is there a specific degree of valve overlap that makes a cam fail emissions? like is 70 degrees max for example?

 

You have the right idea, but you're looking at the wrong specs. The one thing that every emissions-legal cam has in common is NO overlap at .050". So forget overlap at advertised duration.

 

so as long as there is no overlap at .050 than it is emissions legal?

 

close. If there is no overlap at .050", it CAN pass the sniffer test, but may idle noticeably and/or not have an E.O. number.

 

ok so high lift cams (.520 or so) will give you a pretty rough idle. you say that there is a certain sound that it needs to be below to pass?

 

Lift has nothing to do with emissions. There are plenty of reasons to think so, but if you get a modern hydraulic roller, and use high-ratio rockers, you can get past .560" lift and still be clean. They can't fail your car for sound, if you meet the loudness laws as to how many decibels, and your parts all have E.O. numbers and you pass the visual and "sniffer" tests. Now, consider why you want lift: It's to let good heads be fully taken advantage of. On most stock GM heads, there's no point trying to get past about .500", but with something like AFRs, then you should be trying to approach .600"