Here, you can see that with machining, the restriction that the valve guide and it's required bracing would normally cause is removed. The removed bracing and the valve guide itself are moved up, and the overall cross section of flow area is shortened by ~25%.
The image creates a notable issue with valve lift and spring/seat position, which could easily be solved with larger seats/springs that would still allow for the vertical bracing needed by the guides. The upper length (above the guide) could be lengthened as well, to extend valve lift capability and decrease the likelyhood of spring binding and flattening.
Keep in mind, this is a simple representation, and will likely still have issues. I'm not an engineer, I just think like one.
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