Legality of removing exhaust back box, EGR valve etc.,

Back box has nothing to do with emissions, or does it? Thought that was essentially a silencer.

The valve only reduces the emissions, it'll still pass without it. Just another thing to get the emissions down further and give you a cheaper tax bracket.
 
Ricochet J said:
How do these cars pass their yearly MOTs without failing emissions tests? How are they road legal?
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The MOT emission tests are currently really rather basic.

For Petrol, they simply test HC (Hydrocarbon Unburned fuel) content, and CO (Partially burned fuel if you like) and Lambda ratio. Other gasses are not part of the test as such, but they are measured by the test equipment in order to calculate the Lambda value.

Diesel simply tests the particulate emissions under highly unrealistic conditions (Mashing pedal to max revs under no load conditions)

EGR is primarily about reducing NOX,

On a Petrol car, removing an egr valve will increase the NOX in the pre-cat exhaust stream and change the balance of the other exhaust gasses. NOX however is not part of the test and the changes in the other gasses will not affect the Lambda calculation since the changes will balance each other out in the calculation.

In any case an efficient cat will also do a pretty good job of eliminating NOX from the exhaust regardless of the absence of the EGR system, especially under the artificial MOT test conditions

On a diesel car, Removing EGR will not affect particulates as such (Except that it might actually improve them!!) and current NOX is not tested in any manner at all.

People who De-Cat a petrol car will typically have a cat that thy only fit for the test and remove afterwards. removing the cat has no effect on the engine management operation or on engine function.

On vehicles with a post-cat Lambda sensor though the light will come on unless it is disabled somehow. However, this is just a "For information" thing. Currently the Post-cat Lambda sensor plays no part in engine operation.

As for diesels,

Assuming the engine is running OK, a de-DPF'd car should still pass the basic MOT particulate test (Though anecdotal evidence via Mk1 Eyeball suggests that de-DPF'd modern cars, under real-world driving conditions smoke like mortally wounded Battleships! Far, Far, worse than my 25 year old truck :eek: )

Of course, all this might change in the future, if a "Driving Cycle" type emissions test is introduced it would be much easier to spot vehicles that have been modified in this way.

Unfortunately, this would be very expensive, so I suspect that it is unlikely to happen any time soon.

PS, I misread "Back Box" to mean Cats/DPF. But above still apples.

As far as exhaust systems are concerned, as long as they are not excessively load and are gas tight, there is no MOT issue with non-standard exhaust systems.
 
technically you're not allowed to mod the back box/exhaust in general to increase noise levels, but tbh it really comes down the individual mot tester wether he's going to pick up on it.

afaik they dont really have any equipment to test for engine volume anyway so unless you're blatantly running a straight pipe they tend to look the other way. plus on diesels with dpf's the dpf itself is a pretty effective silencer and you'd need to gut the dpf as well to get any kind of major sound change.
 
You can fail on excessive noise if you remove the back box. But there doesn't seem to be any real rules. Just "tester discretion". Even though it is actually illegal to make a car louder than stock afaik.

Emissions testing is sketchy, but that is inevitably going to change and become much stricter quite soon thanks to VW. Once it happens, cars with gutted DPFs etc will start failing.
 
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It's a mot fail if the exhaust has been changed to make it louder - but this is so vague that it's generally not enforced unless the MOT tester is having a bad day. I could imagine someone arguing that without dB readings of a stock car (and some tolerance range) there's no quantifiable benchmark to test against, so there's no actual "proof" it's louder...

It's also a MOT fail to remove any cat, a lot of modern cars have 2 cats and modders tend to remove the 1st when changing their headers and the rear with a simple straight pipe section, or when changing to a stainless steel exhaust. But, the mot test for removed cats is a visual one, so they just look for a bulging bit in the exhaust (which could simply be empty inside) Without knowing every model of car, how are they meant to know the car should have 2 instead of 1? It's time and effort to go looking for cats, so why bother? I think that the present emissions can usually be passed with only 1 in place, especially if it's been warmed up first. But, with a friendly backstreet mot test station, they can test a different car if their palms have been greased, and you still get a pass... Emission testing if open to abuse, but they can't test against "stock" or book values expected for a stock version of that model car, as VW has recently shown, the book/estimated values for emissions can be wildly different to real world.
 
U know how?? U keep stock exhaust and other parts that wont pass in garage. Swap get mot swap over. Used to do on my civic years ago :)

Or You can have mot pass emissions map if You got custom ecu.

