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Discussion Starter #1
Does anybody know how to code the car to omit post cat o2 sensors? Hypothetically speaking, if I was to remove the catalytic converters the downstream o2 sensors would throw codes. Is there a way to work around this or maybe remove the post cat o2 sensors completely?
 

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No benefit to removing the cats as they don't provide any real back pressure issues. The downstream O2 sensors do more than monitor the cats, they also help to fine tune the overall fuel mixture so any issue with the downstream O2 sensors effects overall engine performance.
 

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No benefit to removing the cats as they don't provide any real back pressure issues. The downstream O2 sensors do more than monitor the cats, they also help to fine tune the overall fuel mixture so any issue with the downstream O2 sensors effects overall engine performance.
I thought that downstream o2 sensors (post catalytic converter) only measure the efficiency of catalytic converters and upstream o2 sensors are the ones that directly relate to air/fuel mixture ratios. Correct me if I'm wrong.
 

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I thought that downstream o2 sensors (post catalytic converter) only measure the efficiency of catalytic converters and upstream o2 sensors are the ones that directly relate to air/fuel mixture ratios. Correct me if I'm wrong.
That is a common misconception. The majority the of the mixture correction is a result of the upstream sensors input but the downstream ones help to fine tune it along with monitoring the cat efficency.
 

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What your looking for is called a 02 simulator, The post cat 02 sensors have very,very little to do with overall engine performance more for catalytic efficiency. I have been running catless downpipes with the simulators for proabley 100k miles with no codes or performance issues
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He just did ; ) They have a small effect on fuel air and timing.

I would argue that there is a performance advantage. With the N54 in particular, because of turbo spooling and such. My downpipes need to come off to take apart the exhaust flange, and I intend to core the primaries.

The way around it is with a little electrical trickery. Make a circuit that sends the signals of a good sensor.
Looks like vrsf sells one for $100: https://www.vr-speed.com/vrsf-catless-downpipe-fix-for-07-10-bmw-335i.html

But I know they're each about 5 bucks of parts: http://www.web-nine.com/projects/wrx/celfix/

Ideally you have a switch and only flick on the downpipe fix for safety/emissions so as not to mess with the sensors for longer than you need.
 

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I was thinking of doing a cat delete as well, it def a performance gain by removing cats, esp if you have tune as well.

I Just didnt pull the trigger yet because of the o2 issue.. but like what was said above the cat o2's have little effect on anything, and its the reason why they rarely need to be replaced as well...

the upstream does most of the work IMO.
 

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The downstream O2's are responsible for checking the calibration of the upstream sensors so any issue with the downstream sensors or manipulation of their signals throws the upstream sensors calibration off therefore causing mixture issues.
 

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Anecdotal evidence suggests the effect isn't noticeable.

What would happen in the situation where the primary cat is removed but up and downstream sensors are left intact? My guess is the secondary sensors would read the same as the primaries, causing the car to think the cats are bad, which they are. What then?

I speculate it might be better to fake the sensors are good, which would keep the primary sensors from getting too out of whack. Computer sees the cats are okay and doesn't adjust anything on the primaries.

In any case, the compensation could be reset periodically.
 

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Discussion Starter #10
That is a common misconception. The majority the of the mixture correction is a result of the upstream sensors input but the downstream ones help to fine tune it along with monitoring the cat efficency.
Do you know anything about 2 sensor Lambda control which, I would assume, is an advanced way of calculating optimum mixture for an engine and most likely used by BMW?

Apparently, the Bosch Automotive Handbook says:

Greater accuracy is achieved with two-sensor control.

Here, a slower correction control loop is superimposed on the two-step or continuous-action lambda control described by means of an additional two-step lambda sensor.

The voltage of the two-step sensor downstream of the catalytic converter is compared with a set point value (e.g. 600 mV) for this purpose. On this basis the control evaluates the deviations from the set point value and additionally alters additively the controlled rich or lean shift of the first control loop of a two-step control or the set point value of a continuous-action control.
This, evidently, suggests that the downstream (two-step???) oxygen sensor actually helps to optimize Air/Fuel mixture, thereby increasing performance?

