark IV engine. Coincidentally, the lobe profile for the "140" off-road cam which was developed for use in conjunction with the Cross Ram, was also derived from the big block. Initial design work for the small block Cross Ram began approximately midyear '67 with a target date for completion in December in order to be homologated into the SCCA recognition forms in time for the start of the '68 Trans-Am.

Jerry Thompson, a key development engineer, took the concept above and developed a test manifold using a Corvette fuel injection bed plate. They epoxied in eight tubing runners to which they attached a plenum box fabricated from sheet metal and fitted a top plate that would accommodate two 600 cfm carburetors. Essentially, it was a vertical ram manifold. When it was set up on a developmental test engine the dynamometer results showed an approximate 25.5 horsepower increase over a base run using the same engine with the manifold/carburetor system used on the '67 Trans-Am engines. This first crude adaption set the basic specifications for the Cross Ram that was to follow.

The results of that test manifold were exactly what they were after. The problem, however, was how to keep runner length long for the ram effect, maintain the plenum box on which the carburetors sat, and somehow keep it all under the hood. The solution along with the initial design specifications were drawn up inside the drafting room by Design engineer Charley Turner. He was able to keep the manifold height at a minimum and runner length long by laying the runners down, crossing them horizontally side to side and then enclosing them by building the plenum box around them. Top and side wall design as well as carburetor and linkage details were sorted out by design engineer Chris Madson, followed by the laying up of cores and creation of plugs at Winters Foundry. After the actual castings were produced at the foundry, they were sent to Chevrolet Engineering and Smokey Yunick's garage for evaluation. They found out immediately that the manifolds needed further refinement specifically in the area of fuel distribution. Essentially, the problem was that the massive plenum area of the manifold centrifuged the fuel mixture causing it to separate into its two constituent components leaning out the mixture markedly and causing wet fuel distribution on the sides and bottom of the manifold. As a result of those findings several revisions were made, the most important were being the fuel distribution fixes.

Project Engineer Jerry Thompson explains...

"The first unit would hardly run the mixture was so bad. We ran air flow tests and found it really did pump the air. Using distribution fixes we tweaked it into a reasonable power band and tried a collection of carbs. The 600 cfms seemed adequate, larger carbs just ate up the torque so we stuck with the 600s.

At this point, Roger Penske and Mark Donohue stopped by Engineering for a demonstration; Mark went nuts (9/16" wrench in hand) to get the unit to use. We weren't quite done calibrating at the time so we held them off. As soon as we had the calibration complete, we had the foundry change the molds and sent the prototype off to Traco Engineering (builders of Penske's engines). The production people went on to dot the eyes and cross the tees as they do and the race cars got the pieces they needed."

Initially there were four or five "working prototype" manifolds that circulated back and forth between Smokey's shop and Howell's group. One of the earlier prototype castings

(10-27-67), was installed on the Chevrolet-built '68 Camaro Z/28 test vehicle that was used by Chevrolet Engineering for test evaluation of the HD equipment. This car also doubled as a press car and was featured in many magazine articles (see HOT ROD, May '68) displaying the HD options for the '68 Z/28.

While evaluation continued on the test vehicle, further developmental work was being carried out in engineering where the final configuration for the distribution fixes was made. Because not much work was done at that time with flow bench technology, the distribution fixes were accomplished basically by gas analyzation methods, 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 that were made based on educated guess work. According to Jerry, this particular area of development is quite involved. 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.

When all involved were satisfied with the flow characteristics and the developmental stage of the prototype test unit, it was blessed by Vince Piggins and personally hand-delivered to Traco's shop by Engineer Bill Howell. It was set up on the engine being prepared for Penske's '68 Trans-Am-clinching number 6 Camaro for the beginning of the rapidly approaching 1968 season. Immediately following, specifications with revisions were drawn up for the production version and manufacturing soon followed.

One of the most remarkable aspects concerning the development of this special equipment is that the actual factory prototype units were used on Penske's race cars long before they became production pieces, making it truly a product of race-bred technology.

A special note of interest concerning the early factory press release photographs depicting the carburetors with milled air horns: According to Jim Travers of Traco Engineering, he accompanied the Penske crew to the first race in which the Camaro used the Cross Ram manifold system. They began experiencing some problems related to the induction system. Using a pair of Dutchmans (tin snips), Jim cut off the air horns and filed them smooth relieving some of the problem. "Those first carburetors just wouldn't run right otherwise".

As it turns out, the problem wasn't so much the fault of the carburetor as it was with the first-design air cleaner. Since the element was only 1-1/2 inches tall, it brought the air cleaner lid so close to the carburetor air horn, it restricted and upset flow to the primaries. The height restriction forced air to flow horizontally and then make an acute turn down into the primary. This sharp turning of air caused turbulence and upset proper fuel regulation within the metering bodies. Cutting off the air horns smoothed out the flow and compensated for some of the restriction.

