Everyone always asks, "What do the heads flow?" That's a very important question when looking for the correct cylinder head for your engine, however, it's not the only thing to take into consideration. For one thing, a flow bench is like a dyno: they're all not going to show you the exact same numbers. Manufacturer flow benches may not show you the exact same numbers as your engine builder's flow bench. You also have to look closely at how the heads were flowed. A head may flow more air on a larger bore because the valves are not shrouded by the cylinder walls. Often, manufacturers will use a 4.125-inch bore when flowing aftermarket SBF heads, when in reality, they may get bolted to a 4.000- or a 4.030-inch bore. It makes a difference.
The valve lift also makes a huge difference. A cylinder head may flow 320 cfm, but at what amount of valve lift? If it's at 0.700-inch lift, then how are you going to take advantage of that with your 0.550-inch lift camshaft? One good rule for qualifying cylinder head data is that it's better to choose a smaller port volume head when flow rates are similar. For instance, if you have a 205cc head that flows 300 cfm at 0.600-inch lift and you have a 225cc head that flows 300 cfm at 0.600, then it may be a better decision to choose the smaller port volume. If you can flow the same rate with a smaller port, then it could be a good indicator that the velocity may be much higher. High velocity is a good thing. Remember our discussion on air/fuel charge inertia?
When looking for an aftermarket head, keep in mind that most aftermarket heads rely on stud-mount rocker arm setups (available in both 3/8 and 7/16 stud sizes, which need to be matched to the rocker arm trunion size). A lot of factory heads used pedestal (or rail style) rocker arm mounts. If your plan was to reuse some factory parts in your build, then you will need to make sure that everything is compatible. Also, it's a good idea to have your camshaft in mind when you're buying assembled heads. Flat tappet camshafts call for different valvespring pressures than roller cams. Keep an eye on the valve lift as well, as the springs that come with a specific cylinder head package may coil bind at the lift that you will use with your camshaft. Everything needs to work as a package.
You'll also need to look at the exhaust flange pattern and location. Some heads are only drilled for certain flange patterns (some may not be factory).
When selecting a head, you will also see that there are different chamber sizes available, again measured in cc's. The combustion chamber is a very important component of your compression ratio. This of course needs to be taken into account when choosing a head, as you want to match the combustion chamber size to the rest of your block/piston combination. This will play a role in whether or not you can run 87-octane pump gas or if you have to mix some race gas in your tank. Most manufacturers will offer a few different chamber sizes so that you can take creative license, no matter if you're using a flat top piston or a dished piston.
A lot of the aftermarket cylinder heads have some very large sized intake valves in comparison to factory heads. Usually, aftermarket performance pistons have valve reliefs suitable for these valve sizes, but if you are upgrading an engine and you're using the stock pistons, this could be a temporary show stopper. Pistons designed to work with 1.94-inch intake valves could cause problems with intake valves that are 2.05- to 2.08-inch in diameter.