[Bletchley]

GB, there were at least three different 130 hp Clerget's in use by the British and at least two different AR1/BR1 rotaries, each with a different 'real' hp output, but all nominally 130 hp (Clerget) and 150 hp (AR1/BR1):

Most of the RFC Clerget Camels were, I think, powered by the French-built Clerget 9B engines. These were nominally 130 hp at 1200/1250 rpm but they could develop up to 134/135 hp at this speed, and up to 150 hp for a short period at 1300 rpm.

RNAS Camels were, at first, powered I think by British built Clerget 9B engines produced under licence by Gwynnes (or a mixture of British and French built engines). These were poor copies of the original French 9B (Gwynnes was a producer of marine pumps before it acquired the Clerget licence, and the company does not appear to have been used to the fine engineering work needed to produce rotary engines), and these engines suffered in particular from rather badly made obdurator rings that rarely lasted longer than 10-15 hours and caused a loss of compression. I think the British Air Board figure of 126 hp is probably based on one of these licence built engines (although I do not think there is any indication of this on the data sheet). Bentley was sent by the Admiralty to Gwynnes to "sort them out" and introduced a number of improvements and modifications to the 9B (the main one being aluminium-alloy pistons) that not only improved the reliability of the engine but also raised output from a nominal 130 hp to 140 hp at normal max. speed, 1250 rpm, 160 hp for 'a few minutes only' (Gwynnes renamed it the 9BF and rated it, rather optimistically, at 150 hp). My reference for this is the Gwynnes Ltd manual, 'Clerget patent aero engines, 9b & 9BF', 1917 (reprinted by Camden Miniatures, 2001).

There was both an original lower compression and a later higher compression version of the AR1/BR1. The low compression version was nominally 150 hp at 1250 rpm (Air Board figure), but Andrew Nahum of the Science Museum has quoted 158 hp at 1250 (in his booklet on rotary engines) which I think may have been the output of the later high compression version of this engine, but it could reach 1300 rpm 'for a few minutes only' (Air Board), although I have no figure for the hp at this engine speed. In his memoirs W.O. Bentley referes to this later high compression version, without giving and performance details, and also states that up to a further 11 hp could be gained by drilling a small hole in one of the induction pipes of the BR1, and "We sent the word round and fitters on every B.R. squadron were soon busy with little 2-mm drills" (Bentley, W.O. The autobiography of W O Bentley, Hutchinson, 1958).

So, to sum up:

Gwynnes licence built 9B: 126 hp (?) (rated at 130 hp)
French built 9B: 134/135 hp full, 150 hp max (rated at 130 hp)
Gwynnes/Bentley 9BF: 140 hp full, 160 hp max (rated at 150 hp)
Bentley AR1/BR1 (low compression): 150-161 hp full, max ? (rated at 150 hp)
Bentley AR1/BR1 (high compression): 158 hp (?) full, max ? (rated at 150 hp)

I would assume that these figures are without any reduction for windage.

Bletchley