I understand it's for the strict safety standards and emission standards the US has in place but the Silvia and 240sx are the same cars you would think some USDM bumpers, lights and you would be good to go. The SR20DET burns as clean or cleaner than a Ka24de had mine checked at a friends shop he said it met the standards for the sniffer test. But they still wont allow it in legally.

I know people get them registered just not how they get it done, so I will get a USDM shell and transfer the parts over to have a car I like.