On itr i went with high flow cat for 350 quid and passes no problem also makes exhaust noise more sensible.
 
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DereksDontRun said:
It's a mot fail if the exhaust has been changed to make it louder - but this is so vague that it's generally not enforced unless the MOT tester is having a bad day. I could imagine someone arguing that without dB readings of a stock car (and some tolerance range) there's no quantifiable benchmark to test against, so there's no actual "proof" it's louder...

It's also a MOT fail to remove any cat, a lot of modern cars have 2 cats and modders tend to remove the 1st when changing their headers and the rear with a simple straight pipe section, or when changing to a stainless steel exhaust. But, the mot test for removed cats is a visual one, so they just look for a bulging bit in the exhaust (which could simply be empty inside) Without knowing every model of car, how are they meant to know the car should have 2 instead of 1? It's time and effort to go looking for cats, so why bother? I think that the present emissions can usually be passed with only 1 in place, especially if it's been warmed up first. But, with a friendly backstreet mot test station, they can test a different car if their palms have been greased, and you still get a pass... Emission testing if open to abuse, but they can't test against "stock" or book values expected for a stock version of that model car, as VW has recently shown, the book/estimated values for emissions can be wildly different to real world.
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Yep, the problem with a lot of this is the tester would have to know the car well to know whats missing or altered. Something like an A3 or Golf maybe, they are everywhere. But I took my gt86 to a local garage and he had never worked on one before, so how is he going to know without doing his own research there and then. That and the whole underside is covered by under-trays, which they won't remove anyway.
 
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Nasher said:
That and the whole underside is covered by under-trays, which they won't remove anyway.
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good point about the plastic undertrays with more and more performance cars.

Another factor with cat removal, it's just not common knowledge. I used to have a mk3 MX5 and a common early mod to release a few ponies is the headers (which removes the 1st cat). I was visiting a mx5 specialist (who service, tune and race them - but i don't know if they do mots) and mentioned being interested in headers but was concerned about mot time, and they said it's perfectly fine as the main cat is still there. I explained that afaik law changed in 2012 to include any cat removal, and it seemed to be of news to them, instead of them just playing dumb. So, if a specialist didn't know about the quirks of the recent mot changes, then how can joe public keep up?
 
Clarkey said:
If it still has main cat(s) and it passes emissions then where is the issue?
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The issue is that the current MOT tests the emissions system under highly non-real world conditions. For a petrol engine, 2000 RPM, No load.

Almost any cat system that is working will produce squeaky clean emissions under these conditions.

What it probably will not do is maintain those squeaky clean emissions under a normal driving cycle.

Remove the pre-cats from a modern vehicle and will will easily pass the MOT, but it will almost certainly fail to achieve whatever Euro spec it was originally designed to meet.

Are you bothered?? (Me, not really, I think current regulation is obsessed with chasing a moving target and that we passed the diminishing returns point decades ag, but that is another debate)

As for MOT testing, the only way to test this is to do the full rolling road jobbie. But that would be stupidly expensive for the purposes of annual testing is is only ever likely to happen to a very limited degree.
 
I don't think the 2nd cat is even monitored with sensors most of the time, it isn't on my car. You can pull it out and the ECU doesn't notice.

Someone did a test before and after removing one of the cats and it made practically no noticeable difference to the emissions. There's going to be huge diminishing returns after the first cat. Maybe 2 cats are a requirement in some markets, but it cheaper just to slap them on all their cars than make different sets of manifolds or front pipes.
 
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Nasher said:
I don't think the 2nd cat is even monitored with sensors most of the time, it isn't on my car. You can pull it out and the ECU doesn't notice.

Someone did a test before and after removing one of the cats and it made practically no noticeable difference to the emissions. There's going to be huge diminishing returns after the first cat. Maybe 2 cats are a requirement in some markets, but it cheaper just to slap them on all their cars than make different sets of manifolds or front pipes.
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having some exposure to the automotive sector, I would say it certainly is not cheaper to fit cats to every car, easier maybe, but when it comes to car manufacturing there are so many parts that go in to a car, they certainly wouldb't be fitting additional cats to cars that didn't need them.

My money is on emissions reduction to bring the car in to a lower tax band.
and as said above MOT emissions test is not the same as the emissions tests that these cars have to pass at manufacture
 
I have a hole in my stainless steel back box and I'm not totally sure what'll happen with my MOT?!
 
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