afr reading

[Pappu1]

ok so i was talking to a reputable tuner ands an interesting thing he brought up was that having afr sensor poked in exhaust for tuniing/dyno gives a slightly false reading

he says that poking it in the exhaust can skew the reading to show richer thanif measured right by turbo area

e.g. say at turbo pipe if u put a afr sensor and it shows 12.0 and at the same time having one planted in muffler (like most performance places do) will show say 11.6 or something - richer than reality

i have never heard this before but he has tuned a lota good cars so was wondering if that is true at all??

anyone else know this?

[funkytown]

yup ive seen it here in wellington. They dont like sniffers, prefer something in the up pipe as its much more accurate. I dunno in which direction though.

[WRXONP]

It shouldn't show a different value unless calibration is different - an exhaust leak will make a tail pipe sensor show a false reading - the thing with having it up it's bum is it will react slower (obviously) - with out exhaust leaks you have the same exhaust gases at down pipe or tail pipe

[ADIKT]

Was just thinking... wouldnt an externaly vented wastegate give you a slightly off reading?

[funkytown]

would depend if the o2 sensor was pre or post wastegate

[ADIKT]

If you were testing from the muffler, and it was ext venting then there would be a loss of gases and therfor a false reading to actual afr? and if it was plumbed up then it would be alot more acurate? am i right in thinking about it that way? Would like to see back to back runs

[funkytown]

I'm not expert on themo gaseous stuffz0r but my answer is no.

Reasoning being AFR is a ratio of air parts to burnt fuel parts (or fuel).

if your EWG is venting then its not discriminating between the two parts above and actually venting equal parts of both in the same ratio that is exiting the exhaust normally so will read the same

i still disagree with WRXONP - I've seen quite a few tuners gets different results with sniffers / dislike that method a lot. What causes the difference compared to up pipe measures i dont know.

[ADIKT]

 funkytown said:

I'm not expert on themo gaseous stuffz0r but my answer is no.

Reasoning being AFR is a ratio of air parts to burnt fuel parts (or fuel).

if your EWG is venting then its not discriminating between the two parts above and actually venting equal parts of both in the same ratio that is exiting the exhaust normally so will read the same

i still disagree with WRXONP - I've seen quite a few tuners gets different results with sniffers / dislike that method a lot. What causes the difference compared to up pipe measures i dont know.

Ok, disregard my above comment

[MercuryFree]

 funkytown said:

i still disagree with WRXONP - I've seen quite a few tuners gets different results with sniffers / dislike that method a lot. What causes the difference compared to up pipe measures i dont know.

I would assume that the gas emitted by the engine isn't a "perfect" mixture, so the further/longer/more room it has to seperate out in, the less like the original "compressed/exploded together" gas it is

[Koom]

 MercuryFree']

[quote name='funkytown said:

i still disagree with WRXONP - I've seen quite a few tuners gets different results with sniffers / dislike that method a lot. What causes the difference compared to up pipe measures i dont know.

/quote]

I would assume that the gas emitted by the engine isn't a "perfect" mixture, so the further/longer/more room it has to seperate out in, the less like the original "compressed/exploded together" gas it is

2nd Law of Thermodynamics: Entropy always increases (or stays constant).

i.e. your hot cup of coffee doesn't take in heat from the cooler room, it will loose heat. Same as the mixture won't separate the further down the exhaust, but it can mix more.

The issue with 02 sensors in the tail pipe is to do with air from the surroundings messing with the readings, thats why they stick them in a smaller pipe to try to prevent that happening.

Like this thing;

If you have turbulent airflow exiting the exhaust, there is a strong possibility of eddies around the tailpipe which can pull air in from the surroundings, whereas having the 02 just behind the turbo is going to completely prevent that from happening.

[WRXONP]

what do you disagree with ? it WILL react slower thats simple physics

a leak before the turbo wont effect the air fuel reading from a bum probe

i had same air fuel curve with ewg venting and ewg plumbed in (same tune)

a exhaust leak near the rear of the car sure does - my skyline had leaks near the diff from it scraping - had holes in the bottom from wearing through which made the readings go wack and erratic some times - fixed the leaks and next time it stopped doing it

[Pappu1]

So the consensus wud b that in the tail pipe wud read possibly leaner due to possible outside air drawn in exhaust Nd leaks in pipe pre sensor

Yea??

[funkytown]

Its going to inconsistent possibly in either direction.

might be leaner as your getting eddies of fresh air about the exhaust tip diluting the result

and

richer possibly because your measuring a reasonable distance from the source and your delaying the readings ud see

[Marky]

+1 to the above

It doesn't always show leaner, or richer, it just isn't quite as accurate for that exact reason ^^

Suffice to say - if the car does not use the factory 02 sensor anymore (i.e has gone to aftermarket ECU) or has a spare bung in the up or downpipe, then it is preferable to use it as that will give the most accurate reading.