Disaster Averted

Disaster Averted

Kawasaki addresses valve-spring surging on 2011 ZX-10R

THE MYSTERIOUS SALES HOLD ON THE 2011 Kawasaki ZX-10R ended in late January, and the company has given an explanation for the holdup: the possibility of intake-valve spring surge (coil vibration) under what Kawasaki calls “unique riding conditions, such as on a racetrack.” As a result, the intake camshaft, valve springs and spring retainers of new ZX-IORs will be replaced to eliminate this possibility, “without affecting engine performance.” Computers are wonderful tools, but physical testing continues to be essential for critical performance areas, such as valve-train stability. Valve-spring surge is the rapid bouncing of a wave of compression from one end of the spring to the other, and it is excited by very rapid cam acceleration. Surge results in a swift variation in spring pressure acting against the spring retainer, which is fixed by a pair of tapered collets to the end of the valve stem. This pressure variation can result in the valve’s failing to accurately follow the cam contour, or, as Kawasaki’s press release words it, “The surge could cause the intake valve to seat improperly.” Improperly means hitting the seat at too high a speed, leading to bouncing and damage.

Every manufacturer must deal with the possibility of valve-spring surge, beginning with a computer model, moving to rig and dyno tests, and finishing with road and track mileage. Spring surge is very speed dependent, so much of an engine’s defense against it comes from the fact that, in track use, rpm is constantly changing—never holding constant at the one particular speed that makes surge most likely. I suspect that Kawasaki discovered a possibility that surge could develop if rpm were held at one particular high and constant value, and decided to withhold the machines from sale until even this possibility could be eliminated.

The new cam may be subtly reshaped to suppress surge, the new springs may have a different natural frequency and new retainers are most likely required because the new valve-spring length is somewhat different. All these changes can be accomplished without affecting engine airflow and, therefore, performance.

People are expecting a lot from this new ZX-10R on both street and track, which motivates Kawasaki to ensure that they are not disappointed.

Kevin Cameron