Radiator pressure

[Chuck Arnold <chuck.arnold@oracle.com>]

This last weekend I drove my TR6 in a track training session.  The car 
ran well, but would overheat after about 15 minutes.  The overflow 
bottlw would be full.  The radiator cap pressure is at 6 pounds. 

Is this too low?  What pressure cap do you use?

Thanks,
Chuck