I know it sounds thick but what does a de-cat do apparat from remove the cat? Can you apply to the stand exhaust then add a cat-back later? do you just get more dB, any power gain etc. thanks!
No not thick, if you don't know, you don't know.
This is a picture I did a while ago to show someone else.
I say I've got a decat, I have but it is part of the race manifold, it is all one section. Basically the race manifold does not have a cat in-line. Where the "decat" is labelled on the picture would be the OEM cat with a shorter top section of the manifold.
Yes you can add a race manifold before you add a cat back, but not having the cat on will play havok with the O2 sensor, which puts your Check Engine Light on, and because the decat is a freer flowing system it affects the dynamics of the engine, usually meaning the ECU tries to compensate on it's set perameters sending the air to fuel ratio lean. This is not good news.
So if you are getting a decat it is always recommended to have the car custom tuned, and on FN2s this is usually done with Flashpro. The tuner will also use the Flashpro to basically ignore the O2 sensor that would put the Check Engine Light on, so you don't have that to worry about.
For this reason, you would normally do the decat as one of your last mods before having a custom tune done, because you can put intakes and cat-backs on without needing to have the car custom tuned.
Yes it does give more power and torque as it is freer flowing, the OEM cat being quite restrictive.
Before on mine:
After on mine:
Yes I also have a bigger throttle body and RRC inlet manifold on the second graph, but they will mainly help with top end and a bit of mid range. The point I'm making is look at the difference on the blue line (torque) as with the decat you lose the big restriction and it opens up a lot of mid range.