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MULTIPLE  CARBURETION

 

 

HISTORY

Multiple carburetors are not a 1950’s hot rodder’s invention; multiple carburetors have been available in the USA since at least 1930, when Cadillac used a pair of Johnson updraft single barrel carburetors on their V-16 engines. Cadillac continued the use of multiple carburetor on these engines and also their V-12 starting in 1931, but in 1932 changed brands to the superior Detroit Lubricator carburetors, also updraft.

Auburn and Pierce-Arrow offered a pair of single barrel Stromberg downdraft carburetors, beginning in 1932.

Cadillac discontinued the V-12 engine after 1937, but continued the V-16; and beginning in 1938, the V-16 was fitted with 2 downdraft Carter 2 barrel carburetors.

Buick started experimenting with multiple carburetors in 1935; but their first production multiple carburetor offerings was 1941. Two 2 barrel carburetors of either Carter or Stromberg were offered. Buick called this setup “compound carburetion”. The incentive for this setup MAY have been gasoline rationing during WWII, as the carburetors were setup with progressive linkage. However, the fuel economy with the two 2 barrels proved to be worse running on the primary carb than the 1940 single carb engines, due to A/F ratio consistency among cylinders.

In the 1950’s the use of multiple carburetor setups expanded among USA car manufacturers:

Nash offered twin single barrel carbs in 1951, as did Hudson in 1952. Chevrolet offered triple single barrel carbs on the Corvette in 1953.

The use of multiple carbs on V-8’s proliferated beginning in 1955 with Chrysler and Cadillac

ADDING MULTIPLE CARBURETION TO AN ENGINE - STREET

There are three major reasons for adding multiple carburetion to a single carbureted engine: (A) performance, (B) looks, and (C) added value or universe of potential customers for a sale.

If performance is the goal, then the type of multiple carburetion selected is IMPORTANT! For best results, the number of plenums in the manifold should be an even divisor of the number of cylinders in the engine. As an example, a V-8 would do best with any of the following: (A) twin 2-barrels, (B) twin 4-barrels, or (C) quadruple 2-barrels BUT NOT (D) triple anything. Failure to adhere to this guideline WILL result in inconsistent A/F density to all cylinders, thus decreasing the efficiency of the engine. The efficiency decrease will become larger as RPM’s rise.

If looks are the goal, then probably the best setup would be determined by the desires of the owner. Just be aware that if the guideline suggested in the preceding paragraph is ignored, performance, whether it be power, drivability, fuel economy, or all three, may suffer. So can the guideline be ignored and have a working multiple carb unit? Of course! Hundreds of thousands of V-8’s have come from the factory with tripowers or had tripowers retrofitted. They just cannot work as well as either a single 4 barrel or twin 4 barrels. Why this is my opinion, I offer the following two facts to support my opinion: (1) Pontiac tried tripower on their Super Duty project engines, and immediately discontinued tripower from the Super Duty program, opting instead for single 4 barrel, single 3 barrel, or twin 4 barrel options; and (2) Pontiac paid Carter to sabotage the Carter AFB (secondary side) so the single 4 engine would not outperform the more expensive tripower engine.

If added value or a larger universe of potential customers for a sale is the goal, be aware that the owner may add one type of multiple carburetion, only to find the prospective customer wants a different form. I would suggest being cautious if this is the goal.

LINKAGE SETUPS - STREET

There are basically two styles of linkage: (A) solid, and (B) progressive.

Solid linkage has all carburetors operating simultaneously, while progressive linkage utilizes a “primary” carburetor which functions all of the time, and one or more “secondary” carburetors that do not function until certain criteria or met. This criteria may be: (A) a certain RPM, (B) a certain opening of the primary carburetor throttle, or (C) a certain value of engine vacuum.

And the intake manifold MAY REQUIRE solid linkage. To run progressive linkage, there MUST be equalization ports connecting the plenums sufficiently large to feed the cylinders not directly connected to the plenum of the primary carburetor(s).

This selection will cause as many arguments as the brand of carburetors used (later). FOR BEST STREET PERFORMANCE, VERY DIFFICULT TO BEAT SOLID LINKAGE!

Historically, the first (I think) progressive linkage setup was the 1941 Buick compound carburetion. Because of the lack of normal driving performance and low fuel economy, many (most?) of these compound carburetion systems were removed, and replaced with single carb setups. And in fact, Buick totally discontinued the compound carburetion after only two years.

Fast forward to the mid-1950’s and the horsepower wars, plus the emergence of local dragstrips throughout the country. The racing sanctioning bodies often would allow internal engine modifications (camshaft, compression, etc.) but REQUIRE the use of the original carburetor(s). Thus the car manufacturers would offer twin 4-barrel setups with progressive linkage so that they could use carburetors THAT WERE TOO LARGE FOR NORMAL DRIVING. During normal street use, the engine would run only on the primary carb, and engage the secondary carb only under “spirited” street driving; BUT the larger carbs were present to provide sufficient airflow for engine modifications of larger camshafts, increased compression, etc. Street drivability with progressive linkage will never be as crisp as using solid linkage.

BRAND OF CARBURETOR TO USE - STREET

This issue probably has disturbed more pixels on the internet than any other carburetor question. Before the internet, many gallons of ink were wasted, trying unsuccessfully to convince someone with a different view to change. USE THE BRAND THAT YOU, OR YOUR MECHANIC, IS MOST FAMILIAR WITH, OR WISHES TO BECOME FAMILIAR WITH!!! After all, it is YOUR engine!

Personally, I like genuine Carters, Rochesters, Strombergs, and Zeniths (in alphabetic order).

OTHER ARTICLES

Other articles the reader may find interesting:

Carb sizes

Twin single barrels for inline engine

Triple single barrels for inline engines

Tripower setups for V-8 engines

Carb CFM