Catalytic converter

[DereksDontRun]

It’s all to do with emissions but a plus is that most of the time the afr is at a word I cannot pronounce or spell lol 14 parts of air to 1 part of fuel, the golden ratio where there’s enough air to completely burn the fuel..... more power. Now the car makers need to concentrate on mass production n/a engines with 100bhp per liter.

stoich, and i think it's 14.7:1 for petrol and 14.5:1 for the wrong-fuel

 

[Clarkey]

On modern cars removing the cat doesn't really gain a whole lot anyway. It'll make it louder and stink.

Not really true, turbos hate cats, almost everything is turbo nowadays.

The afr is not set the ecu adjusts the afr using the o2 reading from post cat. Too much o2 and it will increase fuel it’s constantly being adjusted aiming for 14:1

Not necessarily true either, only when running in closed-loop. Unless things have changed since my day, wide open (where good tune is most critical) will be running open-loop.

 

[DereksDontRun]

^ I thought the same. Closed loop is when the engine is warm and not full throttle. At WOT (wide open throttle) it's in open-loop and uses a look-up table to squirt the amount of fuel it "thinks" it needs from the air-flow sensor telling it how much air/o2 is entering, compared with the engine's RPM.

Tuning is effectively adjusting these look-up tables, to squirt a different amount of fuel into the engine for each rpm and air-flow input. Piggybag ecu's do the same by manipulating the air-flow sensor and tricking the ecu into thinking more (or less) air is entering the engine than is actually happening - so they rely on the stock look-up table (which is why a proper remap is better, as it rewrites the look-up table with "ideal" values, instead of manipulating a table and hopefully getting close (not to mention the map is to that specific engine, which will have different real-world characteristics due to tolerances in all the components).

i think...

 

[sideways14a]

I always try and get decats that can be pulled off and put back on quickly for those pesky MOT visits.

There is a lot of power to be had from a decat and remap.

 

[red eye]

I see. So is it easy to just take off and put back on then?

Depends on car some are easy like fiat puntos some are a pain like mk1 focus st.

And if it’s an old cat being removed for the first time then it’ll take a lot of swearing, rounding off the fasteners, snapping studs, heat and maybe 999 on speed dial.

Mild steel, heat and water do not mix well

 

[red eye]

Not necessarily true either, only when running in closed-loop. Unless things have changed since my day, wide open (where good tune is most critical) will be running open-loop.

I would have thought the ecu to be constantly collecting data from at least the after cat lambda without that data it wouldn’t know where it is on the maps ie when accelerating and deceleration.

 

[Liquidfox]

Most 200cel cats are marked for Motorsport use only so they shouldn't really be on either - albeit if it passes emissions it's unlikely to be particularly scrutinised

That'd be the 100 cell cats, the 200s I've seen haven't been marked and pass just fine.

The afr is not set the ecu adjusts the afr using the o2 reading from post cat. Too much o2 and it will increase fuel it’s constantly being adjusted aiming for 14:1

In a N/A engine, sure. A turbocharged engine will be far lower. The map on my Focus aimed for 11.5:1 others ranged from 10.5-12.5:1 depending on the tuner.

 

[DereksDontRun]

It also depends where the cat is.

Some cars have them along the bottom of the car, and easy to access. Others have them on the headers/downpipe (a curved set of tubes coming out the engine to the bottom of the car) - this can locate the cat within the engine bay and a lot harder to get at. And, a lot of cars now have them in both locations (with a main cat under the car and a pre-cat on the downpipe)...

Technically, it's illegal/mot-fail to have either removed, but the mot tester is only tasked with a visual inspection (without the removal of panels) and an emission check. Most cars will pass the emissions with only 1 cat and the one in the engine bay is often hidden by an engine undertray, so it's "safer" to decat that and still pass its MOT (despite the car still technically being illegal). MOT testers cannot be expected to know how many cats there are on every single model of car, so 1 (or what looks like 1) under the car is usually sufficient to pass the visual check.

