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This is the reason a new casting was required to create the Dart 9 Degree head. On the left is a ported 9 Degree while on the right is the Dart 18 Degree casting (which can be ported to create a 13 Degree head). You can see that the intake port has been raised 0.330-inch versus the 13 Degree head. This reduces the radius of the turn the air/fuel charge must make between the port entrance and the valve seat which allows it to flow significantly better than the 13 Degree head.