Try breathing-out through your nose whilst half pinching your nostrils. This is akin to what your engine has to do to get exhaust gases past a catalytic converter! This obviously has an impact on the car's performance. It is a simple matter to replace the cat with a de-cat pipe.It should be noted that the S14a has a second smaller cat in the downpipe from the turbo and a replacement pipe is available for this too.
WARNING! Running an S14, S14a and S15 without a catalytic converter is NOT illegal in the UK. However, not passing emissions testing is, and it is unlikely to without a cat fitted.
The S14, S14a and S15 all require different sized de-cats. Ensure you have the correct size!
Ideally, you need a trolley jack to gain adequate access to the cat. The job should take about ten minutes, so long as your rusted bolts don't shear! You will need a 10mm and 17mm socket and, of course, a de-cat pipe. Depending on the condition of the existing gaskets, you may need replacements.
Using the 10mm socket, remove the four bolts holding the cat shield on. Next, remove the four nuts on bolts holding the cat to the system (Figure 1). Be careful of failing rust! You can now carefully remove the cat from the system.
Figure 2 shows the removed cat. Be sure to keep this safe. You will need to fit it back on for the MoT road test each year! Coat the bolts in WD40 to reduce further rusting.
Figure 3 shows the shiny new de-cat pipe replacement.
Check the gaskets and replace if necessary. Now bolt the de-cat pipe into the gap in the system using the four bolts provided. That's it.