ORIGINAL: acdii

Oy, my head hurts trying to read this. <div>
</div><div>Venturi effect! The low speed ports do not supply any fuel at WOT because the main part of the draw is at the throat of the carb where the venturi is most restrictive. Once the airflow gets past that point the vacuum drops off dramatically. Only when the butterfly is closed or near closed does the vacuum point change. Since you have now moved the smallest portion of the venturi to the hole in the butterfly, you now have the strongest vacuum source, so it will now pull off the low end ports. </div><div>
</div><div>If you look at that diagram, notice the big fat blue arrow, thats your main source of vacumm, where the pull is strongest, past that point you get closer to atmospheric pressure. </div><div>
</div><div>Forced Induction creates a vaccum by blowing air past the venturi to pull the fuel. You can simulate this by blowing air past a straw that is in a glass of water. You never force air at idle, at that point the Supercharger or Turbos are idle. At high altitudes where the air is thin you need the boost to give you the equivalent power at sea level.</div>