old school thought if you move the water too fast through the rad, it does not have time to cool when we races our 340 demon with the 6 blade it over heated put on an 8 blade pump no help, at that time an old guy that had flat head fords told me to drill 1/4 hole in every other blade on the 6 blade pump so we tried it end of problem slowed the water down so it had time to cool in the rad.