Potential engine specification changes for the 25HP model, considering bore, stroke, and overall capacity.
| Identifier | ExFiles\Box 5a\1\ 01-page002 | |
| Date | 2nd January 1931 | |
| E. {Mr Elliott - Chief Engineer} FROM R. {Sir Henry Royce} (At Le Canadel(Henry Royce's French residence).) C. to SC. WOR. {Arthur Wormald - General Works Manager} RG. {Mr Rowledge} C. to HS. {Lord Ernest Hives - Chair} BY. {R.W. Bailey - Chief Engineer} re. 25HP. I have never changed my opinion that, on its own merits, the 25HP. is best as we have arranged it, viz: 3.250 x 5", but all seemed so persistent that it could not compete unless it grew from about 4 litres to 4.5 / 4.7., and I wired my suggestion of scaling up what we had done by 5%, which would be 3.4 x 5.25". I have since come to the conclusion that even this will demand a heavier chassis, and I got a funk that we should spoil a beautiful thing by making it grow just a little too heavy. My final hope is that we shall compromise, and get the proportions and capacity per ton which Derby wish, by saving a little overall weight say between 1 and 2 cwts., and with the .5" we have put up the stroke, and say .100 or .125 on the bore, (I assume this will give something on the valves), be able to keep to the present cylinder centres, or very nearly, and chassis weight, length, bonnet, radiator, back axle, brakes, and clutch, not get out of our class - say not going over 28HP. rating. I don't like 29HP. because we should be called 30HP as we call 3/11 "4/-". I think they are claiming to have shewn a saving in body weights which should amount to nearly 5% on the total. Anyway I think the car will be quite fast enough when we have double carburetters and double top gear, and the greater HP. at higher engine revs. for what is required, and those who want a faster car need not have so large or so heavy a body. I think Derby would soon (or even may now) be saying that the chassis is not large enough in some ways, such as brakes and back axle. Last night I made a calculation that 3.4 is perhaps too nearly exactly just under 28HP. If we made it 3.375 it would just allow for oversize cylinder bores, and pistons, of .025 as a repair service job and still be just under the 28HP. Perhaps Sales and Derby may not think this necessary. If it is not I should adopt 3.4 bore which at 5" stroke gives 4.43 litres. My reason for not putting another .250 on the stroke is the fear of making the engine too heavy, and as we get no more HP. - i.e. make a 5% lower speed engine - we only gain slightly in 5% lower revs, that is, permits of half a tooth more in back axle, but we need bigger flywheel, clutch, and transmission. Naturally there are a few other advantages otherwise we should have a still shorter stroke - i.e. they are as you know rather better combustion chamber, perhaps better mechanical efficiency. | ||
From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).