s13-t25
s14-t28 bigger turbo so the higher hp.
If you plan on upgrading i'd suggest the s13. The s13 has high port flow and s14 & s15 have low port.
Like v-tec the vtc is in another words variable valve timing, just another way to adjust the timing of the cams.
I think the only difference between the red top and black top s13 are the different years. I believe the black tops are newer.
IF ANY OF THIS INFO IS INCORRECT PLEASE CORRECT ME< THIS IS JUST STUFF I READ OFF OF OTHER POSTS. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/sneaky2.gif" border="0" valign="absmiddle" alt=':evil:'>