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CRG Research Report

Camaro Cross-Ram Intake Manifolds

© 2000-2011, Camaro Research Group

Primary Author -
Reviewed by the CRG
Last Edit: 09-Aug-2000
Previous Edit:
Original Release: 09-Aug-2000

Multiple-carburetion intake manifolds were a popular performance item amongst the major auto manufacturers right on up to the Camaro's introduction for the 1967 model year. Due, in part, to some negative publicity that General Motors received in the mid-60s regarding the crash safety results of its cars, GM restrained the image of "all-out" performance by no longer allowing its car divisions to produce cars with more than one carburetor (Corvette and Corvair were the only exceptions from '67 on). Thus, the Camaro started its life without the dazzle of a flashy multi-carb system.

Informed Camaro enthusiasts are aware, however, that a cross-ram intake manifold was made available by General Motors for the Camaro. This manifold owes its existence to the Camaro racing program. The Camaros in the '67 Trans-Am series had been running the standard single four-barrel on the 302-cubic-inch engine. Their main competitors were the Mustangs and Cougars that featured 289-cubic-inch engines and two four-barrel carburetors. One might assume that since the Ford products had more carburetors, that the Camaro was out-classed that first year, horsepower-wise. Not so. According to the April '67 issue of Sports Car Graphic, the horsepower ratings for the main competitors were as follows:

    Dodge Dart        365 HP     (273 cubic-inches)
    Mercury Cougar    390 HP     (289 cubic-inches)
    Ford Mustang      375 HP+    (289 cubic-inches)
    Chevy Camaro      403 HP     (302 cubic-inches)

The Camaro more than held its own, in large part because it had a cubic-inch edge on its competition, not to mention being equipped with a huge, 800-cfm, four-barrel carb. However, Product Promotion Manager Vince Piggins, Chevrolet Engineering, and Winters Foundry wisely didn't rest on their laurels, and proceeded to work on the development of a double-four-barrel (2x4) intake manifold, which would help to keep Camaro at the front of the pack in subsequent racing seasons. With the appearance of a new, larger powerplant from Ford (the infamous "tunnel-port" 302) in 1968, and the new AMC Javelins with their twin-four-barrel-equipped 290s, this was a wise decision.

Since GM had banned multiple carburetion from most of its production cars, a 2x4 manifold would have to be developed with the intent that it was to be sold as a service replacement part. This was a valid procedure as far as the SCCA racing sanctioning body was concerned, as long as the parts were technically available to everyone that was racing, and not just the factory racing teams. The intent was to keep the racing as close and interesting as possible and to allow the independent entries to be competitive with the factory teams.

It should be understood then that no Z-28 was ever factory-built with a cross-ram intake manifold. Not only was there no need (the racing teams were officially able to use the part without having it on the production car), but GM edict said "this shall not be done" - period. The proof that this was not a production item is in the assigned engine stampings for the 302. Flint Engine and the vehicle assembly plants would need a separate code to identify a motor assembly with any unique components. There are no obscure engine codes in the Chevrolet records to indicate that anything but a single-4-barrel-equipped 302 was ever assembled by the factory. It is possible that a dealer may have converted a car prior to delivery, but most of these manifolds were owner-installed.

GM Cross-Ram Manifold
(Click on any image to expand it)
Right Front Left Front Left Side (off car)
GM Cross-Ram installed GM Cross-Ram installed Courtesy of Allan Thomson

The cross-ram intake manifold was developed for the 302 engine using knowledge obtained during experimentation on the MK IV big-block. During the October-November 1967 time frame, the first prototype 302 cross-ram manifolds (with experimental part numbers) were cast. The identification of these early manifold parts can become confusing but, basically, the earliest bottom pieces to the manifolds had no readily visible part number and had a large Winters Foundry "snowflake" (foundry mark) on top of the #1 intake runner. The later bases typically had a part number on top of the #1 runner and a smaller version of the foundry mark. Also, the hole for the vacuum fitting was moved from the base in early units to the lid. A list of the cross-ram components, taken from an internal GM specification sheet, follows:

