By far the biggest news with the new Ninja is its return to a 636cc engine, but that's just the start. In addition, all the engine's ancillary systems were tuned to support the displacement bump. The extra 37cc is achieved via a stroke increase of 2.6mm, for bore and stroke measurements of 67mm by 45.1mm. The engine uses shorter, stronger conrods to accommodate a longer crank throw, and the pistons' longer travel necessitated lowering the cylinder ports, which are in place to reduce pumping losses at high rpm. The camshafts were tuned to suit the stroke increase, with longer intake duration and a 0.2mm increase in intake- and exhaust-valve lift. The piston crowns were reshaped to make room for the valves' increased movement and to reduce compression slightly, which goes from 13.1:1 to 12.9:1. The 2013 Ninja ZX-6R now has an "assist and slipper" clutch, which uses a pair of cams to provide more positive engagement while accelerating, and controlled slip when downshifting. The design transfers load from the clutch hub and basket to the cams, allowing Kawasaki to make the inner hub out of aluminum instead of steel, and to reduce the number of clutch springs from six to three, for a total weight savings of 700g and a much lighter clutch lever pull. Throttle-body diameter remains the same at 38mm, but the 2012 bike's upper injectors were ditched in favor of larger, single, lower injectors. Deleting the upper nozzles allowed Kawasaki to increase the airbox volume by 11 percent, smoothing power delivery and boosting peak output. The move also made room for longer velocity stacks, which help with low-end performance. Most of what resides between the crankcases is the same as before, but first gear is shorter and all the cogs are thicker for increased durability. The retuned exhaust has balance pipes across all four headers, while a reshaped sub-chamber houses a shorter catalytic convertor. The new bike has a narrower yet longer muffler, made from steel rather than titanium as on the previous model. Redline was lowered from 16,500 to 16,000 rpm. The engine's measured output is 112 bhp at 13,300 rpm, and 46.4 lb.-ft. of torque at 11,300 rpm.