You buy the piping kits to fit the motor; ie, for an S13 in either the S13 or S14 chassis, buy the S13 kit, which should also come with the hotpipe. For an S14 kit in either chassis, again, buy the kit to fit the engine (S14). The only difference is in the mounting brackets for the intercoolers. Specify which chassis and ask that the dealer swap the brackets for you. I got a Hybrid kit from HIPRO at
http://www.hiprollc.com and they swapped the brackets for me at no charge.