Removing Catalytic converters, legality ?
Discussion
swerni said:
DrSteveBrule said:
I bought a secondhand car recently and the previous owner didn't mention it had been decatted. When facd a potential bill of £5k for a new oem exhaust system, my garage said it *may* pass with just the pre-cats. I didn't want to risk it so i'm having cats put back on. Longlife Exhausts are building a new system for approx £1400.
Why not get it tested then decide ?Fastdruid said:
swerni said:
DrSteveBrule said:
I bought a secondhand car recently and the previous owner didn't mention it had been decatted. When facd a potential bill of £5k for a new oem exhaust system, my garage said it *may* pass with just the pre-cats. I didn't want to risk it so i'm having cats put back on. Longlife Exhausts are building a new system for approx £1400.
Why not get it tested then decide ?So what if it fails, at least you'd know and if it passes you've saved £1400!
I'd say it depends on the car, but if the sensors are pre-post the 'pre-cats' (actually the main cats?), then you would be fine.
If the sensored cats are missing, then that might cause issues as the 'pre-cats' might just be that and not sufficient to score you a pass.
I'd just get sports cats fitted rather than paying through the nose for OEM ones on an old car. Cheaper and no doubt better.
J4CKO said:
Keep seeign cars advertised as having been decatted but they are newer than the cut off of 1991 (I think ) for the emmissions regulations, can some cars pass an MOT without a cat or would it be a case of refitting them/it come MOT time ?
Is there any legal requirement to have a cat in place as long as you meet the emmissions regs ?
The rules are changing on this but if it's had sports cats fitted then it's fine also if they've been removed properly most can be bolted back in but check first.Is there any legal requirement to have a cat in place as long as you meet the emmissions regs ?
As a sidebar if you take the cats away from an N/A you lose back pressure which depending on what else you've done to the exhaust could result in a loss of BHP.
Yeah I thought it was all about expected back pressure changing the fill rate of the cylinder during that small overlap period, and maybe having more of the fuel air mix going out the exhaust vs before.
So a bit lower pumping loss but also more fuel down the exhaust, maybe.
Complicated stuff... makes you wonder how much aftermarket exhaust people actually consider
So a bit lower pumping loss but also more fuel down the exhaust, maybe.
Complicated stuff... makes you wonder how much aftermarket exhaust people actually consider
ChemicalChaos said:
That's interesting to know, as my RR is on LPG. However worth bearing in mind - a lot of modern cars will apparently flag a dash warning light if you remove the cats as their lambda sensors etc detect values now being out of limits - or so I'm told about the 3rd Gen RR on model specific forums.
And of course, a warning light is now an MOT fail too.
It will usually only come on when you plant the throttle, not under normal driving conditions. Easy enough to clear the warning code with an obd reader.And of course, a warning light is now an MOT fail too.
Are we talking cats here or silencers as the majority of cars only have 1 cat per exhaust & what tends to be removed are the mid silencers to be replaced with a straight through? Cats can also be changed for straight throughs ( I did on my 24v Senator) but I kept the cats & put them back for the MOT.
Mr Whippy said:
So how does back pressure add power to an engine?
I'm not an engineer, this is only me repeating what I've read in the past:Exhaust gases travel back along the exhaust system in compression waves or pulses and the points of higher pressure/lower pressure oscillate up and down the system. Sections of the exhaust are tuned (by having the correct length) so that an exhaust pulse is in the right place at the right time. The right time could be a high pressure spot just behind the valve when it opens so that the cam has to do less work against to open the valve; the higher pressure spot is then bouncing back along the exhaust header away from the engine creating an area of lower pressure that helps scavenging. Reduced power loss. Combined with a degree of valve overlap, the intake valve is also partially open so the then lower pressure in the cylinder compared to the intake helps accelerate the intake charge before the valve is fully open, allowing a better cylinder fill (increasing volumetric efficiency). More air and fuel, more power.
I think. Feel free to correct any inaccuracies.
Winky151 said:
Are we talking cats here or silencers as the majority of cars only have 1 cat per exhaust & what tends to be removed are the mid silencers to be replaced with a straight through? Cats can also be changed for straight throughs ( I did on my 24v Senator) but I kept the cats & put them back for the MOT.
Cats, my Lexus has 2 primary and 2 secondary cats, as do lots of cars.xRIEx said:
Mr Whippy said:
So how does back pressure add power to an engine?
I'm not an engineer, this is only me repeating what I've read in the past:Exhaust gases travel back along the exhaust system in compression waves or pulses and the points of higher pressure/lower pressure oscillate up and down the system. Sections of the exhaust are tuned (by having the correct length) so that an exhaust pulse is in the right place at the right time. The right time could be a high pressure spot just behind the valve when it opens so that the cam has to do less work against to open the valve; the higher pressure spot is then bouncing back along the exhaust header away from the engine creating an area of lower pressure that helps scavenging. Reduced power loss. Combined with a degree of valve overlap, the intake valve is also partially open so the then lower pressure in the cylinder compared to the intake helps accelerate the intake charge before the valve is fully open, allowing a better cylinder fill (increasing volumetric efficiency). More air and fuel, more power.
I think. Feel free to correct any inaccuracies.
But the exhaust cat > back and the concerns probably get a lot less important.
Mr Whippy said:
xRIEx said:
Mr Whippy said:
So how does back pressure add power to an engine?
