"Street heads with flatter valve angles tend to have more mid-lift airflow but sometimes aren't stable at over 0.600-inch lift, which can be undesirable. Valve location in the bore is more important than the actual valve angle, and a head with a flatter valve angle and optimum location will almost always make more power than a head with a steeper valve angle. Port shape, port floor, and roof location have to be optimized for the chamber and valve design. Not getting the valve diameter right for the intended engine size and rpm is one of the biggest mistakes head porters and engine builders make."
Tony McAfee: "The flatter the valve angle, the harder it is for a head porter to increase flow. A 10-degree valve angle in a standard SBC port would never flow any air. Low ports and a flat valve angle are a very bad combination. Once you start raising runners, however, flow improves dramatically. The advantage of a flat valve angle is that valves won't choke themselves against the bore as they open. Some production heads, like the LS7 castings, are as good as race heads. Their high ports combined with flat 12-degree valve angles means that air doesn't have to negotiate a 90-degree turn at the short-turn radius. Furthermore, having flatter valve angles and raised ports means that the combustion chamber is easier to design since so much of it isn't biased toward one side of the bore. Shallower chambers improve flame travel and efficiency."
Jason Neugent: "Diesel engines usually have a flat valve angle and are not considered high flowing heads. In most cases, when you stand the valve up to achieve the flatter valve angle, the intake port is elevated to complement the angle change. The angle that the intake port meets the valve can be critical. This elevated port is often beneficial in making a higher-flowing manifold as well. The shape of the roof and the floor can also be influenced by the type of fuel that is going to be used. Valve location is critical because of the valve's proximity to the bore wall. Having the exhaust valve close to the bore usually doesn't hurt the flow a great deal, but the intake valve needs to be farther away from the bore wall to promote decent airflow. The relationship of the valves to the bore is usually the deciding factor in valve size as well as valve placement."
"The main benefit of a flatter valve angle is that you can run smaller combustion chambers, especially with heads in the 12- to 15-degree range. Smaller chambers promote improved flame travel and combustion efficiency. Cross-sectional area in relation to engine size is more critical than valve angle. If you put 18-degree heads on a 427 small-block Chevy with undersized ports, then you swap them for 23-degree heads with properly sized ports, the 23-degree heads will make more power. Compared to a 23-degree head, the pushrod is moved over in an 18-degree head for a wider pinch point, which increases cross-section. Most 23-degree heads don't have this luxury. This also allows moving the valves over, which frees up space to run larger valves. Ultimately, the overall port design of an 18-degree head is much better than that of a 23-degree head. The flatter valve angle is just one of many variables that need to be optimized to design a high-performance cylinder head."
Kevin Feeney: "Valve angle is only one of many factors that contribute to flow. Air only likes to turn small amounts at a time because it's moving so fast through the ports. The height of the short-turn radius and its relative position to the port opening along with the valve angle all work together. If you were to simply flatten out the valve angle and not make any other changes you would most likely create more problems. It is important to create a 'happy' port by having all aspects of the port working together."