But why? The easy answer to keeping compression up is to increase stroke along with offset in such a way as to compensate for a larger combustion chamber. If you nee more flow, increase valve diameter. What design criterium specifically prevents designing adequate clearance.

My opinion, not to beat around the bush - it is easier to design by increment, and the designers are not too concerned about what happens 120k miles down the line, or even at 30k. They simply throw in a belt or chain swap recommendation, and cover their liability issues, and ignore the interference problem. But I will give them the benefit of doubt if there is a sound technical reason.

Steve