I'm looking for input/experience with dual exhaust.
Buckle up- - - - -the ride could get bumpy. Here's a bit over 50 years' worth of input/experience. Take it or leave if- - - - -but it seems that you're looking for affirmation of some preconceived ideas, not actual facts and figures.
Water heat:
The maximum temperature a cooling system can put into an intake manifold is around 200 degrees F. It takes several minutes of engine run time for that temperature to be achieved. The primary purpose of manifold heat is to keep the fuel in the intake atomized DURING WARMUP when water heat will be at a minimum.
Exhaust heat:
The original heat riser system applies exhaust gas to the bottom of the intake manifold, either directly when the heat riser flap is in the "hot" position, or indirectly from conduction when the gas flow is directed away from the manifold by the flap valve. Exhaust gas can develop temperatures of up to TWO THOUSAND degrees under certain circumstances. The usual way to apply exhaust heat to an intake from Fenton headers is to run stainless steel tubes through a block of aluminum bolted to the intake. This is a terribly inefficient way to heat the intake, but it's marginally better than water heat.
If you choose to run dual exhausts, it's much more effective to split a stock manifold and leave the heat riser system intact. Pipe diameters of over 1 3/4 inches are counterproductive because the exhaust gas velocity from bigger pipes slows down so much that the desired scavenging effect of the cylinder pulses is lost and the overall result is that as much or more back pressure than a stock exhaust system is developed by the exhaust impulses stacking up in the pipes.
The above comments are the result of decades of hands-on dyno testing, not something from an internet search engine. Feel free to refuse to accept them- - - -I don't care.
Jerry
