i got a 98 HX with the VTEC-E motor, im running an AEM full cold air intake, megan racing throttle body spacer, megan racing underdrive pulley, 2.5 inch catback and an MSD external coil ignition. i just ordered headers for it, but i need to relocate my cat which is one unit combined with the exhaust manifold. So what i want to know is, can i buy a magnaflow universal cat and just relocate it under the car attached to the header? and then just extend the O2 sensors to the new cat? Will the car still run the same if not better? should i reduce my exhaust pipe size to 2 inches from 2.5 inches because of the header? any advice is greatly appreciated!

 

I have a 98 HX too, and I recently finished replacing my whole exhaust system. I've got a Megan 4-2-1 header, Megan Drift Spec cat-back exhaust and in between the two I have a Megan resonated test pipe. However, you could easily use a bolt-on high-flow cat in place of the test pipe. Even with the new cat, my understanding is that your secondary 02 sensor will throw a code. I had to use an 02 simulator to trick the ECU into thinking the secondary 02 sensor was reading correctly.

As for your exhaust pipe size, using a smaller diameter cat-back AND putting on a performance header sounds counter-productive. If you have more flow through the header, but then have less flow through the cat-back, you're simply changing the location of the bottleneck. Having said that, a 2.5" exhaust might be a touch on the large side, but going down to 2" might be too small. My understanding is that 2.25" seems to be the sweet spot for a N/A (Naturally Aspirated) setup.

My Megan Drift Spec exhaust is 2.5" and I don't FEEL like I lost any low-end power, but I could be wrong.