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When replacing the stock intake and exhaust manifolds, the heat riser is eliminated which can cause the engine to run poorly. The problem is explained by Tom Langdon as written in a Tech Tip on the Inliners.org website (click on the Learning Resources button to find a link to the Inliners site.) As liquid fuel enters the manifold from the carburetor, the vacuum vaporizes the fuel and causes a chilling effect on the walls of the manifold much like the chilling effect of spraying an aerosol on your skin. Now you have a cold manifold. If you do not supply a continuous supply of heat the manifold will remain cold and even build frost in some conditions. At this point, if acceleration is attempted, the vacuum will drop, fuel will no longer vaporize, and will condense on the cold manifold walls until they are fully saturated with wet fuel – this takes about three seconds, during which time no fuel is going into the engine ( and thus no power or a "sag") After the walls are fully saturated with fuel the air flow finally picks up and floods some of the cylinders but not all of them because liquid fuel is notoriously bad for equal distribution. The solution: Moderate, and continuous heat supply to keep the walls of the inlet manifold warm and fuel in vapor form. Exhaust heat is fast but requires a butterfly valve in one manifold to force the exhaust flow. Water heat is slower but very clean and not corrosive to aluminum manifolds. This method utilizes the water pump to continously supply warm water to a passage underneath the manifold.
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