NA plugs are shorter, think here for minute TTpistons are dished to get 8.5:1 compression whereas the NA is 10.5:1 and is the exact opposite they are domed. So NA plugs will not hit the pistons, now TT plugs in an NA motor WILL hit the piston
Kai 14.765 @ 92.22m.p.h. #4 in 2002 on Z32racing.com in NA drags