Anatomy of a Tri-Power CarbWhat makes this performance trio sing
If four-barrel carbs were good, then six barrels should be better! That was the thinking of automotive performance gurus in the late Fifties and Sixties. Manufacturers such as Pontiac, Chrysler, Chevrolet and Ford put the tri-power option (three two-barrel carbs) on their musclecar order sheets. While each manufacturer had subtle differences in the way the three carbs were activated, in general the process was relatively the same. Here we are using a Pontiac Tri-Power from 1965 and we'll show you the components that make it work. In this case, the only completely functional carburetor is the center unit. The carbs at each end are simply fuel-atomizing funnels to dump gas into the intake on demand. The throttle linkage on the center carb activates the other two. It's important to adjust the linkage so that all three carbs fully open when the pedal is on the metal. Other manufacturers used three completely functioning carbs with vacuum-operated linkage. Regardless of the brand, the end result was usually the same: a loud whoosh, the scream of tires and a big smile on your face.
We're starting with the bare cast-iron intake that has been bead-blasted and painted mid-Sixties Pontiac blue. Composition gaskets are used along with four mounting studs for each carburetor. Pontiac cast the intake (#9778818) and Rochester manufactured the carbs. This combination was optionally available from 1957 through 1966 with various changes incorporated each year. By 1967, four-barrel technology had advanced to the point that a single carburetor could generate more CFM than the three carbs-and they were easier to produce and maintain. This setup provides 780cfm at wide-open throttle. Chrysler continued to offer the tri-power option into the early Seventies and called it the "Six-Pack."
Each manufacturer used different variations on the carburetor theme. Here we see the trio comfortably resting on the intake. Notice the center carb is the only one with a choke housing. The carbs were finished in a light coating of zinc chromate for protection, but over the years most of the finish has worn off. There is a ported vacuum fitting at the rear of the center carb base that supplies vacuum for the distributor, transmission vacuum modulator and interior vacuum gauge if so equipped. Finding a usable set of original carbs is getting increasingly difficult. If you're interested in installing a tri-power setup, try to find and buy a complete unit with the correct carbs. Most of the linkage and other details are available in reproduction.
A large brass T-fitting is attached to the fuel inlet of the first carb. The main fuel-supply line (not shown) enters from the bottom of the fitting. Fuel is then distributed to the center and rear carbs through the two lines show here. The earlier tri-power units used approximately the same fuel-delivery lines, but in 1966 the fuel filter was relocated and the shape of the lines changed. These are aluminum reproductions; the original factory lines were mild steel tubing. Notice the silver tags on the front corners of each carb. These are Rochester part-number tags to identify the individual carbs. The center carb used on the Chevrolet tri-power looks similar and many people have been fooled at swap meets only to come home and find the fuel inlet and throttle linkage are vastly different from the Pontiac carburetor.
The throttle linkage is connected to the butterfly shaft in the rear carb base by an adjustable rod, as we'll see later. Here, below the fuel lines, we see a v-shaped steel rod connecting the front and rear carbs. When the connecting arm attached to the shaft of the rear carb is pulled back, it also pulls back the shaft in the front carb, opening the venturii of both simultaneously. Ingenious in its simplicity, and very effective. There is a small spring-loaded clip at each end securely attaching it to the actuation lever. The rod itself can also be bent in the middle to aid in adjustment.
The last items to add to this side of the tri-power are the choke tubes. There is a passage under the intake that allows exhaust gases to pass from side to side, and this super-hot air passing through the tubes is used to operate the choke and deliver warm air to the carb. The bottom of the tubes slip into two pressure-fit holes in the intake runner. Inside the choke housing is a spring that reacts to the heat, opening and closing the choke plate in the carb air horn. As the spring heats, the tension relaxes and the choke is pulled off. Hey, this was performance technology in 1965!
Here's the business end of the tri-power. The throttle linkage assembly consists of a formed bracket that bolts to the butterfly shaft of the center carburetor. Attached to this bracket is the throttle cable from the firewall (not shown), the actuation rod going to the rear carb and the yellow throttle-return spring. As the cable pulls back the top of the bracket under acceleration, the rod is pushed rearward, activating that v-shaped rod on the other side and all three carbs start to open at the same rate. Stomping the accelerator pedal immediately opens all three at once, and then the fun begins. Most of these linkage pieces are available from the restoration aftermarket, as are the fitting and air cleaners.
From 1957 through '65 the Pontiac center carb had a smaller airhorn than the two outside units. Since they are basically funnels, the larger intake increases the overall cfm. For the last year of production, the '66 tri-power units had all three carbs open to the larger airhorns like the front and rear. This added a little more cfm and standardized parts such as the air cleaner bases and main body castings. If you're looking for a way to instantly differentiate between '65 and '66 center carbs, this is the quickest way of identification.
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