I've been trying to find a reputable source for the cause of this but I'm having little luck.  There seems to be a well known idea that running an engine with just open headers or just a collector with no long exhaust will eventually "burn the valves.(diaphragm valve)"  The idea is that they warp and then lose enough contact with the head for heat dissipation.  I've also seen people saying that exhaust leaks can do this also.  The theory is that upon shutdown, cold air can reach the valves and warp them especially if they are open (unseated from the head).  One counterpoint to this theory is that losing an exhaust causes the engine to run lean, and this is what actually causes damage to the exhaust valves.  Does anyone know the truth behind this?

  It has been my experience that those engines being run with open headers, are being run without an eye to ultimate longevity. i.e., they generally are race engines.  That being observed, do race engines burn exhaust valves more often than non-race engines?  Certainly.  However, if engine longevity is a key consideration for a particular race engine, then many factors come into play, which may effect total exhaust valve life life:  Valve seat width.  A/F ratios.  Camshaft timing. (Including intake to exhaust overlap, and centrelines.) Header tuned length. Header primary diameter, Valve materials. Ect.  This a loaded topic, not easily condensed into one response.  I'm sure others can give you the advice you are looking for, in much more detail.  I for one however remain skeptical that convection of cold air up the header will cause the valves to warp.

The problem as I've heard it described is that momentary flow reversals expose the valves to cold ambient air on the backside, or at least to high temperature differential. If that's the real failure mechanism, a very short exhaust - probably only a few inches - would be sufficient to prevent this. I've never actually seen a failure definitively attributable to this despite seeing engines operating in many conditions that could theoretically trigger the problem.

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