..and another opinion - I'm not the author ...."In upgrading their induction systems, especially with in-line six cylinder engines, people often install both a 4bbl carburetor and headers. The problem they run into with this upgrade is that the intake manifold no longer has a source of heat. Gasoline requires heat to evaporate and this evaporation can greatly cool the airstream, often enough to cause condensation on intake manifolds. Heat is necessary to help keep the fuel mixed in with the air stream. At low loads, the high vacuum manifold vacuum keeps the fuel vaporized, which results in good fuel mixture distribution among the cylinders. However, when the load is high like at wide open throttle (WOT), the low manifold vacuum cannot vaporize the fuel and much of it puddles on the manifold floor causing the fuel mixture to become lean and the engine to loose power. The car manufacturers have always equipped their carbureted engines with intake manifold heat for this very purpose. Only all-out race, propane-fuelled, and multiport fuel-injected engines can really benefit from cold intake manifolds"