Ultimately, it depends on the car and where the main restrictions are - e.g. the mk3 mx5 is known to have a decent improvement with the downpipe changed to a better flowing one (which also removals the pre-cat), and it should still pass the emissions check with the main cat under the car.

I think the main push recently with MOT changes has been to check for DPF tampering/removal, so cats are argually in a samilar ball-park and could be under more scruitiny over time.

 

[Nasher]

Mine (the gt86) has one on the manifold and one on the front pipe. Many people remove the manifold one when tuning and it still passes emissions tests. Though the more expensive catted manifolds do almost as well now.

Maybe just screw a pipe full of activated carbon to the end of exhaust at MOT time

 

[Deleted member 209350]

Depends on car some are easy like fiat puntos some are a pain like mk1 focus st.

And if it’s an old cat being removed for the first time then it’ll take a lot of swearing, rounding off the fasteners, snapping studs, heat and maybe 999 on speed dial.

Mild steel, heat and water do not mix well

What about on a C63s? I plan on getting one in within the next 5 years, and it sounds incredible when its catless. If its easy to remove and put back on, I may have to do that..

 

[Kei]

The post cat lambda probe is normally used to monitor cat effectiveness. The pre cat lambda is the one normally used to make fuel trim adjustments for both closed and open loop though it is used in tandem with the post cat lambda in order to make adjustments for emissions.

I have to say, I noticed no difference going from a 2.5" decat pipe to a 3" system with 200 cell sport cat. That is on a saab 2 litre turbo running at ~270hp. A lot of cars these days come with two cats in the downpipe in addition to pre and post cat lambda probes. (thankfully, neither of which are on my car) Hell, I've seen some cars with a pre turbo cat and two more cats in the downpipe too.

 

[bainbridge]

The post cat lambda probe is normally used to monitor cat effectiveness. The pre cat lambda is the one normally used to make fuel trim adjustments for both closed and open loop though it is used in tandem with the post cat lambda in order to make adjustments for emissions.

I have to say, I noticed no difference going from a 2.5" decat pipe to a 3" system with 200 cell sport cat. That is on a saab 2 litre turbo running at ~270hp. A lot of cars these days come with two cats in the downpipe in addition to pre and post cat lambda probes. (thankfully, neither of which are on my car) Hell, I've seen some cars with a pre turbo cat and two more cats in the downpipe too.

I thought the sensors were purely a device to measure the amount of oxygen being burned. They measure the amount of oxygen being burned both pre and post cat and can then the ECU can determine the fuelling requirements.

 

[red eye]

What about on a C63s? I plan on getting one in within the next 5 years, and it sounds incredible when its catless. If its easy to remove and put back on, I may have to do that..

Two cats each side I’m guessing down pipes and then being a merc 2 more under the car. I’m guessing.
They do sound nice but might be better to delete or swap the backboxes or 300 cell cats or both, but please leave the centre pipe you don’t want a merc that sounds like riced up civic.

Edit I don’t think it would sound like a fart box civic Might have a 6300cc rasp

 

[red eye]

Mine (the gt86) has one on the manifold and one on the front pipe. Many people remove the manifold one when tuning and it still passes emissions tests. Though the more expensive catted manifolds do almost as well now.

Maybe just screw a pipe full of activated carbon to the end of exhaust at MOT time

Removing one cat will now fail even though it’ll pass emissions, being a new car the tester probably don’t know it has two or doesn’t care as long as it passes emissions

 

[sideways14a]

There are usually cats in the downpipes of modern cars as well as primary cats in the exhaust system, heck have heard of more.

All of which are restrictive crap, yeah yeah save the planet ect but the number of folk decatting and driving distance is tiny so it doest matter besides if the cars were lighter and smaller we would be polluting less anyway.
What does matter is the associated power/torque and usually with a remap as well hefty MPG improvement. My 5 saw a ~25 hp rise which aint a lot but it is a 1 ton non turbo car.
My old SX rose from 200hp to 240 with a main decat and 2psi of boost added (it rose on its own after the decat anyway due to less restriction) - that was 40 hp on its own with one decate pipe that could swap in and out in half an hour. After a remap and full exhaust that rose to 290, a massive increase over stock for next to nowt.

