The heat-up with radiator water is almost as fast as the heat-up from exhaust gases. The water is taken from the warm side of the thermostat. After the engine is fully warmed up, the maximum temperature of the heated water will be less than 190 degrees.

The heated water (or air) is needed to keep the intake manifold warm on systems where the intake is not attached to a stock exhaust manifold. When the gas vaporizes there is a significant drop in temperature that would chill the intake manifold and cause the vaporized gas to condense. This is especially important on multiple carb manifolds that do not bolt to the stock exhaust manifold (that is why Offenhauser intakes have internal water journals that allow water to folow from one end of the manifold to the other).

Since there is heat transfer after warm-up on manifolds that are bolted together, there is always enough heat to keep the gas vaporized (even after the stock exhaust "butterfly" closes).

Tim