A further development was permanent compressor bleed routed back into the engine where it contributed to thrust. The Rolls-Royce Conway had a split compressor with intercompressor bleed to the jetpipe. A split compressor with this bypass arrangement allowed the highest pressure ratio of any Rolls-Royce engine, at that time, without the need for variable inlet guide vanes or interstage bleed. It was better known as an arrangement for better propulsive efficiency, a bypass engine. The Pratt & Whitney J58 used interstage overboard bleed for starting and accelerating through low corrected speeds but since it returned to these speeds at high mach numbers the bleed was opened again but this time routed to the jetpipe where it cooled the afterburner and nozzle. This cooling contributed indirectly to thrust in allowing more fuel to be burned in the afterburner. A similar arrangement, but using a split compressor with intercompressor bleed to the jetpipe, was later known as a 'leaky' turbojet, a bypass engine with only enough bypass for cooling the afterburner and nozzle.