The keys to the R engine's high power-to-weight ratio were its supercharger design, ability to run at high revolutions due to its structural strength, and the special blends of fuel used. The double-sided supercharger impeller was a new development for Rolls-Royce: running at a ratio of almost 8:1, it could supply intake air at up to 18 pounds per square inch (psi) (1.24 bar) above atmospheric pressure, a figure known as "boost" and commonly abbreviated as "+"x" lb". By comparison the maximum boost of the earlier Rolls-Royce Kestrel design was +6 lb (0.4 bar), this figure not being achieved until 1934. The high boost pressures initially caused the spark plugs to fail on test, and eventually the Lodge type X170 plug was chosen as it proved to be extremely reliable.