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I do have one on my FZ6 and it not only takes out the "hole" in the power curve between 5 and 6000 RPM where Yamaha de-tuned the engine map to meet emissions regulations, I can also switch to the "fun mode" map which gives a nice power boost (ok, and I lose about 25% fuel efficiency, but you can't have everything!)
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Agreed, these things can/do change the mapping on most fuel injected bikes (as you say at a cost)....... But I've seen them advertised for the fzs which as a carb bike can make sweet fa difference!!