I have a 1991 Nissan 240SX i keep hearing about this S14,S15 what are you talking about and how do you know what type of car you actually have i also have the HICAS stearing.

You have an S13. This was 1989 to 1994. It has to do with Nissan's designation for the chassis, and typically changes with a redesign. They redesigned the 240sx in 1995, and the 1995-1998 cars are the S14. We never got the S15 in the US, and more's the pity; it's a bad mofo.

It's the same deal with other Nissan cars. My 1994 SE-R is a B13 chassis, and the last year for that code. 1995-1999 is B14 for the Sentra and 200sx, and the current Sentra is the B15.

Hope this helps.

As far as the HICAS, I've read that it's a little twitchy and very heavy, and that a lot of people who get into modding eliminate it from the car.