What so you look for when buy heads valve diameter or the flow rate of the heads.
+1 Good write-up.The first thing I usually look at is the combustion chamber size, intake port size, exhaust port size. Then I look at valve train geometry (i.e. stock or not).
Then I look at flow numbers but more importantly i talk to people using the heads to see what kind of power numbers they are making.
Valve diameter doesn't really factor into my decision except when it comes to piston to valve clearance. You want the most flow from the smallest valve possible since this will reduce shrouding, improve velocity, and promote swirl. Also smaller valves weigh less reducing the stress on the springs and other valve train components. And smaller valves give you more room to play with for valve timing events (i.e. camshaft selection).
Lots of people will say that a 2.02 valve is better than a 1.90 valve. but take the edelbrock performer for example. The 1.90 valve head outflows the 2.02 valve head for the first 0.200 lift and keeps up with the 2.02 valve head most everywhere else only giving up a little over 0.500 lift (where the valves in EFI mustang spend little time). Really a 1.90 intake valve doesn't start to become restrictive until you are up around 400+ hp.