Why de-CAT your car?
Discussion
I've seen mention more than once of de-CATing. Can someone explain in simple terms the pros and cons of de-CATing and how it is done?
I'm guessing it refers to removal of the catalytic converter for the purpose of improving performance and facilitating flames from the exhaust, but how is it done exactly? Is an engine re-map needed? What are the implications for getting one's car through the emmissions test during its MOT once a de-CAT has been done? Is it worth doing to any car?
I'm guessing it refers to removal of the catalytic converter for the purpose of improving performance and facilitating flames from the exhaust, but how is it done exactly? Is an engine re-map needed? What are the implications for getting one's car through the emmissions test during its MOT once a de-CAT has been done? Is it worth doing to any car?
MitchT said:
I'm guessing it refers to removal of the catalytic converter for the purpose of improving performance and facilitating flames from the exhaust
The flames are an after-effect and are certainly not the priority (or certainly not for most folks anyway!). The goal is to improve performance by removing the obstruction that the cat creates to the flow of exhaust gases. No cat means the fumes flow straight out with no back-pressure. So extra performance, improved throttle response, a different kind of exhaust note (which most petrolheads prefer), and yes, if your car runs rich enough fire out of the arse.
MitchT said:
But how is it done exactly?
Quite simply, by replacing the section of your exhaust that has the catalytic converter with a section of exhaust that doesn't - i.e. is empty.
MitchT said:
Is an engine re-map needed?
Not mandatory, but recommended if you are chasing every last bhp and lb/ft. In most cases simply removing the cat without a remap will already improve performance and throttle response (the former may not be noticeable, the latter usually is). In some cases you sacrifice low-end torque for top-end power when you decat a car, so it may actually feel more sluggish.
MitchT said:
What are the implications for getting one's car through the emmissions test during its MOT once a de-CAT has been done?
A cat in itself is not required under any law (as long as the lack of one, if it deviates from the standard specification of your car, is declared to your insurers). But a lot of modern cars will not pass an MOT emissions test without one. So though a cat is not technically required, in practice it usually is. "Workarounds" include finding a friendly MOT station that will fraudulently take that particular reading off an emissions-compliant car or sticking the cat back on pre-MOT and removing it again after. In some cars this is easy because the catalyst (and thus, the decat) is a section of exhaust that just comes off. In other cars this is not so easy because the cat is integrated somewhere along the exhaust line and is not sectioned off.
MitchT said:
Is it worth doing to any car?
Depends on the car and what you want. I did it on one of my old cars and it was worth it - much improved throttle response and a better sound. An alternative to consider is a high-flow cat, which is a halfway house between a regular OEM catalyst and a decat. It does what it says on the tin - the mesh is less dense (and therefore cleans the fumes of toxic particles less effectively) and hence gives a performance increase but usually cleans enough to get you pass the MOT.
Edited by -DeaDLocK- on Friday 15th December 07:39
Thanks DeaDLocK – Brilliant reply!
Might not suit my 318is then
I'd like better acceleration from my 318is (particularly at the low end, hence why it might not suit) and to be able to avoid the crippling expense of replacing the CAT should it ever fail. That's always a worry, for me anyway... being a peasant! Perhaps I should have a look and see if it's an easy swap for a non-CAT pipe and maybe try it, remembering to switch back to the CAT for my MOT.
-DeaDLocK- said:
In some cases you sacrifice low-end torque for top-end power when you decat a car, so it may actually feel more sluggish.
Might not suit my 318is then
-DeaDLocK- said:
Depends on the car and what you want.
I'd like better acceleration from my 318is (particularly at the low end, hence why it might not suit) and to be able to avoid the crippling expense of replacing the CAT should it ever fail. That's always a worry, for me anyway... being a peasant! Perhaps I should have a look and see if it's an easy swap for a non-CAT pipe and maybe try it, remembering to switch back to the CAT for my MOT.
