Catalytic converters questions

[HiTechRedNeck73]

I don't know of any back pressure that is good... I could be wrong, but I've always tried to get back pressure as low as possible in my engine builds...

to answer your question about the rear sensors...



the graphs above are from my Intrepid running gas... the top charts are the front (upstream) o2 sensors and the bottom charts are the rear (downstream) o2 sensors... and side-to-side, drivers side and passengers side respectively... hopefully it is clear enough, but you will notice that the chart scales are the same: 0-1.275 volts... the other axis on the chart is simply time in seconds...

from these charts, you can see the extreme changes in the front sensors compared to the rear sensors... basically, the front sensors are watching the exhaust coming out of the cylinders after a spark ignited burn... as exahust comes out of the cylinders and enters the cat, there is an amount of unburnt fuel and oxygen...

the cat, by chemical reaction, uses the heat from the exhaust, the unburnt fuel, and the oxygen to rearrange the chemical arrangement of the polutants... here is the two styles:

A two-way catalytic converter has two simultaneous tasks:
1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
2. Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O

A three-way catalytic converter has three simultaneous tasks:
1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O

now I can't tell you what type you have... but basically, these both reduce the amount of o2, by remixing (for lack of a better term) it with other elements to create safer exhaust gases... because of the lower amount of o2, the sensors after the cat are assumed to be of a lower voltage to reflect that... so the rear sensors should always be less than the front and I'm sure that most engine computers know that and watch for it...

here are some of the error codes associated to the rear sensors in my car: (as an example)

Sensor Shorted to Ground, input voltage below normal operating range
Sensor Shorted to Voltage, input voltage above normal operating range
Sensor Slow Response, sensor response slower than minimum required switching frequency
Sensor Stays at Center, neither rich or lean condition detected
Sensor Heater Failure, sensor heater element failure
Sensor Slow During Catalyst Monitor Test, (this is similar to the slow test above, except this is a portable smog test that actually monitors the efficiency of the catalytic converter... no all cars have this)
Catalyst Temp too High
Catalyst Temp too Low, (I've been told that these two are interolations that the computer does from the o2 sensors... I don't know for sure, but my cats don't have temp sensors so it must be...)
Catalytic Converter Overheat Detected, (I have no idea where it gets the temp)
Catalytic Converter Efficiency, catalyst efficiency below required minimum (there's the proof that it does compare front and rear...)

that is the public list of catalytic-based error codes for my car... I didn't look in my Factory Service Manual, because that list is a lot longer... the way I read all this is, you can't remove them without CEL coming on... but you can modify within reason...

[farmer]

just gotta say i removed my cat last week, fitted full stainless pipe from manifold,got it a size up from standard(can't rember exact size atm) to reduce backpressure -to see if mpg would rise. well massive power increase but no more mpg whatsoever...no less either so make of that what you will.