Full exhaust - lean?

[Cloud]

I was wondering if running a full exhaust with hfc will cause a the M3 with a stock ECU to run lean. I know the ECU can only adjust for so much but will a full exhaust exceed those limits?

[askiles]

It should lean it out some, but not too much. This is one reason it make more power.

-Andy

[MrHarris]

I wonder about this too. Whenever I floor it and my friends are behind me, they say they can smell my car -- and I have a full stock exhaust at the moment. Perhaps a more free flow exhaust would balance this, but this is just a guess.

[hondatrix]

The smell they talk about, is it like rotten eggs?

[MrHarris]

Originally Posted by hondatrix

The smell they talk about, is it like rotten eggs?

No, like exhaust.

[J08M3]

why would it run lean?

[hondatrix]

Originally Posted by J08M3

why would it run lean?

The free flowing exhaust could (should, but some don't)cause gasses to escape faster , and the ecu is programmed to opperate at a certain level of backpressure. It could happen that the parameters go too far out and the fueling than falls behind,causing a lean situation, but it is VERY VERY unlikely.

[J08M3]

doesn't the O2 sensor adjust the mixture?

[hondatrix]

O2 sensors only measure the oxygen content in the exhaust gasses, then sends the signal to the ECU that looks up the pre programmed parameters and then changes the settings , but it all happens faster then you could read the 1st two words.

[J08M3]

Originally Posted by hondatrix

O2 sensors only measure the oxygen content in the exhaust gasses, then sends the signal to the ECU that looks up the pre programmed parameters and then changes the settings , but it all happens faster then you could read the 1st two words.

exactly, so doesn't that mean they adjust the mixture??? or at least are used for it?

[mixja]

Originally Posted by J08M3

exactly, so doesn't that mean they adjust the mixture??? or at least are used for it?

Not sure on the M3 but it seems reasonably universal that O2 sensors are used when the engine is operating in "closed loop". Closed loop simply means the fuel mixture is calculated based upon some reference point, that being the O2 sensor measurements. In essence, a closed loop system has the output of the system fed back into the control point of the system, which consequently adjusts the input to the system. For example, the ECU may have a "closed loop" map with a bunch of target AFRs for various load/RPM points, and the ECU will continuously monitor O2 sensor readings and richen/lean the mixture as needed to meet the target AFR values. Note that closed loop is normally used at light loads and low engine speeds - at a certain point, the engine management switches to "open loop" fuelling, where the fuelling is based upon a pre-programmed fixed value at each RPM/load point and is analogous to having a map that has a whole bunch of injector duty cycles in it, rather than target AFRs. An open loop system does not feedback the output of the system, hence any condition that changes in the system can affect the output.

If the exhaust reduces backpressure and increases VE of the engine, then even in "open loop" fuelling, the additional air mass should be detected by the air mass measurement system (which I believe is MAP-based on M3) and load point at a given RPM will be increased, thus fuelling will adjust. That said the open loop fuelling values at the higher load point in the fuel map will have been calculated from months and months of continuous tuning, measurement, retuning and so on using the stock exhaust system, thus the fuelling will be slightly out once you change to aftermarket exhaust.

[J08M3]

I thought "open loop" was used before the engine got up to operating temperature? Then "closed loop" which uses the O2 sensors to adjust when it's at operating temperature? The Air/Fuel mixture should always be the same once up to temperature. Why would you ever want it too lean or rich? I can't remember the value, but I think it's about 14:1 Air:Fuel? Which ever the number is why would you ever deviate from that number?

[hondatrix]

Thanks, I did not want to write all that.

[mixja]

Originally Posted by J08M3

I thought "open loop" was used before the engine got up to operating temperature? Then "closed loop" which uses the O2 sensors to adjust when it's at operating temperature? The Air/Fuel mixture should always be the same once up to temperature. Why would you ever want it too lean or rich? I can't remember the value, but I think it's about 14:1 Air:Fuel? Which ever the number is why would you ever deviate from that number?

AFR of 14.7:1 (stoichiometric) is OK for light loads but at higher loads in theory an AFR of 13.2:1 makes most torque. However at leaner AFRs the likelihood of detonation is increased, and in practice an AFR of around 12.5:1 is normally the best for making the most power on a normally aspirated engine.

Here's an example of a target AFR map on a Subaru STI (note these are tuned very rich at high load/high RPM sites given they are turbocharged), which shows how a fuel map transitions from 14.7 to whatever the target AFR at peak load should be:

[J08M3]

mixja - cool thanks for the info

[mixja]

Originally Posted by J08M3

I thought "open loop" was used before the engine got up to operating temperature? Then "closed loop" which uses the O2 sensors to adjust when it's at operating temperature? The Air/Fuel mixture should always be the same once up to temperature. Why would you ever want it too lean or rich? I can't remember the value, but I think it's about 14:1 Air:Fuel? Which ever the number is why would you ever deviate from that number?

On your first question, I'm not sure, but I guess it makes sense "open loop" would be used 100% given an O2 sensor needs time to warm up etc to get accurate readings. Once everything is warmed up and operational, then I'm suggesting the car goes into a mode where it uses closed loop operation for light loads/low RPMs and open loop for high loads/high RPMs.

[J08M3]

all I know about it is I've had problems with vehicles that had thermostats too low and the engine never got warm enough for the ECU to switch over to closed loop.

[Cloud]

so stuff like lambda adapters that fool the O2 sensors probably would probably be able to prevent CELs but will cause the ECU to carry out inaccurate calculations?

[J08M3]

Originally Posted by Cloud

so stuff like lambda adapters that fool the O2 sensors probably would probably be able to prevent CELs but will cause the ECU to carry out inaccurate calculations?

I would think so.

[J08M3]

Originally Posted by Cloud

so stuff like lambda adapters that fool the O2 sensors probably would probably be able to prevent CELs but will cause the ECU to carry out inaccurate calculations?

Actually I guess it wouldn't since it's the first set of O2 sensors that work with the ECU for fuel mixture. The adapters or any ECU tunes that delete CEL codes go on the second set of O2's and that is only for checking emissions I believe.

[BMWDEMON]

hmmmm, my friends say the same for me...they can smell the exhaust fumes pretty heavily (I have the full AA system) and they thought it was running rich instead of lean?

M

[Cloud]

I know the second set of sensors are after the primary cats. Where are the 1st set of sensors?