I saw this under the hood of my car. Apparently it's supposed to be activated by vacuum from the induction and close the valve. But what good (or bad) does that actually do short of slowing the car down?

I found this picture on year-one's site on exhaust for 67-81 firebirds, but it looks like the very same kind of part.

Yeah, it's on the end of the "log-style" cast iron exhaust manifold, only on the right one strangely enough...

 

It closes when vacuum is applied. There's a thermal vacuum switch on top of the intake manifold, in a coolant passage, that sends vacuum to it when the coolant is below normal operating temperature.

When the valve closes, it closes off teh head pipe on the right side, and forces that side's exhaust to go through a passage in the head, through the intake under the carb, through an identical passage in the left side head, and then out the exhaust. It warms up the intake under the carb to help the fuel to stay a vapor even when it's cold.

It has to be there, because of the flanges on the parts. You can't just leave it out. You can remove the blade from the shaft, and the shaft from the housing, and plug the shaft holes with pipe plugs; and you can port the body out.

 

Well, mine didn't have a hose on the vacuum port.. nor could I see anywhere for the axhaust to escape to.. Oh well.. can I just throw it away when I'm mounting headers?

 

The hose should come from a thermal vacuum switch mounted on the intake manifold, at the front, next to the water outlet; or on the water outlet itself.

With some kinds of headers, maybe you can do away with it; but with any headers that are designed to fit directly to the stock exhaust, such as Edelbrock, you'll need to keep it.

Like I said, you can port it with a grinder, to where it isn't a restriction with the stock exhaust.