https://www.motortre...-rod-june-1970/
Carburetors operate on the same theories today as they did when it was discovered that the internal combustion engine needed this method of inducing fuel to its innards. It takes more air than gasoline to pack a combustion chamber with compressible fuel and subsequently realize an expanding and burning reaction. Fuel injection is the alternate system to carburetion. Most American-made injectors operate strictly for racing applications. Properly designed and applied fuel injection systems are better than most carburetion systems, but the cost of converting to this method is the biggest reason domestic manufacturers still rely on carburetors. Single and multiple systems are far more advanced than one would have thought possible 10 years ago. They are also costly, in some cases, and the various designs from each manufacturer range from the very elemental to the very exotic. Forgive us if we limit this discussion to only the exotic. Not only are they the ones most useful in upgrading engines, but these are the ones filled with innovations which will lead present-day units into further advances for both general passenger car use and racing engine use.
A carburetor should induce 15 times as much air as fuel for best operation. Liquid fuel is pumped out of the carb float tank through a metered tube and mixed with air as it passes from the metered passage. When it enters the venturi chamber, vacuum resulting from the engine cycling action draws the fuel/air mixture into the manifold and intake port. Venturi chambers increase the velocity through the center (or smaller) diameter, and when the mixture passes this point and reaches the lower, larger bore, pressure increases. Air pressure is greater at each end of the venturi than in the center, and the small center chamber serves as a means of building up mixture speed. It is a velocity tube. Engines operated through a wide range of speed must use the venturi principle, or low speed and idling will be extremely rough. Once an engine is moving at a high rpm, internal vacuum draw will create a fuel mix draw, thereby lessening the need for the venturi principle. On multiple-bore carburetors, when the primary side can no longer supply enough fuel/air mixture to keep up with the engine's needs, the secondary side of the carburetor is put to work. By either mechanical or vacuum linkage, the secondary throttle valves are opened and fuel is induced to these chambers in an almost identical manner as is done on the primary side. Because engine vacuum is greater at this point, there is less need for the venturi principle. Some four-barrel carbs have barely noticeable secondary restriction, and most have easily detectable differences between primary and secondary chambers.
Production-line-built cars carry carburetors that are generally designed with built-in compromises. The four-barrel on a 350-inch engine will be the same as one on an over-400- incher, yet it performs well on both engines. The big engine is nearer using the limit of its carburetion, although still not suffering. The small engine may be over-carbureted, but through the primary and secondary progressive staging principle, proper fuel flow is regulated simply by the engine's needs. Stock parameters include low-speed operation and emission control limits. Small primary bores have been put to use so that an engine will operate on a minimum amount of gasoline at low speed, and large secondary openings for high speeds keep it from starving.
Performance engines, meaning those used essentially for racing, need a different system. Large primary bores are a criterion in designing a carb for this type of engine, and secondary throttle action, linked on a progressive hookup, adds required fuel flow after initial acceleration has been accomplished. Racing carbs don't need chokes, work best on mechanical hookups rather than vacuum, and don't generally need to be fitted with attachments for emission control devices. But there is a middle ground—the carburetor for street and track operation—and this is where the greatest amount of activity is seen right now. Engines well-riddled with horsepower from the showroom need but a little bit of tuning before they're pushing or exceeding the limit of stock induction.
Ah for the good old days; Nowadays top line carbs are extremely adjustble and jet changing is much easier.
Edited by Bob Riebe, 29 February 2024 - 18:13.