The air horn's function is to facilitate the choke mechanism which on these carburetors is absent; not even the holes for the choke butterfly shaft is drilled by Holley. Therefore, there is no need for the horn to remain and it is beneficial to remove it for the sake of better flow. The problem is not as critical with the second design air cleaner with the taller element, the additional height of the element allows a more adequate, less restricted flow; however, for competition this modification serves a distinct advantage. Beyond the air horn, the only other modification Traco Engineering performed on the Cross Ram units was to stagger jet the carburetors for a more even fuel mixture.

After the Traco-prepared engines were installed in the Penske cars they found that the production fuel lines developed for the Cross Ram unit began to present a problem during jet changes. The rigid steel lines and fuel distribution block all tie together on the inside of the manifold between the two carburetors, making access for maintenance/alterations difficult and time consuming. According to Bill Howell, they took the carburetor float bowls and switched them end-to-end which puts the fuel lines on the outside of the manifold, effectively getting them out of the way of the throttle linkage. Then by utilizing flexible Aeroquip fuel lines, they were afforded the convenience of quick and easy jet changes and float adjustments without disconnecting the fuel lines.

The oil splash shield seen on the bottom portion of the prototype Cross Ram unit was developed for testing. It was evaluated and dropped never making production due to it's inability to demonstrate effectiveness during testing. The small block does not have a problem of excessive amounts of oil thrown around in that area as is common with the big block. The bottom of the manifold, which are actually the intake runners, sits quite high and the small amount of oil splash that does reach there does not significantly heat the manifold.

Lou Faux, Chevrolet engineer in charge of cooling and lubrication development on the 302 engine explains...

"On the production vehicle the actual function of the manifold oil shield is to prevent oil from coming in contact with the extreme heat of the manifold heat cross over, the inclusion of which is necessary for good cold weather driveability. Any oil that would come in direct contact with that super-heated area would carburize, denaturing the oil and cause "clinkers" to form and drop down into the engine case. Since there is no manifold heat cross over used on the Cross Ram manifold, there is no need for a shield of that type".

COMPLETING THE PACKAGE...

2x4 AIR CLEANERS- FIRST DESIGN

The first design air cleaner for use with the Cross Ram unit was the "Plenum Air Box". It consisted of an element sandwiched between a top and base plate, enclosed by a special plenum ring. The plenum ring was joined to an opening in the firewall by a special plastic duct in order to draft fresh cool air through the cowl from the vents at the base of the windshield. Engineer Bill Howell states, "the first design air boxes were flimsy and mostly cosmetic, designed basically to complete the package". They were made of ABS plastic and much too thin at best, cracking was a common ailment but worse than that was the fact that the outer plenum rings were of poor fit and had a tendency to slide off from around the element and base plate due to vibration during racing. Most of those who kept the system used duct tape to seal up and hold the plenum rings in place.

During the early testing stages of the Cross Ram, engineers became suspicious that the plenum air box may have been causing or, at the very least have contributed to a bog condition they experienced with the first design carburetors. The element used with the plenum air box, Chevrolet PN #3942572 (Purolator AFP 22) was 1-1/2 inches high by 53 inches in circumference. This yields a total breathing area of 79.5 sq. in. As a reference, the standard four-barrel air cleaner for the 302 is 3 inches high by 44 inches in circumference and yields 132 sq. in.! Yes, the initial 2x4 element was restrictive in terms of being able to flow 1200 cfm at 7200 rpm. In contrast, the element that was used later (second design) in conjunction with the fiberglass hood, Chevrolet PN# 3963825 (Purolator AFP 66) was 2-1/2 inches high by 53 inches in circumference which yields a total of 132.5 sq. in., equal to the single four barrel and having an effective increase of over 66% in area/flow capacity compared to the first-design element for the plenum air box.

In order to over come the problem of restriction due to the limited flow capabilities of the smaller element, some racers like Joie Chitwood altered the production air box to accommodate the taller (AFP 66) filter by splitting the box down the middle, separating it into two equal halves, fitting the taller element between and then used aluminum sheet stock to fill the gap between. Any minor areas that were left open at the lid or near the air inlet where the boot fits to join the firewall duct were closed with the racer's best friend, duct tape. It looks a little sloppy, but races are lost if you wait till the factory gets better stuff out.

2x4 AIR CLEANERS - SECOND DESIGN

The second design "Wide Base" 2x4 air cleaner, more formally known as "Air cleaner to hood unit", was designed to be used in conjunction with the special ducted fiberglass hood. The criteria for design was that it would have to restrict air intake to that which was drawn through the hoods plenum chamber and utilize a production-type "micronic" element. To do so, the base of the air cleaner would have to enclose the element and effectively seal it to the bottom of the hood.

