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).
Comparison of engine head modifications and their performance characteristics.
| Identifier | ExFiles\Box 134\1\ scan0166 | |
| Date | 12th October 1939 | |
| -2- (2) EXS.1384 which is a modification to the original head, yet without a pattern tear up the only alteration being to lower the water space around the sparking plug. The special features of this head EXS.1383 shown on S.90 are as follows (a) Local increase in throat area. (b) Clearance above the exhaust valve reduced. (c) Turbulent lip below the inlet valve to increase the "squish" area. Subsequent modifications to this head includes - (a) Machining away the turbulent lip to EXS.1758. (b) Graduating the increased throat EX.1384. (c) Recessing the sparking plug to bring it nearer the combustion chamber, thus reducing the pocketing to EXS.1809. (d) .100 machined off the face thus raising the compression ratio to 7.0 : 1. The ultimate comparison of these heads are shown on the attached curve and analysed as follows. At 7.0 : 1 compression ratio, the EX.1383 head with the aforementioned modifications loses only 1 - 2 lbs.sq.in. MEP. (from 750 - 2000 r.p.m.) to avoid detonations whereas the standard and sloping valve head at 6.8 : 1 and 6.9 : 1 compression ratio respectively lose 3 - 4 lbs.sq.in MEP. to avoid detonations. The relative non detonating power of the heads is that the EX. head is 4 - 5 lbs.sq. MEP. better than either of the others up to middle speeds. Regarding high speed open exhaust power the figures at 4000 r.p.m. are 140 BHP. with the standard head, 137 BHP. with the EXS. head and 134 BHP. with the sloping valve head. Although the non detonating ignition advance characteristic of all the heads at low and middle speeds are very similar the EXS. head is much less sensitive to ignition advance than either of the other heads, particularly the sloping valve head which is very bad in this respect. | ||