i've heard that the s15 manifold is more efficient, but i dunno for sure. i've never seen an s13 manifold upclose. i'm just including it in the offchance that someone needs it, i know i don't.i'm also not expeting that it's worth much.i have an offer standing for 600(turbo only) but the guy is in georgia and i'm not real sure how serious he is. so if you had the cash handy now, i'de take it.