The Honda CB450 was the first 'big' Honda motorcycle with a 444cc 180° straight twin, dual overhead cam engine producing 43-45 horsepower (more than 100 HP/ liter).
Appearing first in the 1965 four-speed K0 model, and progressing through a series of 'K' models with various improvements and styling changes, notably a redesigned fuel tank and 5 speed transmission in the 1968 K1 model. In Canada the K1 model was marketed as the Hellcat.
The CB450 was first shown in UK during the Diamond Jubilee Brighton Speed Trials of September 1965, traditionally held along the seafront. As the bike was newly imported, the engine was not run-in, so the appearance was a semi-competition demonstration sprint run for publicity ridden by (the late) Allan Robinson, MBE, a Honda staff member, recording a standing-start kilometre time of 30.1 seconds and a terminal speed of 100 mph (160 km/h).
The CB450 was then publicly exhibited at the nearby motorcycle show, held for the first time in Brighton at the Metropole Hotel exhibition centre situated on the seafront.
Although the CB450 never sold up to Honda's expectations, it had excellent engineering for the time, notably 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 basic engine was modified and installed in the Honda N360 car and the exported N600, the precursor to the Honda Civic.