I'm looking to replace my stock impeller and wondering if anyone has suggestions or reviews of some good aftermarket impellers. I have looked at the Solas line but am not sure the best pitch. I live at about 4500ft so am needing a lower pitch for the high altitude... any feedback would be great... even a dumbed down explanation on how to figure out the pitch numbers would help too... Solas show a 14/20 pitch but i think that may be for sea level