| To my knowledge and experience... some Z's came with the ribs and some didn't (both of the same years). It's not that one was better than the other... I think Nissan switched or changed fabricators at some point during the production runs and just decided to liquidate the stock by throwing them in some Z's. I've sat in both and find the unribbed one to be more comfortable but that's just a preference. Some might argue otherwise. 
 
Harry1992 Z Twin Turbo - Stage 4+
 1993 Honda Prelude VTEC
 ICQ # 1553422
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