why won't 91 through 99 headers fit earlier engines?

ok i am fed up looking for an afermarket header for my 90 xj. what is it that keeps me from using a 91 through 99 header? when i bought my xj i had to put a new exhaust on and the preious owner told me that it was a 91. so when i went to put it in the rear section went in fine but only the down pipe was different. so my question is can i just use the later down pipe that will fit to the later header? thanks!!!!!!!!!

The newer header will bolt on to the head no problem. However the newer xj's don't have an EGR setup, so you would have to remove it. As long as you live somewhere without smog nazis go for it.

We have installed the Wrangler 4.0L HO 3-2-1 header on CJ's and XJ's, and just welded an O2 sensor bung to the collector (the factory YJ O2 sensor is on the downtube). It works good.