It is there so you can drive your Corvette in the winter!  When your engine is cold, the valve closes, forcing exhaust through the head, across the bottom of the intake manifold, and under the carb to warm it up.  They make a heat riser eliminator that is just the housing with no holes or valve in it.  There is a gaskey between the manifold and riser, and a donut on the end of the pipe.  Replace them both to be sure.

Larry

Yeah...what Larry said...
You can replace your riser with a new one if it is burned beyond repair, or frozen shut/open, or you can simply install the "eliminator" type. You can't just remove the valve, and bolt the exhaust back up to the manifold, you have to have the heat riser, or the spacer/eliminator.
If your valve is free, as in it moves open/close easily, then you prolly only need the gasket and/or the dognut...
Adams' Apple2008-10-27 10:39:36

Thanks, guys.....now the fun starts....gettin' the bolts loose without breakin' 'em off!

Soak the snot out of them with P.B.Blaster for a couple of days,also soak the bolts that hold on the exhaust manifold.ya prolly aint gonna get all 3 bolts off at the pipe wid out breaking at least 1 off.this way you'll be 1 step ahead when ya have to take the exhaust manifold and take it to a muffler shop an get all 3 bolts replaced,don't forget when you put the stuff back together ,get yourself some of the longer BRASS nuts,made for exhaust mounting just tell the guy ,he'll know what you want.I'm not tryin to jinx you on this job,but ya may as well plan for the worst,ya might luck out ,but I wouldn't bank on it Ron 782008-10-27 12:41:43

First, the heat riser does what Larry says.  The reason is faster warm up of the intake to create better fuel atomization and easier operation during warm up.

If you don't have access to a wrench like kstyer fire wrench, you soak as suggested above, and start the motor and heat the exhaust up (go around the block) keep your hands away...really hot, and with a socket and extension, loosen the nuts.

Wally,