A good summation here
Rolls-Royce R - Wikipedia, the free encyclopedia
As Onceapilot mentions, special fuels were concocted to control detonation. The original engine was rated at 1900 horsepower (at the then unheard of 55 inches Hg boost and 2900 rpm) running on 78% benzole, 22% Romanian petrol and 2 cubic cm of tetraethyl lead per gallon. A Rolls Royce 1927 patent for a ram induction intake to the supercharher also contributed 10% in power.
For the 1931 race they were looking for an output of 2300 horsepower. The failure of crankshafts, con rods, exhaust valves and high oil consumption had to be overcome. Oil was being consumed at a rate of 112 gallons per hour which was solved by improved piston ring design to control blow by. Exhaust valve problems were solved with the introduction of sodium filled valves. For this race the fuel was 70% benzole, 20% Californian petrol, 10% methanol and tetraethyl lead.
After this race an attempt was made on the world speed record with the engine boosted to 70 inches Hg and running on a mixture of 60% methanol, 30% benzole, 10% acetone plus tetraethyl lead, At this point the engine was running at its limit, even stretching the cylinder hold down bolts.