318 exhaust manifold has a buterfly flap??
You find these on the passenger side manifold- they are a diverter of exhaust heat into the intake manifold crossover passage (original equipment iron single plane 2 barrel)
So far so good…the crossover passage is present on all original-equipment and most aftermarket intakes, though, not just on the iron 2bbl items.
to help warm up and atomize the fuel/air mix "better" during cold startup. Think of it as a Jurassic attempt at improving the emissions of your engine for meeting EPA standards of way back.
No, that's not a really accurate description of why the system's there. The intake manifold heat control valve ("heat riser") existed
long before there was any such thing as the EPA. Its purpose is twofold on the Mopar V8s: to provide heat for the automatic choke thermostat to function, and to heat the intake manifold floor so as to vaporize liquid fuel so it can be burned.
This 1935 Jam Handy cartoon, "Down the Gasoline Trail", would be a real trip to watch even if it
didn't contain a sequence (which it does, at 5:37) specifically and correctly answering the question about the manifold heat. Was there any such a thing as an emissions regulation in 1935? No, of course there was not. And yet...engines had heat risers!
[ame="http://www.youtube.com/watch?v=S_bFo2mg9Ss"]YouTube - Down the Gasoline Trail (1935)[/ame]
Most of us take it out and weld closed the ends
Speak for yourself, please. I don't agree that "most" people do that. Some people do, usually out of misunderstanding of why it's there and how it works, and/or out of the mistaken belief that it's an emission control device (which of course must be bad and should be removed just because it's an emission control device) .
That is an unrealistically simplistic statement. It contains a kernel of truth, but there are limits: below a certain temperature, gasoline is a liquid. Liquid fuel does not burn. Only vapor burns. The latent heat of vaporization of gasoline works to cool down the intake manifold even as engine heat works to warm it up. The debate is whether it's necessary to apply heat to the intake manifold. There are opinions "yes" and "no", and there are varying levels of understanding of how the system works and why it's there, and the right answer also depends on the engine configuration and vehicle purpose. I don't mind saying I have a very complete and accurate understanding of how and why the system is there, and my opinion is that on a street-driven vehicle in any climate with a non-race engine, the heat riser should be in place and working correctly for best driveability, performance, and fuel economy. On a street-driven vehicle, a properly-working manifold heat control valve only makes things better, not worse.
The valve is pretty simple in its function: when its thermostatic coil spring is cold, the valve is rotated to the "heat on" position so that some of the exhaust is circulated past the underside of the intake manifold. This vaporizes liquid fuel that would otherwise condense on the inside of the intake manifold. This need continues long after it would seem from outside that the engine is warmed up, because the fuel flowing through the carburetor and manifold exerts a cooling effect. Once things are well and truly heated up hot, the thermostatic coil spring relaxes and allows exhaust flow to push the valve into the "heat off" position, which deflects the exhaust away from the underside of the intake to keep from overheating it and causing hot driveability problems. The heat riser used through 1969 has a rectangular counterweight rather than the round disk you see on '70-up manifolds. The rectangular counterweight on the earlier system is eccentric; when the valve swings over to "heat off", the weight goes overcentre and keeps the valve in that position until the thermostatic spring cools down enough to swing it back to "heat on". The later round-disk type valve just uses exhaust flow to keep the valve in "heat off" until it's cold enough to rotate back to "heat on". The later round-disk setup also has better bushings that seem less prone to seizing up.
The valve is not adjustable, but it requires periodic inspection and maintenance -- with the engine cold, grab the disc and give it a counterclockwise twist. It should rotate easily and spring back clockwise smartly. To keep it working smoothly (or to free it up if it's sticky), get a can of part number 4318 039AC from your Chrysler dealer -- it is not Liquid Wrench or anything of the sort -- shake it well and apply some to each end of the shaft, then rotate the shaft back and forth.