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Under the "Tech Tips" link in the menu to the left is an article by Tom Langdon about manifold heat. Exhaust heat is best, but hot coolant is better than nothing. Most automotive engineering books (e.g., Smith and Morrison's Scientific Design of Exhaust and Intake Systems) also recommend the use of heat. The real purpose of the heat is to vaporize any liquid fuel droplets that get deposited on the floor of the intake manifold, not to heat the air, as many seem to believe. If hot coolant is used, one could install a heater shut-off valve in the supply hose to stop the heat addition if it is felt that heat is not needed. This would be an interesting experiment.
