i was just reading a thread about the compatibility of upgraded cams w/a turbo.. first of all, what is the difference between overlap and duration? i thought they meant the same thing. thanks

 

I'm not trying to dodge your question or be one of those ******** who tell you to go do a search, but this website really helped me understand the whole camshaft, valve, piston arrangement. I don't think I can explain it as well and with pretty pictures.
http://www.howstuffworks.com

Check it out.

 

thanks bro, helped out

 

In a nutshell, duration refers to how long a valve is held open, be it the intake or exhaust valve.

Overlap, on the other hand, refers to how long BOTH valves are open at the same time. Overlap is used during the exhaust stroke and extends into the next intake stroke. Basically, as the exhaust gasses were leaving the preceeding exhaust stroke, this 'momentum' of gasses helps to pull in a fresh charge (since both valve are open...remember) for the next intake stroke...this effect is called scavenging.

This is just a basic description....far from complete.