Very interesting. If my interpretation of this is correct, an Bulleid West Country comes out as 5P7F. I guess the SR did some judicious modification against known performance there. It penalises wide fireboxes quite severely; a King Arthur actually squeaks into 6P by my reckoning, but I don't think anyone would question which of the two classes was the more powerful passenger loco. I can't see how anything can be classified as 8P at all, given the 4480 lb/hr firing rate limit - by the calculations provided a superheated loco can only just make 7P. Maybe 8P was recognition of locos with the capability for feats of high power for as long as the fireman could stand it. Firing rate limit: 4480 x 6.18/20 x 0.00335 = 4.64 tons Actual Merchant Navy: 48.25 x 30.7 x 0.00335 = 4.96 tons Staying vaguely on topic, the L&Y Dreadnought tanks are just below the 5P/6P boundary, limited by their boilers. Oh, and the LSWR D15 I cited earlier as hard done by at 3P - yes, it edges into 4P with cylinder tractive effort the limiting factor.