Flow Power

It’s all about choices. There are probably over 100 different small-block cylinder heads on the market today. Narrow that down to heads for street use and that still leaves a river of heads to wade through when it comes time to pick the right heads for your next street small-block. The good news about aftermarket small-block heads is that almost anything you buy will be better than those old stock iron castings. But there are good choices and then there are better choices. Typically, the decision will more likely be determined by the thickness of your wallet than by any flow evaluation.

Chevy High Performance has been assembling flow data on cylinder heads since 1998. In that time, we’ve flow tested over 35 different small-block cylinder heads on Westech Performance’s SuperFlow flowbench. In fact, all the heads that CHP has tested can be found on our Web site (Cylinder-Head Flow-Bench Database ). With all this information, we’ve had a chance to study these flow numbers at length. While the flowbench is not the ultimate arbiter of a great cylinder head, the bench is the best indicator short of outright dyno-testing.

The biggest problem with flowbench data is that everyone immediately uses the cfm numbers at the highest valve-lift point to compare cylinder heads. However, it’s pointless to use a 0.600-inch valve-lift flow number to decide on a cylinder head when the maximum valve lift you will ever see on your street motor is 0.500 inch.

The smarter move is to evaluate the mid-lift flow numbers between 0.200- and 0.400-inch valve lift. These numbers represent the bulk of a cylinder head’s true flow potential. This includes exhaust flow numbers as well as intake. Another important factor is the intake port volume. This is an area >> where there is plenty of room for choice. Standard 23-degree small-block (excluding the LT1, LT4, and LS1) cylinder heads vary in port volume from 160 to 170cc stock to as large as 230cc. This is a huge port-volume range. Generally, the larger heads are designed for use on high-horsepower, high-rpm engines or larger-displacement 406ci through 430ci small-blocks.

The rule of thumb is to choose a cylinder head that offers the best flow with the smallest port volume. For example, if you can find a 190cc intake-port-volume head that flows the same as a 220cc head, the smaller port head would be a better choice since the smaller port contributes to higher port velocity, which improves torque. Of course, you must also look at the exhaust port in relationship to the intake. The best way to evaluate this is to divide the exhaust-port cfm by the intake- port cfm at the same valve lift.

For example, an exhaust-port flow of 152 cfm at 0.300-inch valve lift divided by an intake port with 193-cfm flow at the same lift will give you an exhaust-to-intake relationship expressed as a percentage (E/I). In this example, 152/193 = 79 percent E/I, which is a good number. E/I numbers between 75 and 85 percent are considered very good numbers.

But you have to be careful because an under-performing intake port can make an exhaust port look good. Price, flow potential, intake port volume, E/I percentages, iron versus aluminum, compression ratio, and several other variables contribute to the makings of a great cylinder head. The good news is that there are tons of cylinder heads from all the major cylinder-head companies that offer a tremendous selection. Bigger is not always better when it comes to a strong street small-block cylinder head. The best news is that all the cylinder heads in this story represent a solid performance investment for a street small-block. All you have to do is decide which is best for your application.