Originally Posted by mcollinstn

Vacuum reading @ WOT will give you the short answer. If you are pulling vacuum, then a larger carb can free up some HP. It's all a juggling act. If we only ran at WOT, we would have short runner intake manifolds leading to a single venturi throttle body with annular discharge metering. A honking huge single barrel.

The need for accurate fuel metering for startup, for idling, for low speed operation, for acceleration, for cruise, and for WOT running is why we end up with several venturis, powervalves, adjustable fuel pump cams, and all the other gizmos we have.

A huge carb is more complicated to jet properly than a smaller carb. There is reversion at low speeds, there is turbulent airflow through the venturi and the column of moving air is large enough that it gets "dead spots" in it.

The smaller your carb, the more "leeway" you have on your jetting. It is more tolerant of environmental changes (humidity, temp, baro pressure).

The result is that the OPTIMAL carb will usually be larger than stock, and require more hassle getting it jetted perfectly.