   Qty  P/N       Description
   ---  -------   ---------------------------
    1   3941124   Manifold assembly, Inlet
    1   3947032   Inlet manifold gasket unit
    2   3941140   Carburetor assembly
    8    120368   Nut, Carburetor
    2   3881847   Gasket, Carburetor
    1   3942593   Pipe assembly, Fuel pump
    1   3942595   Pipe assembly, Carburetor fuel
    1   3942594   Manifold, Carburetor fuel
    1   3942596   Pipe assy, Carb. fuel to L.H. side carb.
    1   3942597   Pipe assy, Carb. fuel to R.H. side carb.
    1   3941160   Rod assy, Front to rear carb.
    1   3928326   Rod assy, accelerator rea[r] pedal
    1   9413182   Retainer, Accelerator pedal rod
    1   3941168   Cable assy, Accelerator control
    1    393292   Retainer, Cable to accelerator pedal lvr. 
    1   3942592   Bracket assy. Accelerator control cable
    1   3921617   Clamp, Accelerator control cable
    1    120706   Bolt, Accelerator control cable clamp
    1   9419727   Screw, Accelerator control cable to dash
    2   3942584   Screw, special 10-32 x .92 socket head
    2   3942587   Spacer, special
    2   9416980   Nut, Carb rod to lever screw
    1   3946801   Bracket, Accelerator pull back spring
    1   3939748   Spring, accelerator pull back
    1   3701777   Gasket, water outlet
    1   3932344   Gasket, ignition distributor
    1   3941132   Gasket, Manifold top plate to base

This list of parts appears to be for the earliest cross-ram configuration, due to the use of the very early carburetor part number. These early Holley carburetors (585 cfm) may have only been used during initial development work. They were quickly replaced by two 600-cfm Holleys (LIST 4210, #3942595) which were dual-inlet units with a single accelerator pump, cam-actuated secondaries and no choke. By the summer of '68, these carbs were replaced again with a different pair of 600-cfm Holleys. These were LIST 4295, #3957859, and again featured dual inlets, cam-actuated secondaries with a secondary accelerator pump, and no choke provision.

The development of the cross-ram intake manifold had the desired results. Horsepower increased to as much as 465, with improvements in peak torque as well, up from 340 lb-ft to 365 lb-ft at 5600 rpm. Cowl-plenum-fed, cold-air induction systems, similar to that used for the 1x4 manifold, were developed and subsequently revised until the change in '69 to the familiar ZL2 hood scoop (a.k.a. the "cowl hood"). The increase in horsepower combined with help in many other areas, not to mention a certain degree of luck, helped Camaro gather the '68 and '69 Trans-Am championship.

Today, it is still possible to find an original example of one of these exotic manifolds. All it takes is a mere $3000 - $5000 and you're set! While they're a beautiful piece to behold, they generally are not street-worthy and typically wind up being more of a nuisance than a pleasure, especially if you're not going to drive the car hard. (See Author's Notes below.)

It's also noteworthy to mention that several aftermarket companies made similar cross-ram intake manifolds (Offenhauser and Edelbrock to name a couple), and while not exact duplicates, they are worthy of consideration due to their somewhat cheaper prices. The Offenhauser manifold is a fairly close copy, with the lower-half being almost identical to the GM part. The upper-half of the "Offy" manifold is similar to the GM part, but with distinct differences, including: 1) the ridge that follows the edges on the top piece is inset further than on the GM manifold, 2) the "Offy" manifold has provisions for a manual throttle linkage (the GM version doesn't), and 3) the Offenhauser name is name cast in two places. The Offenhauser base casting is quite similar in appearance to the GM part; the four photos below show the casting number and date code for each.

GM and Offenhauser Casting Details
Casting Number Date Code
Courtesy of Tom Bogan Courtesy of Tom Bogan
Courtesy of Mark Canning Courtesy of Mark Canning

The Edelbrock cross-ram manifold is a less-faithful copy than the Offy. The bottom-half has the thermostat outlet done in the more traditional, 4-barrel-manifold style, rather than being tilted forward 90-degrees, like the other two. The Edelbrock lid also has no ridge at all around the edge (unlike either GM or Offy), has provisions for a manual throttle linkage (like the Offy), and is cast with the Edelbrock name on it. Also unlike either the GM or Offy units, the Edelbrock has neither a casting number nor a date code.

Comparison of Cross-Ram Castings
Isometric Tops, Apart Bottoms, Apart
Courtesy of Allan Thomson Courtesy of Allan Thomson Courtesy of Allan Thomson
Courtesy of David Pozzi Courtesy of David Pozzi Courtesy of David Pozzi
Courtesy of Mark Canning Courtesy of Mark Canning Courtesy of Mark Canning

Also note that some non-GM "side-by-side" cross-ram intake manifolds were produced (such as Edelbrock's XC8), but these do not have a removable top half and are more easily distinguishable from the GM offering. There were also alternate tops for several of the manifolds, as typified by the single-four version of the Offy cross-ram, shown below. A 3x2 top (not shown) for the Offy was also produced.

Offenhauser Single-Four
Courtesy of David Pozzi

Author's Notes:


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