Taking into account the height and angles of the hood, engineers simply took the existing base plate used with the first design air cleaner and hand built it outward surrounding the element and creating a flange on which a foam seal was affixed. Simple paper modifications reflecting the changes were then made to the original drawings. Like the first design base plate, it too would be a steel stamping. After evaluation/approval the dies were made and production units produced. Because of the raised hood design, air cleaner height was not as limited as it was with the plenum air box and Chevrolet took full advantage of this by increasing the height of the element one full inch to gain additional air flow. The rise of the hood panel easily accommodated the additional height of the taller Purolator AFP 66 element which was then specified.

LIKE ICING ON THE CAKE...

FIBERGLASS HOOD

The special ducted fiberglass hood that was designed for use with the Camaro is probably one of the best examples of style and function combined. The hood was intended primarily for the Trans-Am competing Z/28s. It was designed to take advantage of the high pressure area at the base of the windshield in order to draft fresh cold air to the induction system. This method was chosen over forward facing scoops which create greater pressures and increased ram effect but have problems associated with air turbulence at the carburetor air horn. As for effect, an approximate 1% horsepower increase is gained for every 11 degree drop in temperature providing the fuel mixture is adjusted accordingly.

Larry Shinoda of Chevrolet's design group began development of the hood design approximately May '67- however, the concept never reached the production level until the '69 model year. The delay can be directly attributed to the SCCA. Chevrolet held back the special hood production until it could be homologated into the SCCA Trans-Am events. It wasn't until the '69 racing season the SCCA loosened up and allowed hood scoops, stipulating that they must be of "modest" proportion and in no way interfere with driver vision. The 1969 SCCA recognition forms filed by Chevrolet included the RPO ZL2 single four barrel "steel hood" in order to satisfy the minimum production number of 1000 units. Chevrolet then qualified the 2x4 fiberglass hood as optional heavy-duty equipment.

Initial design specifications for the fiberglass hood were drawn up for use with the small block using a single four- barrel carburetor which was the current limitation for SCCA competition during the '67 season when development first began. Later, when the rules changed and Chevrolet began Cross Ram development, the hood was revised to accommodate the new 2x4 intake system.

The configuration that reached production as optional HD Service Equipment for '69 was set up specifically for use with the Cross Ram two four-barrel system. However, in order not to limit the potential of a great concept, an adaptor plate was developed that would allow the special fiberglass hood to be used with both small and large block single four-barrel configurations. Beefy inner reinforcement structures are included on the bottom half of the hood panel, adding weight but stiffening up the unit considerably. The hood panel was designed to bolt on the extisting hinges and special lighter-tension springs were made available for use. A prop rod was also available for those who chose to forfeit the springs to take advantage of the weight savings. A hood latch mounting pad was included to accommodate the production hood latching mechanism. Holes for hood pins were pre-drilled allowing the option to discard the latch mechanism for weight savings, and special hood pins were made available to use in its place. The special hood system is worth up to 15 horsepower (approx.) subject to application and operating conditions. How many were produced? The only answer that all inside agree on is... "NOT MANY"!

Total weight of the production fiberglass hood was 35 pounds, compared to the metal counterpart released as RPO ZL2 for the '69 model year which weighed 50 pounds. Although it may not seem like a great difference, reduction of only 15 pounds on race vehicle is quite significant, but also take into account:

DEDUCTIBLE WEIGHT

HOOD (DIFF.) 15

HOOD SPRINGS 8

ZL2 STEELHOOD PLENUM VALVE 5

(UPPER AND LOWER)

LATCH MECH. 2 

TOTAL REDUCTION 30 POUNDS

NOTE: Total weight reduction figure takes into account additional weight of hood pins and prop rod necessary with the elimination of the hood springs and latch mechanism.

The additional 30 pounds static weight of the steel hood decreases the power to weight ratio requiring more power to do the same work. Now consider the weight in movement, it is not uncommon for a race chassis to reach 1G force during lateral acceleration. The additional 30 pounds becomes 30 additional pounds of "outward pull" during cornering and will have a profound effect on chassis balance and handling characteristics. Furthermore, not only the weight in pounds must be taken into account but also the location of the weight in relationship to vehicle roll center, which again has a separate and significant effect on chassis performance. The higher and greater the weight above the roll center, the greater the chassis's tendency will be to lean.

It should be clear that although the Cross Ram system is based on a single component, it's only one part of a meticulously well engineered power production package. A comprehensive arsenal that includes the Cross Ram intake, special carburetors, cold air induction system, special profile camshaft, large valve heads and a special tuned header exhaust system -- all developed in concert as a "tuned" system -- result in one of the most potent power production packages in Chevrolet's small block history!

Now, with all of those attributes and accomplishments, it's easy to see how the legendary Cross Ram manifold can so easily initiate that bizarre but enjoyable obsession we call XRAMANIA....!