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Formula One's fundamental ethos is about success coming to those with the most ingenious engineering and best ..............................organization, not to those with the biggest budget. (Dave Richards)
Like most things engine related producing high power is about improving efficiency... so a carb with its best efficiency at an engine's full power will be oversized for best efficiency at a lower (say "cruising") power and therefore smaller would be better for improving cruise fuel economy... smaller is also better because it keeps air velociy high hence improving cylinder filling... obviously at higher revs it will tend to strangle the flow though so you'll decrease peak power potential. (I say "potential" because the peak power depends on loads of other things like the engine's cams, exhaust, piston design, etc...).
A carburetor is a device that depends on pressure differential to function correctly. Inside a carburetor are tiny jets and passages for both air and fuel. To function as designed, the mass air flow through the inlet throat has to be between specific levels of velocity and density. Because of controlled air flow, there are specific sections where air pressure is lower than ambient. Obviously, to achieve optimum fuel-air charge, these pressures, passages, and jets all have to be correctly sized and work correctly.
For a carburetor to work as designed and deliver the correct air-fuel mixture, it has to be sized to the airflow requirements of the engine. Too small and the engine never can receive anough air to breathe. Additionally, the carburetor is not sized to deliver enough fuel to the excessive demands by the engine. Too big a carburetor and the air flow inside the carburetor throat is not high enough to generate the appropriate pressure differential in the regions required. Additionally, the carbureor is designed to handle more fuel and air as required. For instance, if you step on the gas with an oversized carburetor, the accelerator pump will inject much more fuel than required.
If you seek high gas mileage, NEVER get a carburetor sized larger than the engine requirements.
Racing should be decided on the track, not the court room.
Many carburettors for road cars have had multiple throats, where one or two small (primary) throats are used for low speed running. Their size makes them perfect for when you only need a little power, as when cruising along the highway. When you floor the accellerator one or two secondary throat(s) the same size or bigger than the primary opens up, letting the engine get the air it needs to produce high power.
Flummo wrote:Many carburettors for road cars have had multiple throats, where one or two small (primary) throats are used for low speed running. Their size makes them perfect for when you only need a little power, as when cruising along the highway. When you floor the accellerator one or two secondary throat(s) the same size or bigger than the primary opens up, letting the engine get the air it needs to produce high power.
That is true because the engine requires so much air at full power, if the carburetor had just one throat, it would be huge. If it was huge, at idle, the airflow would generate a very low pressure drop, practically useless. Any carburetor throat requires a reasonable amount of air flow to generate the required pressure drop, in order to suck in the fuel. That's why performance carburetors have primary and secondary throats.
Racing should be decided on the track, not the court room.
