Wideband Accurate with Turbos?

[wewing]

I was tuning for WOT on my APS twin turbo C6 and a have been targeting 12 or so AFR. Anyway I got to thinking that on a turbo car, the waste gates are open and spewing air into the exhaust. How accurate is my wideband in this case?

I am running about 6psi of boost and I realize if my theory is true that my car is actually richer than the wideband shows which is fine I guess, but is there a rule of thumb that I should tune it leaner due to wate gate dumps?

Attached is a scan with a couple of WOT runs in closed loop.

[Chris@HPTuners]

wastegate only allows exhaust gas to bypass the turbine (and hence control boost), no outside air is allowed in.

[wewing]

Cool. Sorry for the dumb question. Thanks. So is it correct to shoot for the 12 AFR during boost?

[wewing]

Never mind, I did a search and found out 12.2 AFR for boost is good. As you can see from my scan, mine varies from 11.3 to 12.2 depending on RPM, so I'll keep after it, thanks for the help.

[Country Boy]

12.2 is pretty lean for boost, IMO. I shot for 11.6

[Chris@HPTuners]

on my s/c i ran 12.0:1 @ 6psi and also 12.5:1 @ 6psi both results were within 5hp on the dyno so i stuck with the 12.0. Max timing of around 18-20 deg.

Now i run 11psi turbo's and 11.6:1 with timing around the 14-15 mark.

in both cases i found i have to be quite agressive with the IAT retard to prevent knock, althought the turbo's run the IAT a lot cooler than the s/c did.

[eficalibrator]

The output from a wideband will only be skewed if the pressure at the sensor is too high. This can happen when measuring between the head and turbo rather than after the turbine discharge. If that's the case, "rich" will look "richer" to the wideband, so you'd really be running slightly leaner than indicated by the wideband depending on how much backpressure you have at the sensor location.

I have also seen some of the cheaper controllers (DIY, Innovate, others...) read incorrectly at these richer conditions below 12.0:1. Because of this, I STRONGLY recommend going a little richer (~11.6:1) on your target AFR for safety. A little bit of extra fuel can go a long way toward knock resistance.

Moral of the story: Install the wideband downstream of the turbine if possible and use a good controller.

[wewing]

Sounds good, and yes I have my sensor after the turbo, it is in the rear bung of my down tube. Here is my latest dyno run with the AFR at the bottom. I know its kinda hard to see, but it starts at about 12.2 and as the rpm rises it goes toward 11, so I think I'm ok.