Complex sentence: Breathing was not greatly important for early production cars because engines could not run long and reliably at high speed, and all engines had poor combustion anyway, so this was a minor concern given the benefits in simplicity.
Split into simpler sentences: Breathing was not greatly important for production cars, in those days, because engines could not run long and reliably at high speed. In the early days all engines had poor combustion anyway, so this was a minor concern given the benefits in simplicity.