- My model of MX-5 is: NC 2.0 Sport__
- I’m based near: North West
- I’m looking for technical help or recommendations on: I have searched all the threads on exhaust mods etc and cannot find a definitive answer to my question. I have recently completed my first track day which I thoroughly enjoyed. At present, my 2.0_2006 Sport is power and exhaust wise bog standard. I am looking for some more power and maybe some more noise. I cannot afford the extortionately priced supercharger or turbo kits so was thinking of manifold/exhaust and remap route. My question is, as a road car, can you remove either one of the cats and still pass emissions tests? If I was to buy a catless SS manifold and retain the second cat, would that work?? Or am I better getting a manifold with cat and removing the secon cat?? And what problems might I encounter either way?? Thanks in anticipation!!
Mot rule is that if the car came with 2 cats there should be 2 cats there. That said most mot testers don’t look for them and are more interested in the emissions result, which the cars tend to pass no problem with one cat missing.
It’s your choice. If you remove a cat just might be worth keeping it incase you need to put it back
Thanks for that, but do you think the second cat will do the job if I get a cat free manifold and retain the rest??
The centre standard one works fine and no it’s not a failure if you remove one. Despite what people say. From experience extensive research and indeed confirmation from DVSA.
Saying that I am sure people will tell you it is. 
Interested to see your DVSA confirmation because their web site says:
It is not always obvious to vehicle owners that some modifications will make the vehicle illegal to use on the road, irrespective of whether the insurer is prepared to provide cover. For example, removing a catalytic converter or diesel particulate filter will almost certainly result in a vehicle’s emissions exceeding **emission type approval limits and make the vehicle illegal to drive on the road. Engine re-mapping may also result in non-compliant emissions.
It is the driver who is responsible for ensuring compliance.
It’s unlikely that an mot tester under the current regs would notice and almost everything passes the mot emissions test but I suspect that things will continue to be tightened and the requirement is changed to meet the type approval limits as the years go by and pollution becomes even more of a problem.
Catalysts work better when they are heated to operating temperature. This is why we generally have two in these cars.
If you do an emissions test with just one of them and the car is cold it will fail the test. However the test instructions specify the car must be warmed up to operating temperature to do the emissions test.
Bypassing the first catalyst means you need to move the oxygen sensor after the second catalyst - if the ECU flash an issue on the dash that’s an MOT failure. IMHO its not worth the hassle.
Should this topic be closed now or do we give it a week? 
That’s good news.
All I see is people saying the tester can fail it on a visual check if one is missing. Can you share details of the info you have? I think this would help many people including me.
Thanks
Robbie is correct, this topic has been debated to death before and indeed a few beers were taken afterwards.
Mine with front catalytic converter removed and other exhaust mods gets a probe up its rear end every year and passes the “Emissions Test”.
At the end of the day is that not what it’s all about? (in my opinion).
VAG (and indeed others) were very, very, very naughty boys for the vehicle Type Approval.
People should do what they feel most comfortable with.
I can give my take on it here if you so wish, (probably not).
So! Just to conclude then, you pays your money and takes your choice. But, in essence, these catless manifolds ‘should’ only be used on the track and not for road use. Never mind the insurance issue…if you do not inform the Insurance, then your policy is in effect nullified. So, again, without spending thousands on turbo/supercharger/throttlebody kit, then there is little I can do to noticeably increase the power of my 2006 2.0 sport!!
i recently asked an MOT tester about removing one of the cat’s and he said “aslong as i can see atleast one its ok and i will pass it”
however he did point out that this was his personal ethic and another tester might say differently.
lets be honest that chances of you getting a tester that knows that your NC should have 2 cats is pretty low.
so aslong as it passes emissions you should be alright.
that being said.
i myself am not prepared to risk it so i will be getting a catted header from BBR, its “only” £200 extra and for me its easily worth the 200 extra to not have to spend a couple of weeks worrying about it before each MOT.
but again this is personal choice.
Good point. So would you remove the second cat?? 
me personally NO.
i suffer from anxiaty and not having both cats would cause me to worrie about it before every MOT, so for me the extra 200 is worth it, if only so that i dont have to worry!
So the BBR catted manifold is the way to go. BBR say it is MOT compliant while the MX5parts IL one does not!! As for the second cat…mmmmm??
i think your looking at the decatted one mate!
the catted one says that it is mot compliant
Yep, that’s what i said. BBR is compliant but MX5parts say this…
PRODUCT DESCRIPTION
Produced by IL Motorsport, we have here a seriously high quality, fully stainless steel 4-1 manifold with a 200 cell race catalytic convertor. Tested in house on their own dyno, this manifold has been designed to give maximum air flow and in turn maximum horsepower. Comes supplied complete with a manifold to downpipe gasket. (Manifold to head gaskets are supplied seperatly should you need one).
Fits all Mk3, Mk3.5 & 3.75 models, 2005>2015
Please Note: Due to current regulations these are being sold for off road use only and may not pass a UK MOT.
If you consider your original question about power, it’s definitely not worth removing the cat as any power difference is negligible. If you want more noise, you’re better looking just at the back box.
Unfortunately, increasing power pretty much means forced induction.
Hello Drewby
My 2014 Sport Tec Recaro has had the Roddisons Motorsport modifications:-
4 into 1 manifold, 200 cell cat, stainless steel centre pipe with small expansion box,
plus a custom made final box with centre exit twin pipes. (Jota type system).