Edited by MitchT on Friday 15th December 08:02
victormeldrew said:
My car is decatted yet gets through its MoT without a need for fraudulent MoT testers. I've never informed any of the testers that the cats are missing but it passes anyway.
but yours doesn't need a cat does it..? because the chim had already been in production etc etc blah blah.
anyone please (politely!) correct this!
MitchT said:
...how it is done?
I once had 4 litre Jeep.
The catalyst started breaking up and rattled at tickover.
I crawled under the car, unscrewed the lambda sensors, and removed the exhaust.
Then I knocked the guts out of the catalytic converter using a big screwdriver, a hammer and a jack handle. The crap that came out was unbelievable.
Then simply replaced the exhaust, including the empty catalyst chamber, screwed back the lambda sensors and the job was done.
The Jeep already had a Powerflow cat-back exhaust, so with the catalyst removed it sounded completely wild. Whether it was faster or not was hard to tell, but the pops and bangs on the overrun were great.
I sold the Jeep before it was due an MOT, so I have no idea whether it would have passed the emissions test, but the law doesn't demand a catalyst.
I am looking forward to the catalys failing on my 11 year old BMW so that I can repeat the exercise....
)baSkey said:
victormeldrew said:
My car is decatted yet gets through its MoT without a need for fraudulent MoT testers. I've never informed any of the testers that the cats are missing but it passes anyway.
but yours doesn't need a cat does it..? because the chim had already been in production etc etc blah blah.
anyone please (politely!) correct this!
It's also entirely possible with a TVR that the tester might not know exactly what the car is, and test it based on it being a kit car or amateur built vehicle - and some may suggest even if they DO know what the car is they might argue it falls within this category anyway
. These require only a visual test (exhausts, yes I see two, check!). At the end of the day I have the original catted Y-piece tucked away and could fit it in a half hour if it did fail, but that hasn't been required in the last eight years.
MitchT said:
I'd like better acceleration from my 318is (particularly at the low end, hence why it might not suit) and to be able to avoid the crippling expense of replacing the CAT should it ever fail. That's always a worry, for me anyway... being a peasant! Perhaps I should have a look and see if it's an easy swap for a non-CAT pipe and maybe try it, remembering to switch back to the CAT for my MOT.
A cat for that car would surely only cost about 100 quid which can't be that much more than a cat pipe. I can't believe you're going to gain anything tangible by dropping the cat and you'll want to measure it to be sure because it strikes me that many people mistake an extra bit of noise for an extra bit of power. If you want to try it on the cheap than get down a scrappy and get an old cat. Cut the ends of and weld some pipe between them then fit it and see if it accelerates from 50-70 any quicker. If it doesn't then you've not lost much money.
victormeldrew said:
baSkey said:
victormeldrew said:
My car is decatted yet gets through its MoT without a need for fraudulent MoT testers. I've never informed any of the testers that the cats are missing but it passes anyway.
but yours doesn't need a cat does it..? because the chim had already been in production etc etc blah blah.
anyone please (politely!) correct this!
It's also entirely possible with a TVR that the tester might not know exactly what the car is, and test it based on it being a kit car or amateur built vehicle - and some may suggest even if they DO know what the car is they might argue it falls within this category anyway
. These require only a visual test (exhausts, yes I see two, check!). At the end of the day I have the original catted Y-piece tucked away and could fit it in a half hour if it did fail, but that hasn't been required in the last eight years.
ok thanks for explanation. icouldn't remeber the years that emissions changed and i did actually think (incorrectly) that models already in production were subject to the std of the year it was introduced.
The whole catalytic converter thing seems dubious to me anyway.
youre not reducing the crap being produced, youre just storing it up in one place (the converter). Fine if they get disposed of differently, it would actually result in less crap in the atmosphere, but from what ive seen they all end up just sitting in a huge scrapyard with everything else, so wheres the benefit? youre just putting all the waste in one place and not treating it differently.
youre not reducing the crap being produced, youre just storing it up in one place (the converter). Fine if they get disposed of differently, it would actually result in less crap in the atmosphere, but from what ive seen they all end up just sitting in a huge scrapyard with everything else, so wheres the benefit? youre just putting all the waste in one place and not treating it differently.
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