I ran a McGurk dual intake, similar to Road Runner's, for many years (1956-60) on a 261 with a Corvette six exhaust manifold and an operable heat riser. It warmed up and ran as smoothly as the stock one-barrel. I later replaced the McGurk manifold with a Harper "Stubby" with three 97s, which was still fairly driveable. Of course, with the Harper there is no flat "floor" for the fuel droplets to accumulate, as the droplets will run straight and down into the head where they should quickly evaporate, once the engine is warmed up.
The late model "Fenton" headers do not seem to have the real heat riser, just bypass ducts. How well do those work?
In the Tech Tips to the left Tom Langdon explains some of the physics involved with the need for manifold heat. The purpose of the heat is not so much to heat the air, but to warm up the floor of the manifold so that any fuel droplets that hit the floor will quickly evaporate and not accumulate, especially when there is a sudden drop in manifold pressure. The late model "Fenton" headers do not seem to have the real heat riser, just bypass tubes. How well do those work?
If the heat riser valve sticks in the position to force continous circulation of hot gas through the intake manifold, the air can be heated too much, reducing power at wide-open throttle and lowering the knock resistance, as I believe Panic was indicating.
