i have a stock exhaust manifolds with a tubi muffler. The previous owner removed the exhaust valves. While the sound is pretty cool, the noise level gets very tiring going down the freeway. For those that have the valves, is it closed during cruising down the freeway? If not, is it harmful to get a manual control and close the exhaust valve at freeway speeds? I see exhaust valves on eBay, would those work?