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).
Engine pinking complaints on Bentley cars and the influence of fuel quality and octane rating.
| Identifier | ExFiles\Box 150\1\ scan0068 | |
| Date | 11th November 1935 | |
| To Hd.{Mr Hayward/Mr Huddy} (FOR DEPOT SHEET) 81282 Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}6/KSW.11.11.35. Pinking. We note that in the last Quarterly Complaint Sheet there were 43 complaints of pinking on the Bentley. It is clearly quite impossible to form an opinion as to the cause of these complaints unless we have further information. There are, however, several factors affecting the trouble which may not be fully appreciated. In the first place, the most important item in reducing pinking on a car is the fuel. Nowadays there are on the market petrols of widely varying knock rating. These are classified according to octane numbers. Car petrols at present available lie between 60 and 80 octane. Most No.1 petrols are round about 70 octane. Benzol mixtures obtainable from pumps are only a little better. The Bentley is designed so that in a clean condition it will run on these fuels with only light detonation when the ignition is left in the fully advanced position. After about 5,000 miles, detonation will be noticeable, and can be removed by control of the spark. At 10,000 miles detonation will be very marked and the car will have to be decarbonised. If, on the other hand, it is run on Ethyl, we should expect the engine to be reasonably free from detonation up to 10,000 miles. Ethyl petrol is now 80 octane; i.e., very much better than any standard No.1 for anti-knock value, and as good as a No.1 spirit mixed with 50% Benzol. Ethyl is obtainable almost everywhere; 50% Benzol mixture is, as far as we know, unobtainable except by special mixing. Realising the advantage of these petrols such as Ethyl with lead in them, the Bentley has been specifically designed so that it can run on them. If a Bentley owner refuses to run on Ethyl for some prejudice, there are two other alternatives; he can have his car decarbonised frequently, or he can run with his ignition retarded and so lose performance, when he has carbon in the engine. | ||