It'll be because the engine NEEDS back pressure.

No back pressure is fine at high rpm but at low rpm there is a longer time when both the inlet and exhaust valves are open at the same time allowing air to pass right through the engine without doing any good!

It's a delicate balance.....

You can have no back pressure if you have allot of air coming in... ei. a turb charger.

No problem mate

Shit mate, glad u read this!
A bag of spanners, wouldnt be happy with that and the sound? no way, i dont like loud and farting noises from exhausts, a nice burble from a magnex is all id want.

Well cheers Ash think i'll scrap that idea!

Burt :-)

I've got a full s/s system, no centre box, just cat and backbox...

my cat went a few months back, all the internals got blown out, so was actually just an empty box - the car ran like a bag of spanners til i replaced it, then performance was better than ever. If I were you, I'd either leave the cat, or get a proper sports decat pipe.

On another note, an empty cat sounds really shitty, get nasty farting noises from the exhaust all the time. Hope this helps.

Ash :thumbs:

MikeH im probably askin a stupid Q but do u mean a standard cat or one which has been gutted of its internals?
If its a normal cat then it would still cause a blockage/restriction no matter where it was put on the system

Has anyone tried fitting a cat further back along the exhaust system?

could be chief?
Seein as i have the spare cat and it didnt cost me anything i think i'll try it out just to see. plus cops mite not check ur emmisions if they see u still have a cat??Although they havnt started that over here yet anyway, thank fak....

mind u i have noticed that the sportex one has a small silencer in where the de-cat is, also so to janspeed dont they? maybe its for this reason?

i know what ur saying broster but i found this on GMC motorsport site who are real boffins, they quote "and expansion box in the same place is better than removing the cat altogether" they say its all about gas harmonics and pipe length. apparently having an expansion box actually creates a vacuum and sucks the exhaust gases out quicker.
Ive heard about this resonace thing b4 with inlet and exhaust manifolds aswell???
Just a thought. :-)

if u have an expansion box in, i guess u mean an empty cat box? then ull gause the gass to be turbulent there, hence slowing it down and causing a restriction, granted it wont be as bad as the cat but still not as good as a de-cat?

Hello mateys

im gonna bring up the Q of decating again. we all know that taking the cat out will give more BHP approx 5 or so, and this is because u remove the restrictions inside the cat. i've been searching about for more info on this.....

This what i found
The same problems regarding pipe sizes and lengths apply to exhaust systems, this is why all manufacturers (until Citroen started with VTR) have the Cat or before Cat’s came into being, had an expansion box or silencer just about 3 feet away from the manifold (or a multiple distance of same).

So taking off your first silencer and putting in a straight pipe is unlikely to do anything for performance, unless you happen to be lucky that the rear silencer position just so happens to be the correct length away.

This is most unlikely as rear silencers are fitted anywhere that room allows. So what about taking off the cat from under the car, the car goes a lot better without it, yes, it does but this is due to getting rid of the restrictions of the Cat, nothing to do with pipe lengths and gas harmonics.

So whilst taking off the cat is good it is not as good as taking it off and replacing it with an expansion box in the same place


I know its getting a bit technical but Just thought it might be something to try, or has any1 tried it yet??

Im lucky enuf to have a spare cat waiting (i know they are expensive) for when i get my 16 sport, so im gonna clear everything out of it and try it with just the empty cat (expansion box) and see what happens...........:o

Burt