Extractor wideband Vs dyno tuners tailpipe wideband

[1uzfevolvo]

Had the car at a dyno tuner over the weekend and was interested to have him say that my extractor fitted, two LC-1 (Bosch) widebands were not accurate enough for him to use. He indicated his tail pipe sensor took into account the distance and the catalytic converters and was more accurate. The difference in readings was mine showed approx 0.5:1 richer.

I expect the tuner is happier using his sensor but is this kind of variance normal.

[jsmcortina]

Reading downstream of a cat surely tells you little about the engine AFRs. It is good to confirm environmental emissions only.

However, on a dyno you have the ability to tune for peak power with real numbers instead of aiming for an AFR and hoping.

James

[Yves]

I can see a sensor in the tailpipe would get some more free air entering with pulsations in the exhaust or is this to much of an assumption ?

I remember something about JAM here not getting a reading on the O2 in the collector of a header without exhaust system...

[Matt Cramer]

I can see a sensor in the tailpipe would get some more free air entering with pulsations in the exhaust or is this to much of an assumption ?

Very common. Sometimes they're unusable below 2000 RPM because an exhaust tip sampling setup is getting free air at that point.

[1uzfevolvo]

After a good drive and some logging today, I got the following AFRs using my sensors (5th gear, 3.54 diff)
Idle (680rpm)- 15.0-15.6
80kph (2188rpm) - 14.7-15.1
100kph (2563rpm) - 13.6-13.8
110kph (2913rpm) - 13.6-13.8

Now if my sensors are reading rich by 0.5 then the figures the tuner would have seen are as follows
Idle - 15.5-16.1
80kph - 15.2-15.6
100kph - 14.1-14.3
110kph - 14.1-14.3

Because of the suggested variance, I've turned the EGO correction off but the above figures seem to be backwards to what I thought was acceptable.

[DaveEFI]

I dunno your engine, but am surprised you're getting a reliable idle at 15.6:1 I'd expect it to be more like 14:1. Is that figure from TunerStudio, or from the wideband gauge?

[Matt Cramer]

I dunno your engine, but am surprised you're getting a reliable idle at 15.6:1 I'd expect it to be more like 14:1. Is that figure from TunerStudio, or from the wideband gauge?

It could be an aggressive cam, or if the sensor is very close to the exhaust tip, a reversion issue giving a false reading.

[kaeman]

the readings at the tail pipe after the cat will always read leaner than the engine is running because of the cats doing their job. my new dual wide band setup instructions specifically point out that the sensors should be mounted before the cat because the way the cat works, the sensors will give you false lean readings. your o2 sensors are a better judge of the afr of the engine than a sensor stuffed into the tail pipe. good luck

[1uzfevolvo]

The readings I've highlighted above are logged from my 2 widebands, in Tunerstudio, before the cats, but the VE table was set up by the tuner using the tailpipe wideband.

The motor is a standard 1UZ Toyota motor with extractors and an air intake from outside the engine bay. As it stands at the moment, the power figures are good and the motor runs smoothly, but I don't know what would change if I went back to using my wideband readings. AS I've paid for the dyno tune, I must assume the tuner knows what he's doing, but now the timing is more dialled in I could always try adjusting my AFR table a bit and turn the auto tune on and see what happens. If it all goes pear shaped, just reload the tuners tables.

[jsmcortina]

If it's making good power and running well.. I'd be very tempted to go out and enjoy it instead of tweaking the tables!

James

[bradyzq]

I second what James said above.

I wonder though, how much of a difference measuring before and after a cat should make under various sets of conditions. Cats both use and produce oxygen, which is what O2 sensors obviously measure. I imagine that they must do so in somewhat equal number, or else there would be an accumulation of stored oxygen, or its complete permanent depletion, within the cat.

Stoich should be stoich, pre and post cat. For rich and lean conditions, who knows?