If you can,get a sports cat - it gives most of the benefits of a decat while filtering out the crud.

 

[red eye]

That'd be the 100 cell cats, the 200s I've seen haven't been marked and pass just fine.



In a N/A engine, sure. A turbocharged engine will be far lower. The map on my Focus aimed for 11.5:1 others ranged from 10.5-12.5:1 depending on the tuner.

11.5 is lean, my understanding is lean is bad for turbos

 

[sideways14a]

Depends on the load, where your at in the map and a whole lot of other things.
11.5 at full power though is a bit too lean, used to shoot for 12.1 although going to rich will sap power and use more fuel.

 

[miikeeyy92]

^ I thought the same. Closed loop is when the engine is warm and not full throttle. At WOT (wide open throttle) it's in open-loop and uses a look-up table to squirt the amount of fuel it "thinks" it needs from the air-flow sensor telling it how much air/o2 is entering, compared with the engine's RPM.

i think...

The ECU will go closed loop as soon as the pre-cat lambda sensor is up to temperature (they heat themselves to remove all moisture from in/around the sensor). This is in the order of seconds, not minutes in the case of engine coolant temperature. There will be a closed loop area on the fuelling maps where outside of, like you say it injects a given amount of fuel that's based on engine speed and in modern ECU's, the air flow model / MAF sensor. Old engines used engine speed & throttle angle or engine speed & MAF/MAP sensor as lookups. In closed loop, it does use these maps, but the lambda sensor feedback "adjusts" the value (as opposed to creating a brand new one).

In a N/A engine, sure. A turbocharged engine will be far lower. The map on my Focus aimed for 11.5:1 others ranged from 10.5-12.5:1 depending on the tuner.

Fuelling will be enriched near peak power to keep exhaust temps low to protect the turbine, not everywhere in the engine speed / load range. Vast majority of normal usage will be at lambda 1.

11.5 is lean, my understanding is lean is bad for turbos

An AFR of 11.5 is rich. Correct that lean (~lambda 1.3 ish) will provide very high temps that would melt the catalyst / turbine at peak power, and as said above, enrichment is used to reduce these temperatures. AFR < ~14.7 is less air than stoichiometric, thus rich.

 

[Deleted member 209350]

Two cats each side I’m guessing down pipes and then being a merc 2 more under the car. I’m guessing.
They do sound nice but might be better to delete or swap the backboxes or 300 cell cats or both, but please leave the centre pipe you don’t want a merc that sounds like riced up civic.

Edit I don’t think it would sound like a fart box civic Might have a 6300cc rasp

I see, thanks for the help!

Well I havent even got the car yet, but im thinking of getting it within the next couple of years though. I dont plan on overdoing anything, but these pesky emission rules really are ruining naturally aspirated sounds and engines so just want it somewhat close to those days instead of most of the turbo nonsense we have now

 

[Kei]

To convert lambda to AFR requires multiplying by 14.7. So a lambda value of 0.8 = 11.76
To convert AFR to lambda requires dividing by 14.7. So your AFR is 11.76 / 14.7 = 0.8

Rich = AFR less than 14.7 or <1.0 lambda. See post 41 for the graph illustrating it.

I doubt all cars follow the same protocol on when they switch between open and closed loop, but mine switches based solely on manifold pressure which can be set in a table on the ECU against rpm. Mine like most half decent management systems has adaptive mapping where the ECU has trim adjustments for most of the various tables. It's rather useful as you can get ball park settings correct then download the trim data from memory and implement it into the base map.

Also note that modern direct injection engines run far leaner in their default maps pushing AFR to 15 and holding 14.7 for as long as possible to try to reduce fuel consumption and no doubt reduce CO2 emissions at the expense of oxides of nitrogen. My mini cooper S from what I've seen doesn't drop below 14.7 until ~ 4krpm and even then, it only drops to 13.7. (pushing 1.4-1.6 bar boost) By comparison, I mapped my saab to run at 11.5-12.0 and usually runs around 11.8. (pushing a mere 1.2-1.4 bar boost)