I'm not an engineer, this is only me repeating what I've read in the past:Exhaust gases travel back along the exhaust system in compression waves or pulses and the points of higher pressure/lower pressure oscillate up and down the system. Sections of the exhaust are tuned (by having the correct length) so that an exhaust pulse is in the right place at the right time. The right time could be a high pressure spot just behind the valve when it opens so that the cam has to do less work against to open the valve; the higher pressure spot is then bouncing back along the exhaust header away from the engine creating an area of lower pressure that helps scavenging. Reduced power loss. Combined with a degree of valve overlap, the intake valve is also partially open so the then lower pressure in the cylinder compared to the intake helps accelerate the intake charge before the valve is fully open, allowing a better cylinder fill (increasing volumetric efficiency). More air and fuel, more power.
I think. Feel free to correct any inaccuracies.
But the exhaust cat > back and the concerns probably get a lot less important.
I don't know about more important/less important.
ETA: there's a section about "mufflers" at about 20 or 21 minutes in.
EETA: at 26:30ish he says the tailpipe isn't massively important.
Edited by xRIEx on Friday 1st April 15:36
DrSteveBrule said:
Because I like my cars to be as original as possible. I'd also rather face an MOT knowing there's a good chance it will pass rather than a slim chance.
Ask around car forums for your area and I'm certain you'll find an MOT tester who will pass your car's emissions regardless of whether it has a cat or not. The probe will be going nowhere near your car's exhaust that's for sure. Not condoning it, but I know for a fact plenty of it goes on. Like you say in the OP, how else do so many de-catted cars pass MOTs.Krikkit said:
Nope, still stupid. If it passes the test with the pre-cats in place why spend £1400 rather than £0?
Because as well as being decatted he had fitted a stainless steel centre section which is a little too loud for my liking. When I bought the car I thought it would be an easy fix to put back to stock although I wasn't counting on the cats being missing too.It's not just the emissions that are driving the purchase, I want to get rid of the sound too. Even it passed the test as it currently stands, I still want the exhaust back to how it should have been originally.
JagerT said:
DrSteveBrule said:
Because I like my cars to be as original as possible. I'd also rather face an MOT knowing there's a good chance it will pass rather than a slim chance.
If you want it to be as original as possible why are you not fitting an OE system then ?£1400 is comparatively cheap to get a near-OEM spec system fitted. That's everything from the pre-cats back that's being replaced.
Section 7.1 of the mot testers manual
7.1
Exhaust System
Information
Method of Inspection
Reason for Rejection
The overall security of the exhaust system
1. Check the security of the exhaust system. 1.
An exhaust system not adequately supported.
should be assessed; one or more exhaust mountings missing may not be a Reason for Rejection.
2. With the engine running, check the exhaust 2. system for leaks.
A major leak of exhaust gases from any part of the system.
A durable repair to an exhaust system which effectively prevents leaks is acceptable providing the system is structurally sound.
3. On spark ignition engine vehicles that qualify for 3. a full catalyst emissions test and all
compression ignition vehicles check the
presence of catalytic converter(s) and
A catalytic converter or particulate filter missing where one was fitted as standard.
A minor exhaust leak from, for example, a connection joint or a pin hole, is not a Reason for Rejection.
particulate filter(s).
Note: To ascertain whether a spark ignition engine vehicle qualifies for a full catalyst test, use the flow charts in Section 7.3.C disregarding the result of the Basic Emissions Test.
4. During all parts of the emissions test relevant 4.
for the vehicle, assess subjectively the
effectiveness of the silencer in reducing exhaust
noise. similar vehicle with a standard silencer in
Note: For vehicles not subject to an emissions test, the engine should be revved to around 2500rpm or approximately half maximum engine speed if this is lower.
average condition.
5. On Class 5 vehicles only, check that exhaust 5. fumes are not entering the driver’s or
passenger area.
Exhaust fumes entering the driver’s or passenger area.
A silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a
7.1
Exhaust System
Information
Method of Inspection
Reason for Rejection
The overall security of the exhaust system
1. Check the security of the exhaust system. 1.
An exhaust system not adequately supported.
should be assessed; one or more exhaust mountings missing may not be a Reason for Rejection.
2. With the engine running, check the exhaust 2. system for leaks.
A major leak of exhaust gases from any part of the system.
A durable repair to an exhaust system which effectively prevents leaks is acceptable providing the system is structurally sound.
3. On spark ignition engine vehicles that qualify for 3. a full catalyst emissions test and all
compression ignition vehicles check the
presence of catalytic converter(s) and
A catalytic converter or particulate filter missing where one was fitted as standard.
A minor exhaust leak from, for example, a connection joint or a pin hole, is not a Reason for Rejection.
particulate filter(s).
Note: To ascertain whether a spark ignition engine vehicle qualifies for a full catalyst test, use the flow charts in Section 7.3.C disregarding the result of the Basic Emissions Test.
4. During all parts of the emissions test relevant 4.
for the vehicle, assess subjectively the
effectiveness of the silencer in reducing exhaust
noise. similar vehicle with a standard silencer in
Note: For vehicles not subject to an emissions test, the engine should be revved to around 2500rpm or approximately half maximum engine speed if this is lower.
average condition.
5. On Class 5 vehicles only, check that exhaust 5. fumes are not entering the driver’s or
passenger area.
Exhaust fumes entering the driver’s or passenger area.
A silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a
DaveCWK said:
Worth mentioning also that any car converted to LPG, regardless of age, will only be tested to the old non-cat standards (which pretty much any fuel injected car will achieve if it's working properly) - 1200ppm HC / 3.5% CO at idle iirc.
I'm going to drag this back up again. If I take my 94 rr which has cats and replace the exhaust with a stainless system complete with lambda sensors etc without cats and present it for MOT running on LPG will I fail?It will pass the emissions no problem but does the physical lack of cats mean an automatic fail?
I've read the regs and as far as I can see it should (and would if running on petrol) but on lpg its not having a cat test so they don't go looking for cats.
Or do they?
Confused.
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