Holley Carburetors

In 1896, Bradford, Pennsylvania was like any other small town with one major exception - it was home to the Holley brothers, George and Earl. Together they started a business to produce a small single cylinder three-wheeler they named the 'Runabout'. Given that this was the dawn of the motor age, the 'Runabout' was no slouch producing a top speed of thirty miles per hour.

Following their early success, the brothers established the Holley Motor Company in 1903 and introduced the 'Motorette'. By the end of 1907, 600 Motorettes had rolled out of the factory in Bradford. Henry Ford was so inspired that he requested Holley Motor Company to develop and produce the carburetor for his infamous Model T. The carburetor, fondly known as the 'Iron Pot', rapidly became an automotive success and became standard equipment on many Ford vehicles as well as those manufactured by Ford's rival Oldsmobile.

In 1907 the Holley brothers relocated the factory to Detroit, Michigan and focussed their manufacturing efforts on ignition systems and carburetors for classic American motor companies including Ford and Buick. Reflecting this change of focus, the Holley Motor Company changed its name to The Holley Brothers Company.

The 1950's saw the introduction of the first true performance carburetor, the Holley Model 4150 4-barrel, on the 1957 Thunderbird. This marked the beginning of the modular Holley 4-barrel as it is known today. The 4-barrel became standard equipment on many high performance automobiles. Over the past 100 years, Holley has established itself as an industry leader in carburetors. Its carburetors have powered every NHRA Pro-Stock champion and NASCAR Sprint Cup team and for the past forty years.

The Carburetor and the Internal Combustion Engine
The internal combustion engine essentially converts a controlled explosion of fuel in the engine's cylinders into kinetic (motion) energy which propels a vehicle via the crankshaft, gearbox, drive shaft and axles.

For the controlled explosion to occur, three elements are required: fuel, oxygen and an ignition source. The carburetor controls the fuel and oxygen by drawing in air from the engine's inlet manifold and combining it with a precise amount of gasoline. The carburetor then vaporizes the resulting mixture before it is drawn into the awaiting cylinder and subsequently detonated by the ignition system. Carburetors work on Bernoulli's principle: The faster air moves, the lower its static pressure, and the higher its dynamic pressure.

A carburetor is basically an open pipe, commonly known as a 'barrel' through which the air passes into the inlet manifold of the engine. The pipe is shaped as a venturi, i.e. it narrows in section and then widens again, which causes the airflow to increase in speed in the narrowest part. Below the venturi is the throttle valve which is a simple butterfly valve comprising a rotating disc that can be turned from fully open (full throttle) to almost fully closed (idle). The throttle linkage (accelerator) activates the valve and other carburetor mechanisms to control the flow of air being pulled into the engine. The speed of airflow, and thus its pressure, determines the amount of fuel drawn into the airstream.

How to Choose Your Carburetor

It is tempting to buy the biggest carburetor you can find but, as with all other engine modifications, you need to choose the carburetor size that matches your engine and vehicle's performance level and your driving style.

To determine the correct carburetor size for your engine, you must know its volumetric efficiency (VE) which is an indicator of how well your engine can breathe. The better the engine’s 'breathing ability', the higher its volumetric efficiency.

VE is expressed as a percentage indicating the ratio of the actual mass (weight) of air taken into the engine at your expected operating RPM compared to the theoretical mass which the engine would take in if there were no losses. If your VE is not stated in your owner's manual then you may need to talk to your dealer or have your vehicle tested at a garage or local auto club. At maximum torque an average street car with a low-performance engine will have a VE of approximately 80% at maximum torque, a sports car with a high-performance engine will have a VE around 85% and the VE for a car with an all-out racing engine can reach as high as 95%.

Carburetor size is generally expressed in terms of 'cubic feet per minute' (CFM ) which is a measure of the volume of air/fuel it can provide. CFM is also the measure of your vehicle's consumption.

CFM is calculated by the formula ( (CID x RPM) / 3456 ) x VE

  • CID is the cubic inch displacement of your motor
  • RPM is your expected maximum RPM.

Engines with none to mild modification and up to 327 CID may run well with 525 to 725 CFM carburetors. Engines with moderate engine modifications and 350 CID or more may run optimally with a vacuum-operated four-barrel carburetor of up to 750 CFM. Some larger, highly modified, engines may run best with a 850 CFM carburetor. Extremely modified engines exceeding 454 CID may require larger carburetors in excess of 850 CFM.

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