You'll be alright without it, just, but you won't be seeing any real benefit of running a K&N filter and pipes without a cat. You're effectively running an already lean running bike, even leaner. The ECU will compensate somewhat when in the closed loop, but that's only at low rpm and cruising on part throttle.

Fit a Kev mod and O2 controller and you will not only make the filter/pipes work for you, but you'll really notice the difference in how the bike pulls. No longer will it shudder and protest below 3000rpm and get weezy above 6000rpm. You'll get a better spread of useful power from 2000rpm to the red line - that's a useable rev range of 5000rpm, as opposed to the miserly stock bike's 3000rpm.

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