Displacement. The "displacement equivalency" of a rotary engine is 2x, thus your "1.3L" Renesis is actually considered to be 2.6L and is thus ineligible for STL absent the specific allowance(s). You are allowed to run it in STU as an ITR car.
In my personal opinion, the philosophy of Super Touring engines is one of allowed intake/engine modifications and classification based purely on displacement. Thus, in my mind an unmodified rotary engine does not fit into the philosophy of STL on two fronts.
But, obviously, that's only one person's opinion.
GA
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