Power Valve #'s

Yeah, you're looking at the date codes stamped on them, and the opening rate numbers. IN terms of the design...I've heard a bunch of things, even two seperate things from Holley people themselves via phone. From what I have picked up, the rating is the amount of vaccum required to keep the valve fully seated (closed). Most engines will be fine going by that "1/2 the vaccum reading in gear at hot idle" bit. But, some packages will run better with variations of that. The stiffer the spring, the more vaccum it needs to stay shut, and the sooner (in terms of load being applied) they will open.