Interesting turn.
 

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then why is there a huge crowed that deletes their cats / and or add headers for more performance ?
It's an old school way of thinking that does not apply to modern day technology...kind of like a car with a loud exhaust must be fast or larger diameter exhaust pipes produced more power. Don't get me wrong there can still be some benefit from these modifications but one really needs to take into account how the engine is going to be operated because these types of modifications are not a one size fits all.
 

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Do you know anything about 2 sensor Lambda control which, I would assume, is an advanced way of calculating optimum mixture for an engine and most likely used by BMW?
I know that modern day engine management systems can and do use both upstream and downstream sensors to control mixture.

I was 1st introduced to this on Fords back in the early 2000s while I was scoping all four O2 sensors on a vehicle I was having an issue with. I noticed that at times both upstream sensors would indicate either full rich or full lean at the same time but fuel trim numbers would remain steady. I thought I had a pcm issue until I got in touch with a Ford factory trainer and he informed me that this was a way of checking the downstream sensors for functionality but when this happens fuel control would be switched to the downstream sensor so the driver wouldn't notice it. Obviously the systems have become much more advanced adn if you look at scan data on enough vehicles you will notice that more of the them have post cat lambda readings and also post cat fuel trim correction readings...this ain't there for the hell of it and actually can be very useful information IF you know what you are looking at along with the other readings.

True story...just 2 days ago I had a 2002 Toyota Camry in the shop for a emissions failure that also had a P0420, cat efficiency code, stored in the pcm. More often that not, esp on Toyota's this means the cat is bad BUT you also need to make sure the engine is in fuel control and not too rich or lean. This particular engine was running very lean at idle but dam near perfect while driving according to the fuel trims but the rear O2 sensor was switching like crazy so it popped a cat code but the upstream AFR sensor was pretty much normal so I needed to check that out. Obviously the 1st thing to look for was vacuum leaks but there were none and all other systems checked out. To make a long story short there was a hole about the size of a knitting needle in front of the rear O2 sensor and this was causing oxygen to be drawn into the exhaust and across the sensor which caused the sensor to read lean and the pcm was trying to enrich the fuel mixture because it thought the engine was running lean due to the downstream O2 reading. After welding the hole closed and a test drive of about 5 miles fuel trims returned to normal and the cat passed with flying colors.

Ultimately this strict control is to maximize performance AND emissions and by doing a little something here or there you can make more power but something will suffer and that is either driveability or emissions and being that aftermarket tuners don't care about emissions they don't try to achieve a balance between the two. I honestly don't think that they have the knowledge to make it happen anyway. As long as people can say " I have been running catless downpipes with the simulators for proabley 100k miles with no codes or performance issues" that's all they choose to care about but there's more to performance than what the seat of your pants is telling you. Don't get wrong I love fast cars just as much as the next person but I'm a bigger fan of clean air.
 

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I1tech: +1. Just to add another thought, do you really think these small companies know more about fuel systems and engine management systems than the company that invented them and probably spends more on r&d than all the other companies combined? Just saying.
 

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On the N54, the MHD stage 1+ tune is for fmic, and stage 2+ requires downpipes as well.

Based on what you're saying, I should be able to run the higher tune without adding downpipes and it should be fine?
 

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Discussion Starter #17
On the N54, the MHD stage 1+ tune is for fmic, and stage 2+ requires downpipes as well.

Based on what you're saying, I should be able to run the higher tune without adding downpipes and it should be fine?
Logically thinking, on a turbo engine it's possible to increase air beyond natural compression of air (obvious statement). If you increase boost beyond factory specifications, you must also increase exhaust flow capabilities. Otherwise the exhaust becomes a bottleneck for the entire system. Naturally aspirated engines, on the other hand, have no capability of compressing air, hence there is no necessity to increase exhaust flow. Correct me if I'm wrong.
 
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