The guys who invented the heat riser valve probably knew exactly what they were doing when they put the thermostatic spring on it. Manifold heat is neither necessary nor desirable after warmup. It leads to detonation and engine damage, and loss of power due to an overheated intake mixture. Whether you heat the manifold with water or exhaust gas after the first 5 minutes of engine operation, you're on a fool's mission. The most effective method of manifold heating is with a vacuum motor operating an exhaust gas heat riser valve and a coolant temperature sensing vacuum control like the late 1980's GM vehicles used. The bimetal spring on the original-equipment stovebolt heat riser is the next best option.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!