Mine spent about 45 minutes at 70 mph on the highway after i rebuilt my trans with only a pair of manifolds on it. It was a incredibly loud ride, but no, it won't damage your engine. If this was true, you would'n see race cars that just run headers. Motorcycles and old airplanes run very short, open exhaust. If you are worried about cold air hitting the exhaust valve, think about what happens every time a engine with a cold air intake takes a gulp on the intake stroke. Cold air is intentionally pulled into the cylinder. Then look at people running nitrous, when it is sprayed in the intake, it drops the temp of the incoming air by like 80 degrees. That air is frigid and does not do damage to the valves. With the amount of pipe you will have before your cut-outs, there is no way this will cause any problems. Also, if this is true, how do vehicles with air injection manifolds not warp/burn valves? They FORCE cold air into the exhaust, and if you have ever had one of these manifolds off, the tubes stick up in the heads ALMOST touching the valves. If anything was going to cause a burned/warped valve, that would do it for sure. The only burned valves i have ever seen were caused by a mechanical problem where they are not closing properly, and the pressure and heat from the burning fuel escaping around the valve seat had burned up the head of the valve.