I have had the car for 4+ years, get the exhaust hot, it sales through MOT’s
Roddisons Motorsport 0114 244 5300 Sheffield S9 1US ask for Paul.
Regards
Keith
ps. I am based near J34 M6 if you want any further info.
sorry my mistake i misread your comment mate.
sadly it happens quite alot because of my dislexia.
ok il motorsport header!!!
ERRRRRRRMMM!!!
bit of a head scratcher that one!
its £300 cheeper than the BBR.
it does have a cat, so it should be ok for MOT
so why are saying it might not meet mot standards?
im taking a wild guess here, a VERY wild guess but maybe the cat thats on it just isnt very good?
then again aslong as you midpipe cat is ok, your emissions should be a pass.
cars can pass emissions with only a mid cat anyway so a header with a fake/■■■■ cat shouldnt make a difference!
This should get the tongues wagging and PM’s sent! 
The first question you have to ask yourself is this.
“Why are catalytic converters fitted”?
To reduce the final emissions output from the vehicle into the atmosphere to comply with the standards set at the time of the type approval.
The type approval (emissions wise), is a lengthy robust (allegedly) process to measure the final output from the engine.
This is all dependant on the build of the engine, induction system, fuel system, exhaust system, oil used and indeed fuel used.
Therefore manufacturers will fit either one, two (or more) catalytic converters to achieve the type approval.
Fuel has come on a long way since the initial unleaded fuel and indeed the mandatory introduction of the CC in 1992.
Are Supercharged, Turbo charged MX 5’s all illegal then because the exhaust systems have been modified from standard?
No is the short answer.
Notice BBR will tell you it’s illegal to put a decat header on your MX5, but willingly sell you a stage one or two BBR Turbo kit at great expense which has completely different manifolds on from standard.
Does fitting the 2.5 engine and indeed exhaust system make your car illegal because you have modified it from standard?
Are the MK 1’s and 2’s all illegal because people have swapped the OEM expensive CC’s?
No of course not.
One could argue that modifying your brakes, suspension, steering or whatever else makes it illegal because you have modified from the standard type approval.
As long as you declare the modifications to your insurer they are fine with it all. Some will cover you, some will not.
The MOT tester is NOT allowed to use pre knowledge of a vehicle for the test.
So he/she will see that a catalytic converter (as by law, date and vehicle specific) is there and at the end of the test conduct the emissions test as per the current UK rules, (which are compliant as the EU by the way).
My reply/s from DVSA regarding questions about modifying exhaust systems in general and not specific to the MX5.
I guess some people will interpret differently to others, that’s life.
How on earth can a tester know and indeed measure the emissions that were part of the type approval process?
Impossible, because every engine will be slightly different for one thing and how will they know what the type approval process was anyway.
Obviously each owner has to make their own decisions regarding what they do/don’t do.
Good luck. 
The MOT emissions test cannot possibly replicate the type approval emissions test which is far more complex and involves laboratory testing as well as a chassis dynamometer test. Type approval also tests for more pollutants than MOT test, such as oxides of nitrogen (NOx) and particulate matter.
The MOT test for emissions is therefore a simplified test of only certain emissions that can be readily conducted in a workshop. The emissions values used are the same across the European Union to comply with the European Roadworthiness Directive.
For the purpose of establishing official fuel consumption figures, there used to be a 40 second engine warm up period before emissions sampling began. This warm up period was eliminated from the year 2000. There is also a 40 second timing requirements for the exhaust catalyst to reach a certain temperature.
Best regards
Shaun Martin | Policy Specialist (MOT Testing Service)
Driver and Vehicle Standards Agency | Berkeley House, Croydon Street, Bristol BS5 0DA
"Exhausts can be a problematic area in the MOT test. This is largely because the test is based upon the relevant legislation and this sometimes makes it difficult to assess compliance where modifications have been made or aftermarket components are fitted.
Firstly, it is illegal to replace or modify an exhaust so that it increases the noise made by the escape of exhaust gases (Road Vehicles Construction and Use Regulations 1986 -Regulation 54)
It is also illegal to make modifications to a vehicle that increases the levels of exhaust emissions (Road Vehicles Construction and Use Regulations 1986 -Regulation 61A). This is not just the emissions checked at MOT test, but all emissions that that must be met as part of the type approval process.
Therefore, if an aftermarket stainless steel exhaust is fitted, the MOT tester will need to assess that the exhaust noise is not unreasonably above the noise level you’d expect from a similar vehicle with a standard silencer in average condition. They would also check that no emissions control components are missing, obviously modified or obviously defective.
It is important to note that the regulations do not prevent modifications or the use of aftermarket components, including exhausts. The components are merely required to perform as well the OE components in respect of noise and emissions. If the design of the exhaust uses one catalyst where there were originally two, this is not necessarily an MOT failure, provided the system still meets the same standard for noise and emissions as the OE components did when in a serviceable condition. It may be necessary to show a receipt or other documentation to prove this, but like you say, most testers will not know how many catalysts were originally fitted.
In all cases, where there is doubt about whether a defect exists, testers should give the benefit of the doubt and pass the vehicle.
I hope this helps ”
Best regards
Shaun Martin | Policy Specialist (MOT Testing Service) Driver and Vehicle Standards Agency | Berkeley House, Croydon Street, Bristol BS5 0DA
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