PROPER CARBURATION

The guidelines for good carburetor selection are not well understood in the aftermarket, often resulting in poor drivability especially at lower (under 40 M.P.H.) speeds. To operate well at low speeds, the physical limitations of the carburetor require it to be small for good fuel delivery at part throttle. This is in conflict with the large air flow demands of the engine in an effort to produce maximum power at higher speeds and under full load at full throttle. These are fundamental facts that are well respected among engineers who did this as a profession for the industry.

As applied to our vintage inline sixes, these basic principles help to understand that original (one barrel) size selection was to make the carburetor as large as possible while preserving good drivability and fuel economy. The compromise was (in those days) inevitably a loss in maximum power. The solution to this compromise would have been a progressive system that operates a small carburetor at low speed and then opens an additional carburetor at heavy load. Oldsmobile did this in 1949 when it introduced the four barrel carburetor for its premium V8 engine. This is the classic and most common progressive system. Unfortunately for the sixes, this was not commonly done until 1966 when Pontiac used a Quadrajet four barrel on its Over Head Cam 230 cubic inch 6 Cylinder engine.

There are other systems that employ this basic idea:

  • Buick straight eight “Compound Carburetion”
  • Three two barrel carburetors (with progressive linkage)
  • Two four barrel carburetors (as used on early Corvettes) with progressive linkage.
  • Ford Pinto four cylinder with “Holley Weber” progressive two barrel (Weber 32/36)
  • Ford Escort four cylinder with Weber 32DFT progressive two barrel

Back in the 1950’s when our sixes were most popular, the aftermarket intake manifold manufacturers used the carburetors that were most expedient. Most applications used two original equipment carburetors which resulted in 100% “over carburetion” and the resulting poor drivability.  By using two original carburetors, the total venturi area was so large that at part throttle low speed there was insufficient signal to draw fuel after the idle circuit stopped working.  Add to that the common practice of NOT adding heat to the inlet manifold and you virtually guarantee major drivability issues.

Fast forward to today, and the typical Rochester One barrel carburetor has been rebuilt several times and violated by virtually every rebuilder. This common combination of issues guarantees an unsatisfactory product.

For those six cylinder enthusiasts who want the vintage look of a dual carburetor system, we would recommend a system of two, two barrel, progressive Weber carburetors, as used on early Ford Pinto, Chevrolet Vega and Ford Escort vehicles. We offer our new “Carter Weber’s” for this application and also triple applications where space permits.  These carburetors use a very small primary venturi for good driveability and supplement it with a larger secondary to employ sufficient air flow for maximum power at higher speeds and under full load at full throttle.

Using one small carburetor is just fine if you do not mind giving up 20 or 30 horsepower. Using multiple large carburetors on straight linkage usually works fine at full throttle for racing but suffers for day to day normal street drivability. If you want both, you must use a progressive (or “staged”) system.

Tom Langdon

31 Jul 2012

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