O2 sensor placement on turbo application.

[TheSilverBuick]

Where is a good place for a sensor on a single and/or twin turbo set up?

If after the turbo is there a problem with readings with external wastegates? Is heat still an issue in the exhaust collector at the turbo?

[TC]

Where is a good place for a sensor on a single and/or twin turbo set up?

If after the turbo is there a problem with readings with external wastegates? Is heat still an issue in the exhaust collector at the turbo?

In the collector or right after the turbo....... The closer to the exhaust port the better reading you'll get...... I've read on drag race cars 8 inches from the exhaust port is optimum, were as in Circle track type cars in the middle of the header tube is optimum........ though in those applications they are probably running an O2 sensor in each header tube and tuning each cylinder individually..... That's why I said in the collector, because it gives you an average of all the cylinders.......

[Beagle]

how critical do you think it is Randal? Not trying to be rude, just wondering how exacting you want to be. A wideband,down stream of the turbo - the wastegate should dump into the exhaust anyway. Pre cat should be good. HEGO shouldn't care how exactly close it is really... they seem to handle long tubes okay, the downpipe from a turbo probably won't be any further downstream than a set of 30" pipes and getting it on the collector there.

Are you building a F1 car or street driven? I'd wonder if individual EGT would get you more than the wideband... Scott was doing some interesting stuff with some of the folks here. I'd think if you are sweating that last .10 horsepower, it's a big deal. I'd spend more time worrying about pipes leaking than O2 placement probably.

[TheSilverBuick]

My personal feeling is put it down stream of the turbo as I don't think the oxygen ratio will change much from the exhaust port to the tail pipe, even if some exhaust volume is bled off or bypasses the turbo and down stream of the turbo will keep the sensor from cooking. However, the one thing I don't know is how NOx plays into the equation with the temperatures seen at the turbo. If oxygen is still binding with the nitrogen at the turbo, or if the pressure and temperatures are too low to make it happen. Because if the reaction is going on you'll see a different AFR after the turbo than you would before it. That's just one factor I'm thinking of, there could be a dozen I don't know about, or it could be completely moot.

[TC]

My personal feeling is put it down stream of the turbo as I don't think the oxygen ratio will change much from the exhaust port to the tail pipe, even if some exhaust volume is bled off or bypasses the turbo and down stream of the turbo will keep the sensor from cooking. However, the one thing I don't know is how NOx plays into the equation with the temperatures seen at the turbo. If oxygen is still binding with the nitrogen at the turbo, or if the pressure and temperatures are too low to make it happen. Because if the reaction is going on you'll see a different AFR after the turbo than you would before it. That's just one factor I'm thinking of, there could be a dozen I don't know about, or it could be completely moot.

O2 sensors work off "heat", the farther from the exhaust port the cooler the exhaust gasses gets, just saying.......

Here this may help.

Also, the placement of the O2 sensor - the distance away from the exhaust valve - is critical. The closer the sensor is, the fewer the amount of slits the sensor will have to decrease the amount of heat the spinell layer will see. Sensors in the manifold will have only a few slits and sensors further downstream will have more slits or holes. Some sensors have slits that face clockwise, others face counterclockwise. When replacing the sensor, be sure the slits are faced in the proper direction or exhaust flow into the sensor will be changed. You can see, too, that all this is much more sensitive and important on OBD-II engines.

So when it comes to oxygen sensors, there isn't a generic "one size fits all." Be conscientious and watch the application ... it can save you a comeback.

http://www.asashop.org/autoinc/may98/techtips.htm

So it may depend on the sensor your using.......

[Beagle]

Just about any O2 sensor is heated by itself any more. Maybe I'm getting lazy, but I'd check wher Subi or Mitsu put theirs? They've mathed on it a lot and have a few years with hot dawg turbo motors and EFI. Porsche may not be a bad indicator too, but their exhaust is pretty wacko.

It's a hell of a question, when I get through brute forcing stuff in today, maybe I'll do the goo-glechanic / goo-glengineer deal. Has to be some good reading on this somewhere.

[DanStokes]

On our Diesel dyno motors we generally ran an O2 sensor downstream after the single turbo (Cummins ISB). They make TONS of NOx and the O2 seemed to work OK. But we also had an O2 in each runner so this may not be real applicable - we were able to check them against each other. Still, they were OK as I recall and agreed pretty well when you averaged the individual values and compared to the downstream sensor. BTW - the downstream were on either side of the after treatment (2 sensors used) and the post-after treatment sensors were wacky.

Dan

[TheSilverBuick]

but I'd check wher Subi or Mitsu put theirs?

To complicate things, from what I gather Subaru's have one BEFORE and AFTER the turbo! Why would they do that?!?!?

[dieselgeek]

Eh, don't (permanently) put the O2 sensor before the turbo. None of them will live at those kind of EGTs for long. Usually you can find the manufacturer's recommendations for maximum exhaust gas temp the sensors can tolerate - if I remember right, the NTK wideband O2 sensors worked at higher max temps than the cheaper Bosch wideband O2 sensors. This info should be in their documentation.

[TC]

To complicate things, from what I gather Subaru's have one BEFORE and AFTER the turbo! Why would they do that?!?!?

They could do it for the same reason they do it for a catalytic converter....... to monitor exhaust gas temps....With a cat if the temps get to high it signals the computer to richen up the fuel mixture to cool the cat back down, same could go for the turbo.......Or at least it's plausible.......