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Hello Walt,
I have worked with many different mechanical valve trains, with factory specs from 0.004" to + 0.020", and wondered the same questions. Valves, especially exhaust valves, need to get rid of heat and most of that goes through the seat when the valve is sitting down and closed. A good example of this can be found on the Porsche designed TAG turbo engine, where the exhaust valve seat had a groove to circulate coolant and 10% of the water pump output was directed to the exhaust valves. Porsche's recommendation of 0.004" is just about the tightest I know of, and considering the air cooling of the heads, recommending tighter clearances is just bad advice. If someone is that anal about valve noise, they should get something with hydraulic valves.
Factory specs for valve clearances, like many things, involve several issues. Jaguar, for instance, used 0.006" and 0.008" for many years, winning LeMans 5 times, testing engines at WOT for several days, but when the road cars came up against US cars with hydraulic lifters, customer complaints made them tighten the specs, then drill the lobes for oiling, then redesign the lobes for quieter running at 0.012"-0.014". These cams have exactly the same duration at 0.050", but are 40 degrees different at zero lash and have the same performance. Quieter valves with wider clearance and entirely a marketing decision based on customer desire. OEM's have to balance noise, adjustment intervals, and longevity into their specs. Most OEM's that make recomendations for racing (few) specify wider clearances to help get rid of the extra heat with more seat time.
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Paul
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