I have read, with interest, Mr. Ireland’s letter and have referred back to your own article on the old Riley factory.
Perhaps I can help clear up the question of power outputs on various Riley 9-h.p. models. Although Riley (Coventry) Ltd. did not make a practice of quoting brake horse-power in their sales literature, obviously from time to time such queries were raised, and the figures quoted by the old company were as under.
9-h.p. Standard series, single carburetter, 28 b.h.p. at 4,200 r.p.m.
9-h.p. Special Series, twin carburetter, 35 b.h.p. at 4,800 r.p.m.
9-h.p. Imp (7.0-to-1 c.r.), 37 b.h.p. at 5,000 r.p.m.
The standard Imp engine was not fitted with twin exhaust camshafts, although the fitting of an exhaust camshaft in place of the inlet was a procedure sometimes adopted to obtain greater power output at the higher revolutions. This, I think, was the engine for which 41 b.h.p. at 5,000 r.p.m. was claimed, but it should not really be regarded as a standard production engine.
Incidentally, for any enthusiast who may fit twin exhaust camshafts, the object was to give a greater valve overlap and the ideal timing would be for inlet to open 25º before top dead centre and exhaust close 25º after t.d.c. A timing fairly close to the ideal was obtainable by turning the inlet camshaft wheel one tooth beyond the standard marking, in a clockwise direction.
A. Farrar, Hon. Sec., The Riley Motor Club – Abingdon-on-Thames.