S30 body, 3.1 litre normal-aspiration, no computers, no carbon fibre:
Bit more to it than that eh count
you should read your own books.
Like access to an entire range of tuning goodies that we just cant get over here unless we pay through the nose to have them imported (import tax, shipping agents fees tralalalala) and then if it goes wrong or doesnt fit, your a little stuck to say the least.
Just looking through a magazine or two, Items such as
Exhaust manifolds (couple of pages), cams (whatever grind you like), valve springs (whatever rev and cam lift you need), valves (what exotic metal do you want), pistons (what shape, size and material do you need) state of the art head and chamber design (heads to make DJ cry with joy), inlet manifolds, head gaskets, tyres (not that these make much of a differance
).
We get some of these items, but no where near the selection, quality and relative cost that those guys do.
Plus and entire machine shop and tuning culture that understands the engine, who aside from a very select few in the UK can be bothered with a single cam normally aspirated 6 pot?
You get me the items I want, Count (at UK type prices and favourable shipping rates) and I'll run 12 seconds.
Last but by no means least, one of the biggest factors is
competition --- few people in the UK want to take their precious S30 onto a nasty track in case they scratch it, over there they are plain mad and do it all the time.
But you know all this dont you
BTW there are several US cars running 12's --- one definitely is on twin SU's