The Honda CB450 was the first "big" Honda motorcycle with a 444cc dual overhead cam engine producing 43-45 horsepower (more than 100 HP/ litre). Appearing first in the 1965 four-speed K0 model, and progressing through a series of KX models with various improvements and styling changes, notably a redesigned gas tank and 5 speed transmission in the 1968 K1 model. K0 models are often known as "Black Bomber" models in the U.S. and notable for their distinctive large, square gas tank. In Canada the K1 model was marketed as the Hellcat.
Although the CB450 never sold up to Honda's expectations, it had excellent engineering for the time, notably including reliable electrical components, an electric starter, and a horizontally split crankcase, all features distinct from the British twins of the era. The most radical feature was the valve springing. Instead of the conventional coil springs, it used "torsion bars" - rods of steel that twisted to provide the spring effect. The engine was modified and installed in the Honda N360 and the exported N600, the precursor to the Honda Civic.