I have adsense on my higher CPC sites but I have some sites that get more traffic but the cpc is pretty crappy. Anyone know if I add adsense to the sites with lower cpc and higher traffic, will that lower the CPC of my good CPC sites? I tried it before and it dropped but it could have been a coincidence.

I never heard it. That possible a coincident. I have some low as well as some high CPC site but I never see such.