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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 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 4 bbl. 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 smaller elements limited flow capabilities, 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 fire wall duct were closed with the racers 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 automotives best examples of style and function combined. The hood was intended primarily for the Trans-Am competing Z/28'S. 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 Chevrolets 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 hoods 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 it's 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 STEEL HOOD PLENUM VALVE 5

LATCH MECH (UPPER AND LOWER) 2 

TOTAL 30 POUND REDUCTION

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.

Well folks, that completes the legendary Cross Ram package...

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- resulting 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....!