Swengel (ref 1, p.244) describes these P&LE 2-8-4s as the last of the “Big Mike” philosophy. He includes the comment “while no boosters were fitted, these engines had all modern features and paid special attention to the combustion system”. Given their timing, I think that it would be surprising had NYC not included its previous learnings into its design. Perhaps the “Selkirk” front end is some confirmation of this.
Lucas (ref 2, p.91) includes a line drawing, although it is not as detailed as the usual RME or RA reproductions. It shows the BMOD as 100 inches, ID at the front ring being 90 inches. The firebox looks to be on the shallow side without much slope, maybe not of too much consequence in and of itself, but perhaps not too consistent with the original 2-8-4 precepts.
Why a 63 inch 2-8-4 in the late 1940s is hard to fathom. At least from the empirical evidence, this configuration was something of a dead-end judging by its relative fate on the roads that were initially enthusiastic about it. Perhaps NYC HQ said no to a 69 inch 2-8-4 on the basis that it had long ago decided on 4-8-2, not 2-8-4 for that slot.
Also why a 100 ft² grate? Lima had started with that number, true, but it seems to have been viewed as a bit over-the-top; It was trimmed to 88 ft² for the Mopac version, and 90ft² was deemed to be enough for the van Sweringen 69 inch model. Perhaps it was envisaged that the locomotives would normally operate for extended periods at drag speeds, full throttle and late cutoffs, and so needed abundant steam. But for such operations, their inherent inefficiency would probably negate any benefits that might derive from finely tuning the whole air/gas/steam cycle.
Regarding use of the boiler parameters for comparative purposes, the following excerpt from one of E.S. Cox’ books (ref 3, p.19 is interesting. Note that free gas area is considered to be an important measure.
“Similar care is needed in using published figures for boiler-heating surfaces as a measure of the steam-raising capacity of different boilers. Even within a single railway successive engineers may have had different ideas on how best to obtain a given steam production, by long tubes or short, by spacing them well apart or by packing them in regardless, and by different proportions between flue tubes and small tubes. Comparison of one engine with another can be misleading in that a lesser surface with good design could usually produce more steam than would a larger surface with poor design. Internationally the comparison was further vitiated in that some administrations measured their heating surface on the gas side of the tubes and firebox, while others based it upon the area on the water side.
“It must not be deduced from the above that no figure means anything and that the student must throw overboard all attempt at comparative analysis, but the more one lived with and understood the steam locomotive the more one realised that dimensions published in good faith always required a little consideration before their full acceptance.
“It is a matter for regret that while ambivalent figures such as the above were included as a matter of course in every official locomotive description, from no matter what country, there were other values bearing upon the very heart of locomotive power potential which were seldom made available. Of such were the free area for the hot gases to pass through the tube bank, and its relationship to the grate area, and the relation between cross sectional area and total internal surface area of the tubes. These proportions were vital for maximum steam production and high boiler efficiency and their values gave a remarkably accurate assessment of the power potential of a given boiler. Similarly, diameter and steam lap of the piston valves and the maximum travel were indices of how expansively it was possible to use the steam produced and were thus a measure of relative economy in working. Other vital figures, only obtainable from extensive testing work, were maximum steam production and steam consumption per indicated horsepower hour.
“Values such as are outlined above are not available to us on an international scale, even although they may be readily forthcoming in particular countries at particular periods in history. It is for these reasons that seeking to compare an American 4-8-2 with a Russian 2-8-4, or a British Railways' Britannia with a German 03 class Pacific, for example, needs a measure of patience and perspicacity.”
(1): F.M. Swengel; The American Steam Locomotive, Vol.1, The Evolution of the Steam Locomotive; Midwest rail Publications, 1967; LCC 67-29846.
(2): Walter A. Lucas; 100 Years of Steam Locomotives; Simmons-Boardman, 1957; LCC 57-12355.
(3): E.S. Cox; World Steam in the Twentieth Century; Ian Allan, 1969; SBN 7110 0079 4.