"decatting" or removing baffles from the backbox...

[grainqey]

Hello,
I have a couple of stupid ideas - as you do when your a yobo with a W215 CL

firstly, how much do the precats actually do compared to the main cats? are they sufficient to pass an mot with a hot engine for example or do you need the main cats?

secondly, i've not got resonator boxes - just cats then the backboxes which i've had opened up with the intention of making it a bit louder but i welded them back up and have decided to dig a bit more before vandilising them - i've got a spare catback coming so i can chop at will soon...
can you just remove the baffle in the middle of the backbox between the two halves or what? i thought about cutting out the "S" of perforated pipe inside the backbox and replacing with a straight piece of perforated pipe
OR
would the removal of the main cats increase the volume without bringing the engine management light on?

thanks in advance
Alex

 

[Howard]

Currently it will pass on just the pre cats.

However , the new MOT rules coming in ( may already be in ) say that a car must have the same amount of cats it left the factory with ( not sure how they know how many every car has , maybe a list somewhere they refer to ? ) .... if not , it should fail.

Way around this is to cut the cats open , take out the innards and weld them back up.

 

[grainqey]

thanks for that Howard
i've cut cats out before so out comes the rear cat innards lol
thanks
alex

 

[MicB]

On my SL I removed the mid boxes as well as the secondary cats......passed three MOT's since.....in fact the difference in the emissions is not discernible by standard MOT testing equipment.

Mic

 

[Howard]

Changing soon though, cats need to be physically there.

 

[Gkinghorn]

Changing soon though, cats need to be physically there.

I have heard a few folks say this... I don't believe it. I done same as Mic and its passed enough... Go talk to folks at Halfords/kwik fit etc who do the MOT they haven't a clue how many cats were on your SL when it left the factory. And I did ask the guy who did mine the last time if he knew.... All he said was he saw a bit of pipe work had been replaced but emissions were fine and that was all he cared about.

And supposing you replaced all 4 of your cats with a new system which was better but only consisted of 2 cats there is no way it would fail simply because it came out of the factory with 4 ... This test is about emissions ....

Sorry but I hear this and think its just an over zealous interpretation of the rules (which are in btw and my car has passed since) ...

 

[Howard]

I didn't know they were in already , didn't know that Hellfrauds or Krapfit did MOTs either ....

Only takes someone to fail it though and you've got a big bill for buying cats to put back on suspect there will be a crackdown at some point , and a reference manual for each car issued.

Safer to empty the cats out and leave the shells in place surely ?

 

[grainqey]

thanks for the replies, ive heard about the rule change but unless they have a big pdft file or book then i dont see how they can enforce the rule. Thanks. Alex p.s ''posted on my stupid nokia that doesn't do formatting text''

 

[Gkinghorn]

I didn't know they were in already , didn't know that Hellfrauds or Krapfit did MOTs either ....

Only takes someone to fail it though and you've got a big bill for buying cats to put back on suspect there will be a crackdown at some point , and a reference manual for each car issued.

Safer to empty the cats out and leave the shells in place surely ?

If you are running scared then sure empty them ... But the MOT is about emissions and capability not the numbers of things... As I said have heard it all before ...it's a wrong interpretation of a badly worded rule...

In terms of Halfords or kwikfit doing an MOT is easy enough for them... Knocking them for some things is fine but they are capable of doing mots ..after all the staff are certified to be able to do to and an MOT is car independent ....

 

[hotrodder]

FWIW Section 7.1.3 of the MOT testers manual says...

On vehicles that qualify for a full cat emissions test, check the presence of the catalytic converter.

and gives the following as reason for rejection...

A catalytic converter missing where one was fitted as standard

Exhaust emissions for spark and compression ignition are under sections 7.3 and 7.4 respectively

They've got lots of charts and a computer database at their disposal, hell for many post '02 cars the emissions are tested against vehicle specific data that the 'puter spits out when the tester logs in to start the test. Whether a 'number of cats' database has been added yet or is 'in progress' i don't know and of course it depends on how zealous the tester is... big chains more likely to be 'by the book' imo

MOT Testers Manual Note there are various pdfs scattered around the thing with charts and special notices section in the tabs at the left

 

[Howard]

If you are running scared then sure empty them ... But the MOT is about emissions and capability not the numbers of things... As I said have heard it all before ...it's a wrong interpretation of a badly worded rule...

In terms of Halfords or kwikfit doing an MOT is easy enough for them... Knocking them for some things is fine but they are capable of doing mots ..after all the staff are certified to be able to do to and an MOT is car independent ....

Ha ha ! Running scared ! Of the man eh ?

Just being sensible achieve the same outcome with no risk , what's not to like ?

Unless you like to show the underside of your car off on ramps ? ' look at my shiny straight through pipe ! ' ?

Anyway , you appear to know best , so you do it your way and I'll do it mine

 

[BlackC55]

We have removed centre Cats for about 50 odd cars now and not one has failed the MOT. When the ss pipes get a little older they look OEM and you would be hard puhed to tell if it has been de-catted at all.

 

[marc.l]

Just gutting exhaust boxes and cats never work very well , will slow down exhaust gasses and more than likely to cost hp , much better to replace with a straight pipe .

 

[grainqey]

what about welding a piece of pipe inside? lol

...sounds like hard work to me but would be the best for air velocity - think i'll just cut it open and abuse it with a hammer tbh.
the only adverse effects i've witnessed when gutting a cat was on a T5 where you got alot of wooshing resonating in the empty cat from the turbo spooling up
thanks
alex

 

[Gkinghorn]

Ha ha ! Running scared ! Of the man eh ?

Just being sensible achieve the same outcome with no risk , what's not to like ?

Unless you like to show the underside of your car off on ramps ? ' look at my shiny straight through pipe ! ' ?

Anyway , you appear to know best , so you do it your way and I'll do it mine

Didn't say I knew best..simply pointed out my experience and views... Tbh your response is a bit on the condescending side to someone who simply contributed information to your question... Disappointing..

 

[DSB SL AMG]

my old CL500 went straight through the MOT last August (new rule in place) no issues, car had secondary de-cat straight pipe...i think the rule will be hard to enforce unless MOT station can verify the design of each vehicle system...i think this would be hard to execute.

 

[covlad]

Currently it will pass on just the pre cats.

However , the new MOT rules coming in ( may already be in ) say that a car must have the same amount of cats it left the factory with ( not sure how they know how many every car has , maybe a list somewhere they refer to ? ) .... if not , it should fail.

Way around this is to cut the cats open , take out the innards and weld them back up.

agreed take the inner stuff out and then hope it passes but it will be noisy