The exhaust valves are regulated by a governor as I understand it, not exhaust pressure, so the kink in the pipe shouldn't affect the valve operation. However, if the pipe is kinked, it does increase backpressure which isn't good as it can greatly affect performance. You want some backpressure to keep intake gasses in the cylinder (rather than the exhaust), but not so much that it pushes intake gas back out of the cylinder.