What do these two terms mean when related to exhaust systems? I was looking at this on ebay.
Half way down it says: System type: Cat back system including de-cat
What does that mean?
Cat back means the system attaches to the catalyctic converter a de cat system means your getting an extra bit of pipe that will replace the cat so as to make the exhaust more free flowing. The cat restricts flow and has filters in to reduce emmisions.
So cat back means all of the system back from but not including the catalytic converter - Yes?
De- Cat - So is that legal then not to have a cat? If you replace the cat with a de-cat (as I understand you) will the car still meet the emissions test on the MOT?
No, If you “de-cat” you will have to re-fit the CAT for MOT. Plenty people do this, but personally I couldn’t be bothered with the hassle.
First point yes thats right second point a decat system will probably fail emmissions on MOT but if your car is registered before a certain date (Im not sure of date) you dont have to have a cat. mine qualifies because of that being a 1990 model. Yours wont so if you want a decat then when your MOT is due you will have to swap over the decat part and put your cat back on.
Hope this helps.
Yes great - thanks very much.
So I’d be alright with a Cat back Stainless exhaust, but not the de-cat bit (unless I could be bothered with swapping about every year)
Indeed you would try MIJ engineering in Walsall I got mine from there superb sound and cheap they make it for you while you wait cost £270 ish
Be careful what you buy. Check there is a small “CAT sized part” if you go to de-cat, which would then be easy to swap for MOT. Beware of a lengthened centre section to incorporate centre + CAT, as this whole section would need re-fitted for MOT (centre and CAT).
Personally, I would do “cat-back” option - fit and forget 
Yes - Thanks to you both.
I’m just going to go for Cat Back only now you’ve put me straight.