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 (a cycling rate that would allow it to bleed enough exhaust so that more 20psi is not made at the turbo).

Is there a potential inaccuracy in the WB o2 reading? I am looking at this from the sense of really fine tuning, the last 10% of power being extacted and what margin error to plan for.

suganuma

This has been leering over my head as well, but i sure don't want to vent my wastegate back to the DP. I guess the real question is how homogeneous are the exhaust gases coming through that whole system to eventually meet your O2 sensor in the DP. I would venture to guess at those high exhaust velocities that the gases are pretty homogeneous and the heavier fuel in that air mixture is not going to "fall" out or take a certain path seperate from the air. I think as exhaust gas velocities become slower this might become more of a realistic issue.

someone correct me if this "guess" logic is faulty somewhere.....

Badog

But I am thinking that at higher pressures, where the WG is open more than closed, there is a period of time, called it seconds, where the sample hitting the WB O2 sensor is not the same as what is coming out of the engine.

RMEGEDDON

i think sugnama is right, how is the ex. gas going to differ between the wastegate and o2 sensor......besides, my car's o2 sensor isn't even hooked up (haltech).....if you are tuned for high boost with a wideband, which i am, then i really don't see how you could lean out......but what do i know...

weaklink

I'm going to go with suganuma as well. As long as no new gasses are introduced, the mixtures are the same. It would be similar to having a y in a straight pipe. Readings at the ends of the y would be the same.

Zero R

iTrader: (1)

I'm thinking that the difference isn't going to change, while you may be sampling less gas I don't think it would change the mixture of air to fuel.

peejay

I think you might be overthinking it.

If the exhaust was that badly non-homogenous, then you would have problems with the A/F readings fluctuating even with the wastegate completely shut.

One thing that *could* cause problems, though, is if the exhaust exits in such a way that the O2 sensor can get exhaust gases on its outside. If I am not mistake the WB O2 is just a narrowband O2 that can have its accuracy range altered electrically, which is why it needs special circuitry and a controller. Narrowband O2's generate a voltage by sampling the ambient atmosphere on one end and the exhaust gases at the other end, this discrepancy generates a voltage, the mechanism of which I am at a loss to explain right now but that is the gist. Blow exhaust gases on the O2 sensor's exterior and it reads leaner than actual, because it has a lower O2 content on the atmosphere side than it is calbrated for and thus generates a lower voltage. Again, this is definite for narrowband O2's (and I have diagnosed a running problem that wound up being due to cracked exhaust manifold causing just this situation) and I would imagine this would have the same effect on a wideband as well.

Wargasm

Good information here!

http://www.sts.sae.org/membersonly/t...xygen13-17.pdf

shawnk

Disclaimer - I know very little as facts but have a decent amount of common sense (I think anyway).

The A/F ratio sensor has to measure a ratio of the fuel to air since it has no idea what specific volumes it is dealing with. (meaning it cant just measure the amount of fuel it detects because it doesnt know how much air is present and vice versa). The sensor doesnt care what size exhaust you have or what pressure the exhaust is at so it has to be independent from those factors.

If that is the case then spliting the exhaust stream in 2 (by opening the wastegate) would not change the ratio of each individual stream. Therefore the composition has to be the same in each stream. So this will have no effect on what ratio is passing out the downpipe.

The problems I see that could with this logic...

There may not be enough sample for the sensor to detect.

Or the sensor may not work like this at all...

Anyway - I went to a tuner that claimed he couldnt tune with a dumptube. I didnt think much of his abilities and really thought it was just an excuse - I mean there have to be plenty of people who have dumptubes that have had their car tuned at the reputable shops right?

Just my $.002.. (and yes I know I typed 2 tenths of a penny...)

Shawn

Wargasm

Actually AFR sensors don't measure the AFR directly. They measure the relative concentrations of oxygen ions inside the exhaust and outside in "normal" air. The difference in concentrations generates a voltage and you can figure out what the AFR is based on that.

It would seem to me that if a wastegate dump tube is dumping out some of the mixture, the concentration of oxygen ions inside the exhaust stream is now reduced, and you would get a reading on your wideband that would indicate slightly leaner than it really is.

You can read up on the Nernst equation here

http://www.science.uwaterloo.ca/~cch.../nernsteq.html

This equation is used to calculate the voltage potentials of cells and it is used in oxygen sensors to figure out what's going on.

Brian

mako

iTrader: (5)

I really dont think it would make much difference. I have not run that high of boost, but I did not see much change in a/f ration when i had the wastegate dumped and then routed it back into the downpipe. so much exhaust pressure will be coming out that it should not change in readings if your venting some of it.


also maybe I am wrong but the higher boost you run the more closed the wastegate will be right? it would be putting more exhaust pressure on the turbine wheel then bleeding off to the wastegate. I would think at lower boost you would bleed more excess exhaust out the wastegate due to trying to run a lower amount of boost. ie more air going out the wastegate to not spool the turbo.

on a side note i noticed boost would run 1~2 psi higher than the spring pressure when dumping the wastegate to the air, compared to routed back in to the downpipe. to loud and annoying for an everyday street car.

James

peejay

The concentration inside the exhaust pipe wouldn't change. There would just be less of the same micture, as opposed to feeding the wastegate outlet back into the pipe.

TailHappy

The thing I was wondering, which I don't think was mentioned, is that the issue might not be the measurement of the A/F ratio, but that the ratio could actually change due to the dumptube. It seems to me, if you set your wastegate for say 10psi. When you floor it, lots of excess exhaust dumps out the wastegate making for a very unrestrictive exhaust. You tune for this so that you can follow one of your P lines in your maps. Then you move up to 12 or 15 psi. Well now, when you are in the P-lines for 10psi, the wastegate is closed, and the exhaust is going through the more restrictive exhaust route, which chokes the engine some and changes your A/F's (richer, right?). Make sense?

I finally decided that the best way is to tune with the above method, and then when your wastegate is set for higher boost, the AFR should be lower (and safer) since the total flow is less with the more restrictive exhaust. If that makes any sense at all, does that sound reasonable? Or is the whole theory flawed from the start?