Badog

You have a wastegate with let's say a 15psi spring. This wastegate vents to air, not rerouted back into the exhaust. You're runing for let's say 20 psi, with a wideband. How acurate is that WB between 15 and 20 psi if a sizeable amount of exhaust is never getting to the WB O2 sensor?





So you have highflow of air coming out of the engine, wastegate venting (more like cycling open and closed) and the turbo building to 20 psi. At 20 psi you would assume that the WG is wide open for most of the time to hold boost and not make more.

Badog

Think of it as sampling rate of the WB o2 sensor where it sits in the exhaust and what it is NOT seeing when the WG is open (venting outside te exhaust.)

djgiantrobot

but would you agree that the same exhaust is going out both the turbine and the wastegate? What i mean by this is that the O2 sensor reads a sample of the exhaust gas, so regardless of what percentage goes to which place, the mixture of Fuel, oxygen, Co2, nitrogen, etc in the exhaust is the same and it will have the same reading.

rxrotary2_7

Tony, i would have to agree that my thinking process would lead me to believe the same....

Badog

In theory yes, I think I can agree. This is assuming steady state flow, right? So what happens when the flow changes? Pressure goes up while gas mixture changes, and the rate at which the WG is open goes up? I think this makes the WB O2 reading some % off for that moment in time, until things equal back out.

djgiantrobot

i just don't see how the exhaust divided between two outlets could have a different reading based on the rate of change in the flow of the division. A/F reads at idle and at WOT and everywhere inbetween, so we know that the flow rate doesn't affect the sensor. I think you can model this assuming steady state flow because with a wideband you are talking about an instaneous reading anyway.

Badog

But is it divided equally for the whole period of time? I think when the WG changes cycling to mainly open that for that period, a majority of the gas is going out the WG, until it closes. When it closes it goes back through the turbo and exhaust. Since the turbo path is restrictive, when the WG opens under pressure the exhaust is going to go where it is less restrictive.



I think we're talking about very small %'s of deviance in the accuracy of the WB 02 samplings. But I want to figure if there is one that I need to be aware of.

1Revvin7

I seriously doubt that it will cause a problem.

djgiantrobot

well i can't say at all what % division goes to the wastegate and what goes to the atmosphere, but you have to think that even with the wastegate fully open, at 20psi there is still quite a bit a flow out of the exhaust. I guess its complex to visualize for me cuz we are talking about a pretty complex closed loop system, and fluids were never really my thing, however i am standing by my assumption that the exhaust for both outlets comes from the same place and should be the same mixture regardless of where its directed.

Badog

Comes from the same place, yes. But pressure before the turbo and after it (in the exhaust stream) AND the through the wastegate is different that two, equal paths, say two exhaust pipes that Y- off. So, what I am saying is that when that shorter, lower obstruction path is open, a majority of the exhaust is going through it.



Do you think that if the wastegate was held wide open, that almost all the exhaust gases takes that path?