Decat

[jacksargent93]

Just a quick question - has anyone decat their car? Your experience? Do you sketch out a little about the coppers? MOT? Thanks :)

[jacksargent93]

Also thoughts on sport cat

[stevegtuk]

The decat is only needed for cars which have emissions tests, so diesels dont need them on, (I took mine off).

The coppers wouldnt do anything unless you stopped them and told them you didnt have one on, even then i dont think they would really bother.

[jacksargent93]

So getting my Petrol decat would be okay - as long as I informed insurance? Would it pass the MOT? And most of all, is it a nice mod to do?

[b1g_dav3]

Won't pass an MOT no. Sport cat is the way to go, unless you remove the innards of a standard cat, which is the same as removing.

[stevegtuk]

As above no it wouldnt pass an mot because it does require a cat. Some have tried taking the guts out of the cat ,but it doesnt really give much more improvement as there is now an open void for the air to flow into, circulate around and the leave through a small hole, so causes turbulence.

The sports cat sounds the best option, unless your good at welding, then the other option left would be to get a decat pipe, take the cover of fthe original cat and weld it onto the decat pipe to make it look like the original cat and your sorted.

[jacksargent93]

So whats the difference between a sports cat and decay? Is it a mod you guys would do?

[FOCA]

So whats the difference between a sports cat and decay? Is it a mod you guys would do?

Just to underline what other FOC members have said, a decat is a completely different proposition for a petrol than it is on a diesel

On a diesel there is often a small, passive cat, that works in a similar way to a DPF (diesel pariticle filter) by "passive" i mean it has no electrics/ electronics or sensors, its a mechanical/ phisical thing, it can be removed and replaced with a straight tube on a diesel with no problems, no EML (engine management light) should go on, and as Steve has explained, will still pass a UK MOT, (only particulates(smoke)is measured) Technically, there is a "construction and use" thing that the Police could enforce, in theory (this is not my area) but it would be difficult to prove and they probably are not interested, unless the exhaust is really loud etc, i have a straight-through exhaust and a decat on my diesel, it has been checked by the boys in blue, and given the "thums up", they were very helpful and even tested my lights as well (thats service for you )

A petrol engine cat is a tottally different kettle of fish, 1st, the MOT is much stricter, the exhaust must be much cleaner, removing the cat would put the emmisions up and fail the MOT, there is also a lambda sensor on the cat, it is "tied into" the cars' ECU, the engine will not run properly without the cat with the lambda sensor connected, unless special measures are taken

On top of that the cat may not be the most restrictive part of the exhaust, (there may be more restrictive silencers or small diameter pipes in the system that could be upgraded for potentially better, easier results) or the engine runs better with a certain back - pressure (normally NA (Naturally asperated/ non turbo)) engines and removing the cat LOSES power

Tuning a car/ engine should be looked at as a whole system, not just one little part, when i think about gas-flow i consider the air intake, through all the pipes,filter, inlet manifold etc all the way to the tailpipe - with the exhaust, i consider the entire system, from the manifold to the tailpipe, the cat is only a small part of this, but a part that is expensive/ a lot of hassle to change

Instead of asking about cats/ decats, or what color of "go faster stripes" are the fastest, better to ask "what is the best/ most cost effective way to make my car faster/ louder/ better"

Personally, I would think about a large diameter free- flowing cat-back stainless system first (and probably other mods, like a remap etc/ depends on the car) before considering a cat upgrade

[jacksargent93]

Thanks alot, that has cleared alot up - now I can see that is not for me :)

So would you suggest upgrading the whole exhaust system? How much would that cost roughly including fitting ( I know its a stupid question )

Any other cost effective mods? Would any K&N filter fit my focus?

[FOCA]

No worries, what model/ engine capacity etc is your Focus?

I forgot to add, you can get replacement sports cats for petrol- engined that are free- flowing, again, better to sort out the rest of the exhaust system / look at other mods 1st

it is possible in theory to remove the cat on a petrol car and still pass a UK MOT, (as long as the emmisions are low enough - which is unlikely) but the the engine might need special setting up with a very clean exhaust output, and probably, a very "frendly" MOT tester, even a petrol rally car has a cat (often a huge one in the tailpipe)

More likely is the cat (petrol engine) gets fitted specially for the MOT and removed afterwards once it has passed, which is, of course a bit naughty and the EML (engine management light) may come on without a signal from the lambda sensor

[wase16ll]

just to chuck my two penneth in, plus clarify a few points.

on a petrol engine

a fully working catalytic will only ever reduce emmissions by an absolute max of 0.5%..and even that is pushing it.

all the work is done by all the other engine managment system bits and pieces...the cat just gives it a final clean...in other words if everything else is working correctly, emmissions will still fly through mot without the cat

having said that, if cat is removed or seen to be tampered with, it will fail, hence why people gut them rather than remove...

Steve made an excellent point in gutting a cat isnt always beneficial due to turbulence created...all depends on size of cat compared to size of pipe exiting.

A single pre-cat lambda sensor system, will not throw up a management light, but a pre and post cat sensor system can...though not always,..there is a way round this with a special sensor wired into the after cat...im doing one currently on a tricked up honda integra

de-catting was quite a popular mod on a lot of early cat systems, but on more modern your much better off with sports type cats, as FOCA suggests, even this is only really benficial if part of a complete system change...the manifold is probably far more important than the rest of the system, but then the rest of the system needs to expel those gasses quicker anyway..pointless having a freeflow manifold on a standerd set up, much the same way as only fitting a sport cat to a standerd set up

should add, as often asked...if a fully working cat only reduces gasses by 0.5 max, why would a failed/collapsed cat increase gasses so much....simple answer....it would be the same as putting a spud up the tail pipe...will totally throw out readings with the lambda sensors

[stevegtuk]

it is possible in theory to remove the cat on a petrol car and still pass a UK MOT, (as long as the emmisions are low enough - which is unlikely) but the the engine might need special setting up with a very clean exhaust output, and probably, a very "frendly" MOT tester, even a petrol rally car has a cat (often a huge one in the tailpipe)

I dont think it will matter how good the the emissions are, unless you are very friendly with the tester, a cat needs to be fitted to modern petrol cars.

as quoted below by VOSA

"Clarification that the check for the presence of a catalytic converter only applies to petrol vehicles that

qualify for a full catalyst emissions test regardless of the result of the Basic Emissions Test."

from this site. (bottom of page 2)

http://www.dft.gov.uk/vosa/repository/Special%20Notice%203-12.pdf

I have thisquote and link saved for when the deisel owners query whether they need a cat