Cat Question

[nimike]

Is there any requirement for a Cat to be fitted to a petrol engine car for MOT? Why I ask, there is a rattle coming from the exhaust system, somewhere at the front (I dont know where) and I think it could be the Cat on its way out. I have the car running on LPG and I can get the emissions test done while running on it, so do I need to have a Cat on the car for anyother reason? E.g. MOT requirement or anything else.

A friend had a 106 xsi which blew the Cat and he ended up replacing it with a straight through pipe. It has went through MOT no probs.

Thanks

Mike

[DaiRees]

Yes, post 1993 or 4 IIRC. There were some early 90's cars produced with cats that weren't legally obliged to have them (a mate's Nova springs to mind ), so you could decat them, but I don't reckon yours would be legal without one.....

I'f I'm talking bolox someone'll come along and correct me shortly

[nimike]

anyone else?

[munfred]

I have known quite a few people with newish cars passing with 'decat' pipes. Really depends on the MOT station. I think they are supposed to fail if it isnt there, but as long as the data that goes to DVLA on emissions reads ok then they have passed them. I have also heard that they are not as important on Diesels, but am not sure if this is true or not.

On the saxo I always put my cat back on for MOT's and take it off again after. Emissions are almost 0 with the CAT on........
With the MG it has a straight through pipe and no cat, but then again it doesnt need to have paid road tax either

Basically I would say check with your local MOT centre that you usually go to and see what they would pass you with, then go with the option they can pass you with.

[Longintooth]

There are some misconceptions with the emission control on vehicles - to understand this you have to view it in two basic parts to keep it simplified.
The CAT is a device that alone can deliver most of the emission control to pass the tailpipe test during an MOT, however, it is not good enough to do this long term without some help from engine controls to limit the things that cause excesses during the driving cycle long term. These controls are sensors you will recognise as - Knock, Lambda, MAF, MAP, TPS and EGR etc. To put it broadly, any one of these sensors can go faulty and although cause a drivability problem can still pass the tail pipe emission test at the MOT station. This is because the CAT will cover the excesses for a short term. So you can regard the ECU OBD2 controls as an early warning of problems that would ultimately cause the CAT to fail, though in most cases because these controls are essential to the proper running of the engine and they will pose running faults such as poor idling,loss of power etc. Conversely the CAT could be replaced with a straight pipe and the ECU could theoretically produce a clean exhaust - enough to pass the test if everything is working perfectly. So as you can imagine the two together act as a belt and braces because nothing is perfect.

So you may read on this site and others, examples of where some contributors will say " well I replaced the CAT with a through pipe and it passed the test" or the "MAP sensor was left off and the engine light was on" but is still passed the test. This is quite true and valid because here in the UK (for now) we only test the emissions with a final tailpipe test whereas in America they will use the OBD2 fault codes to fail a vehicle and they have dedicated test periods for emissions.
So our MOT test has allowed some scope for the tester to overcome some deterioration and climate conditions at the point of test. As the climate change lobby increases we are likely to see much more effort going into prevention than cure and this is why OBD2 should be taken seriously by us. At the moment we have to buy a scanner to read this data but it will come that this will be fitted as standard to all vehicles everywhere - not just a little warning light that "something is wrong"
Hope this helps

[turbolag]

The physical absence of a cat is not a failure point as along as, as aformentioned, the car passes the requisite tailpipe emissions tests (which it is not so likely to do without a cat). Diesels are easiest as they're subjected to a smoke test and the removal of the cat makes zilch difference, but makes a big difference to the speed at which the turbo spools up.

[Longintooth]

but makes a big difference to the speed at which the turbo spools up.

Really - perhaps you would like to explain to everyone how it does this.

[steve_earwig]

Less confrontational please.

I'm guessing less back pressure? Or is it one of those things that're counter-intuitive? A prize to the first person that manages to use the name "Bernoulli"

[turbolag]

I'm not sure, perhaps less pressure between the output side of the turbo and the atmosphere, never thought about it, care even less. Either way, I've decatted my D9 HDi, 306 XuD and Rover 420 diesels and all enjoyed a pepped up turbo response because of it.

I still got the Gtech meter dowloads from the 406 on my old laptop and from memory it knocked something like 4 seconds off the 40-70mph figure in top, solely cos you're not waiting for the tubby to wake up - the improvement is real and measurable.

[Longintooth]

counter-intuitive?

Ok I'll run with that steve. When I polish my 406 bonnet up like glass and clean the flies off the screen it goes like the clappers, or it seems to do!

[teamster1975]

[nimike]

back on subject please.

[steve_earwig]

My brother used to swear that his car was faster when it was clean. I suppose that's why he spent more time cleaning it than driving it

Anyway, that's not really chewy encounterive. I was expecting something along the lines of "if the air flows faster Bernoulli says that the pressure must decrease, so there's less pressure to drive the turbo." If you remove the honeycomb bung out of your eggsauce of course it's going to be faster! Unless it's a 2.1td

[steve_earwig]

back on subject please.

Sorry, I thought your next step was to ask at your MOT place and get back to us

Altough it seems a bit daft to fit something that expensive to a car if it doesn't need it, when you could leave it off and drop the list price a bit (yeah, I know it's bollocks)

[nimike]

.... there is a rattle coming from the exhaust system, somewhere at the front (I dont know where) and I think it could be the Cat on its way out.

back on subject please.

Sorry, I thought your next step was to ask at your MOT place and get back to us

Altough it seems a bit daft to fit something that expensive to a car if it doesn't need it, when you could leave it off and drop the list price a bit (yeah, I know it's bollocks)

Just seeing if there was anyother opinions or experiences out there in the meantime, until the MOT centres open.

Thanks for the replies so far!