Pro stock bikes here in OZ use a lot of old tech compared to some other bike types . They still use carbs ( Lectrons ) , and wheelie bars . I am not sure about engine rules but the US Suzuki running steel rods would have to be a compromise compared to what the twins can run to level the field . Some of the no bar bikes are quicker nowadays , which I find amazing .
This is pic of AU $16K billet cylinder head that a fella I know bought for his pro stock build .
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