CAT replacement pipe
A Cat Replacement Pipe, or CRP, is a device used to replace the Catalytic converter on an internal combustion engine. Somethimes refered to as a de-cat pipe or decat or straight catpipe, it's nothing more than a straight through piece of tubing!
Alternatives to this are a SCRP or Sports CAT.
Noise
Large increase in noise, pops & bangs on overrun.
Annoys the neighbours!
Performance
About a 3 BHP gain on the non-VVC engine.
MOT & Legality & Insurance
It is illegal to use your car on the road without a CAT and not just because of the emissons. This may cause you a problem in the event of an insurance claim. The procedure to change your CRP is quite simple.
111S & VVC Engine
All tuning is a case of balance and this is still the case with exhaust systems. Some engines require some back pressure and the system may have been tuned with it in mind so a Motor sport version maybe more favourable than removing it completely.
Some further reading and testing on similar engine configurations can be found on this MGF owners site.