Chamber Shape & Flow
Air and fuel flow through an intake port at a pretty good clip. I don't know exactly how fast, probably slower than the speed of light but faster than grandma getting off her rocker. I'm sure there's a study on intake air velocity somewhere, but that's not important for you to consider when choosing a set of heads.
In a test I did a while ago, I dyno'd an engine with a pair of mildly CNC-ported heads right out of the box and then sent them for all-out aggressive CNC porting after the day's tests were done. When I got them back, the port volume had increased by 12cc and dry airflow dropped off at the bottom of the lift curve, but it picked back up at the top, surpassing the original set. More importantly, the shop that did the CNC work hit the chambers with their mill and reshaped them into what they believed to be a better design. The differences were subtle, but you could see it. Anyway, I bolted the heads back onto the same engine, went back to the same dyno, and retested it with no other changes at all. And I found more power. Not a huge amount, about 2.5 percent average overall, but it was there. After talking with the CNC shop, they pointed out that airflow measured on a dry flow bench does not indicate where the air is going as it enters the chamber.
My longtime friend and cylinder head expert, Joe Mondello, has developed a "wet flow" bench to find out what the air and fuel does as it enters the chamber. Many bigtime engine builders and cylinder head companies are using it to find things they'd never thought would occur. That's because air and fuel mixed together become a fluid, and fluids can move in strange ways.
Much of this effect comes from the chamber's shape and how it relates to the intake port. When the air/fuel column is rushing past the intake valve, especially at higher rpm, it's important for the cylinder head's combustion chamber to give it some direction. Note in the photograph how far back the chamber has been milled away from the intake valve. The arrows indicate the direction of air and fuel travel as it enters the chamber. You can see that the chamber's shape will at least give the mixture some direction of travel. This typically creates "swirl," which has been determined to be a good thing in combustion chambers.
Now, check out this stock chamber from an older iron head. See how it's got no shape to induce swirl? Nothing in this chamber is designed to direct the column of air and fuel anywhere. Its only purpose seems to be keeping the combustion pressures in the block.
For the longest time, it's been said that matching your intake manifold's ports to your cylinder head's ports is the only way to make real power. And that's not far from the truth, but it has to be taken in context. If you're building an engine to make max power at high rpm, then you'd better have your ports aligned perfectly. But on a street engine where you're trying to get the best power on a budget, it's kinda tough to perfectly match the ports. You can get close, but that's usually the best you can do.
Port-matching a set of cylinder heads is really not matching the ports completely. Instead, the ports are typically matched to an intake gasket size, and then the intake manifold can be opened up to match that same gasket, or left just a bit smaller. But by doing so, you could effectively put your head's performance out of the range you need.
What I like to do for stock or ported heads is to take the existing port shape in the cylinder head and straighten its walls and smooth in the entry just a little bit. Don't worry about opening it up to match a gasket exactly unless you're going all out. Then I do the same thing on the intake manifold and bolt them together. It seems to work pretty well and is a lot less hassle.