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Old 11-28-2012, 01:42 AM   #1
PEI330Ci
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Simple Questions

Gents, I have 2 simple questions.

What happens to O2/AFR/Lambda sensor readings when pressure is above 1 BAR? (Atmospheric)

Is the answer to the above considered in your ECU tune?


There is a reason why some O2 sensors are $200, and some are $2000.
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Old 11-28-2012, 04:34 AM   #2
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Originally Posted by PEI330Ci View Post
Gents, I have 2 simple questions.

What happens to O2/AFR/Lambda sensor readings when pressure is above 1 BAR? (Atmospheric)

Is the answer to the above considered in your ECU tune?


There is a reason why some O2 sensors are $200, and some are $2000.
I've never heard of anything close to 15psi exhaust pressure post-turbo. Is this from some racing class with silly modification rules like unlimited boost on stock exhaust?
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Old 11-28-2012, 04:52 AM   #3
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I've never heard of anything close to 15psi exhaust pressure post-turbo. Is this from some racing class with silly modification rules like unlimited boost on stock exhaust?
I should clarify: The 1 BAR being referenced is atmospheric pressure, or 0 psi.
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Old 11-28-2012, 07:57 AM   #4
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After doing some reading, it appears that for the bosch LSU wideband sensor at 6psi backpressure (extremely high for a performance exhaust) an uncompensated display would be off by 0.17 AFR at 12.30... I guess I don't see the need for a $2000 sensor when .17 is going to be a fraction of most tuner's factor of safety.

(Note: My understanding is that the raw voltage output of the sensor would be off by ~8%, but that same signal -- once converted to an AFR reading -- is only off by 1.4%)

Last edited by SPDu4ea; 11-28-2012 at 07:58 AM.
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Old 11-28-2012, 02:48 PM   #5
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After doing some reading, it appears that for the bosch LSU wideband sensor at 6psi backpressure (extremely high for a performance exhaust) an uncompensated display would be off by 0.17 AFR at 12.30... I guess I don't see the need for a $2000 sensor when .17 is going to be a fraction of most tuner's factor of safety.

(Note: My understanding is that the raw voltage output of the sensor would be off by ~8%, but that same signal -- once converted to an AFR reading -- is only off by 1.4%)
What if the back pressure measured (via a second sensor) is just an average of exhaust pulses?
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Old 11-28-2012, 05:07 PM   #6
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It is just one of 1000 imperfect assumptions I'm making. These assumptions are why I conceed 5-10% from maximum power by backing off timing & boost and add extra fuel as a factor of safety.

If l was tuning for an ultra-competitive class then I'd be motivated to slash that factor of safety (and look for ways of doing so), but I'd start my improvements with the larger errors ($10,000 ecu package instead of $2500 -- or $30,000 ecu instead of $10,000).
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Old 11-28-2012, 10:12 PM   #7
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It is just one of 1000 imperfect assumptions I'm making. These assumptions are why I conceed 5-10% from maximum power by backing off timing & boost and add extra fuel as a factor of safety.

If l was tuning for an ultra-competitive class then I'd be motivated to slash that factor of safety (and look for ways of doing so), but I'd start my improvements with the larger errors ($10,000 ecu package instead of $2500 -- or $30,000 ecu instead of $10,000).
I've just been digging into some stuff that Harold Bettes had written a while ago...and it started jogging my memory on various topics. This was one of them. The Bosch Automotive Handbook has a lot of really good info on this and other instrumentation methods....

Agreed on the ECU package.
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Old 11-28-2012, 10:20 PM   #8
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What if the back pressure measured (via a second sensor) is just an average of exhaust pulses?
I would venture that such a condition is also present with O2 sensors taking readings that are "averages" of actual values during exhaust pulses.

You and I had exchange some words about some fancy and expensive sensors and logging components capable of taking higher resolution manifold pressure readings.

I would also even venture that sensor resolution specifications is likely to be the main difference between the 200 and 2000 dollar sensors.

I don't see how using expensive sensors makes sense in tuning an AEM (or any inexpensive ECU); if someone is going to fork out for high resolution sensors, they will obviously want a more adequate or "professional" ECU that can leverage the higher capacity. Heck, they probably want better injectors, pressure regulators and other such components as well. The highest level of control that can be obtained is likely limited by the least precise sensor or component used.

It's certainly worth looking at how custom cams, ported head as well as more optimized intake and exhaust relate to your goals before going for the fine-tune in such a setup. If you do it the other way around, you need to re-finetune if you start swapping hardware.

Diminishing returns considerations has me scratching my head.

But before any of that, a builder should set his objectives. A highly tuned setup normally has a specific purpose and operating parameters.

+1 to examining larger error/safety margins!

Last edited by FragNasty; 11-28-2012 at 11:20 PM.
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Old 11-29-2012, 12:12 AM   #9
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It all comes down to resolution, and how close to optimum you want to get.

The crank position for example can have a fairly high resolution "Map" of say 0.1 degrees. Basically, it calculates control functions in 1/10 of a degree increments. But that "map" is not built on crank position sensor pulses, but rather corrected based on crank position sensor data that outputs position (for the S54 for example) in 5.8 degree increments. So from a resolution perspective, you can build a projected model of what you want to see, and then correct it based on actual response, and have interpolated resolution that's really high.

So an example of this might be a $X Lambda device, which is getting at the maximum 25 samples per second accuracy. The O2 sensor just isn't able to produce changes of signal faster than this, so it's really a hardware issue more than anything. But, that $X Lambda device may simply output voltage readings exactly as it receives them. So you get a jagged 25 step output if there are large changes in 1 second. A higher end Lambda device can have a much higher frequency "map" going on and output an interpolated signal that is updated at the crank position sensor sample rate.

Anyway...my point was to encourage some discussion on sensors...as there seems to be a lot of focus on EGT lately without much discussion of sensor latency and the use of this data by an ECU.

Last edited by PEI330Ci; 11-29-2012 at 12:15 AM.
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