Given that GCRy and apparently UP have installed Lempor Exhausts on engines-it makes me wonder-given modern metallurgy perhaps overcoming some of the limitations of the past-and betraying my mechanical ignorance-could rotary cam poppet valves be installed on an existing locomotive with redoing the whole thing?

Rotary cam poppet valves can be applied, however, you are quite correct in noting the cost. It would involve a complete new cylinder block, complete revision of the valve actuating machinery, reversing machinery, and quite possibly addressing the balance.
The end result, from all evidence so far, is that the clearance volume involved with poppet valves is what prevented it from out performing the reciprocating valve.
There were other issues with poppet valves in general, all of which, it is believed could have been corrected with time and of course money.
A fine reference on this issue can be found in Vern Smiths book, the correct title escapes me at the moment. He worked with Franklin Railway Supply in the R/D lab, specifically on the valve gear for the PRR, T1 .

Respectfully,
Dave Griner
www.wasatch-rr-contractors.com

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L. D. Porta was very much of the opinion that poppet valves were not as great as you'd think. In one of the last technical papers he wrote (on the subject of compound locomotives designed to his philosophy) he devoted an appendix to the subject of poppet valves.

Basically, he says that indicator diagrams show no advantage (as far as steam distribution) for poppets over well-designed piston valves. According to Porta, test data showed that poppet valves were leakage prone. Finally, he points out that they are relatively delicate and are very difficult to manufacture compared to piston valves.

Porta highly advocated light-weight, welded construction piston valves with a large number of narrow sealing rings. The advantages he cites are good steam distribution, long life with minimal leakage, and ease of repair.

BTW, since One Man's Locomotives is commanding these exorbitant prices, why in the world don't they reprint it?

Draughting improvements give big gains and almost instantaneous pay back. A modification to the extent of poppet valves would give small gains and take more than the rest of the locomotives life expectancy to pay back, if they did. At no point in all of my draughting work has such a modification been discussed as it fell outside the desired result. Such a feature on a new build is unlikly considering the current achievable results of a Porta valve.

"From Steam to Diesel" command similar asking prices. I'm not sure why Princeton Univ Press doesn't do a second edtion.

You actually have to confirm that someone is actually PAYING these prices first. An AbeBooks listing only shows off someone's delusions, otherwise.

Vernon Smith was also employed by the Santa Fe where his project was the Franklin valve equipped 4-8-4 #3752