"When running open headers, you should always stuff a rag or something into the collectors so cold air won't get in and cause the valves to warp."
LOL! I hear this doosey every now and then and it makes me laugh every time. People spew this kind iof crap as if it's facts. The REAL fact is; when running open headers, cold air isn't going to "rush in", go up the collector, follow the pipes around all of the bends, go through the exhaust ports of the heads and get to the valves and still be "cold air", even if you stuck a vacuum cleaner on top of the carburetor to create some sort of "reverse evacuation air current" to DRAW air in from the exhaust and do that! And even if you did, the valves STILL wouldn't "warp". Jesus, they aren't made out of thin, crap metal! Not to mention, at any given time, most of them are closed, rather than being open where they could possibly warp around the rim / face area if you shot maybe below zero air right at a 1,500 degree exhaust valve.
For air to get in there and go all the way up to the valves, it would have to be displacing (or compressing) the air already in the pipes, cylinders, etc, and that isn't going to happen. Even on dragsters where we run "zoomie" exhaust pipes that are all of about 10 - 18 inches in length with no collector or anything, cold air doesn't get in, and valves don't warp. Yes, you'll see all kinds of things plugging-up the ends of zoomie pipes, but not when they are scorching hot right after a run which would melt the plastic, rubber, tennis balls, or whatever you have in the pipes. Those things are for keeping dirt out when it's just sitting, not "cold air" out right after a hard run.
Another factor is; IF cold air did get into the pipes, just how far do you think it would go up those pipes before it went from cold air to "hot" air due to the scorching hot steel pipes? It certainly ain't going to stay cold all the way up the pipe and into the head runner where the valve is. Please, when you hear some dork saying stupid stuff like this, call him on it so he doesn't keep sound like an idiot.