 Higher octane fuels allowed boost pressures to be increased without the risks of pre-ignition or knocking.Rather than use inches of mercury, the British measured boost pressure in psi.A reading of +6 meant the air/fuel mix was being compressed to 20.7 psi (6 psi more than one atmosphere) before entering the engine; +25 meant the air/fuel mix was being compressed to 39.7 psi.