I have all 3 heat ranges and like to stick with the 9's for normal use and plug chops.

The 8's work well for street use if your engine is not running on the edge, they tend to help prevent fouling for those who run a tad rich and not change their jetting often.

I would use 10's for the track, but dont frequent the track here much.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
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