New theory on the "Ski-slope" AF ratios

[NetworkingGuru]

Guys,

I've been thinking about this, and I just have to ask to see what the consensus is. The tuner I took my car to for the dyno said that air must be re-entering the exhaust and throwing his AF sensor off because of how much the readings varied at different RPMS. The more I think about it, the more I think this theory makes some sense, and would explain our "ski-slope" AF ratios. The theory is that the negative pressure waves in the exhaust at low RPMs is pulling air back into the exhaust, possibly as far as the H pipe and then recirculating it out on the possitive wave. To my thinking, if this were true, the AF at low RPMs would be off considerably since fresh air is being mixed with the exhaust gasses. However, at high RPM's, the positive pressure would be so great that the air doesnt have time to recirc and the reading would be more accurate. The tuner suggested that the next time I bring it in, we pull out one of the stock O2 sensors and put the AF sensor in that hole to check. I think this might actually give us true readings. What do you guys think?

 

[Got Incon?]

Are you saying that he just stuck his probe in the tailpipe????


Don't ever pay for someone to do that, they are not accurate. Not only is the exaust cold (not gonna give an accurate reading) but your also reading any and all leaks in the system all the way down.


The best way to do it is to weld in a new bung so that your O2's continue to funtion. Whats the point of taking it out, then the o2 will tell the computer theres a fault or if you just dangle it in the air it will report it is running lean.

If he welds in a bung, get him to put it in the correct orientation so that if you ever buy a WBO2 for yourself down the road, you can use the bung he made. (The bung either needs to be on the top or the bottom, I forgot which. It is to prevent mositure from seeping into the base of the sensor. I know the info is on the DIY-WB page) I'm sure you would have to find this out yourself as most people wouldn't know there is a right and wrong way.

 

[Got Incon?]

Also make sure when he welds it in (which will be out of the car) keep watch over the obvious that whatever orientation he puts it in has room for it to actually be there when its installed.


NAPA sells bungs, and you can get plugs too for when theres no sensor in it.

 

[Got Incon?]

Now that I am thinking about it, I'm pretty sure the sensor tip needs to be pointed down, so that moisture doesn't condense on it and then sit on the sensor.

 

[Elapidae]

I doubt there could be any vacuum in the exhaust significant enough to overcome the positive exhaust gas pressure as to draw air all the way to the H-Pipe. Just hold a feather or a piece of styrofoam on a string behind either and see if there is any air movement inward. I'll bet there's not.

 

[NetworkingGuru]

Originally posted by Got Incon?
Are you saying that he just stuck his probe in the tailpipe????

Yep, that's exactly what he did.

 

[slow lane]

Ski slope is real

See attached stock dyno (with UPR CAI as only mod).

Wide band O2 sensor installed in driver's side H-Pipe bung.

 

[Got Incon?]

What did you do with the stock o2 on that run though?

 

[slow lane]

It was unplugged.

With the chip in the A/F line looked perfect, with and without the chip at part throttle the A/F stayed dead flat at 14.5 to 5000 RPM, so clearly the computer / chip can recognize that the O2 sensor is disabled and read the fuel map in open loop without it and compensate for the missing sensor in closed loop (part throttle) operation.

Also to note... my car seems to have a wide band sensor in that position.

 

[NetworkingGuru]

Originally posted by slow lane
It was unplugged.

With the chip in the A/F line looked perfect, with and without the chip at part throttle the A/F stayed dead flat at 14.5 to 5000 RPM, so clearly the computer / chip can recognize that the O2 sensor is disabled and read the fuel map in open loop without it and compensate for the missing sensor in closed loop (part throttle) operation.

Also to note... my car seems to have a wide band sensor in that position.

Slow Lane,

How sure are you that it's a wide band? That would be great since the last post I saw on it said a WB O2 sensor was like $700 each (OUCH!).

 

[slow lane]

The dyno guy recognized the part number as one of the options for his SuperFlow WynDyn eddy current dyno. He said his price to buy one would be in the $500+ range. He also pointed out that one he was using was significantly more expensive, but didn't say how much. Then checked the number on the passenger side rear O2 sensor, same thing.

They held my car for nearly a month before shipping (built 5/17) without ever saying what the quality issue(s) were. They released the first 100, then held the next 150 for 3-4 weeks, and delivered many of those with bad gas and fouled cats.

 

[mineral03]

I dont think the oxygen sensors will affect anythign when running a car on the dyno... if youre at WOT, the computer isnt going to use the 02 sensors for anything. Its only really when youre idleing/cruising that the computer uses the o2 sensors to monitor the exhaust. On heavy throttle runs, the computer ignores the o2's, and goes into open loop. If youre tuner didnt know this already, then id be worried about youre tuner!:nono: thats why most places just remove one of the 02 sensors when the do dyno runs with airfuel monitering, cause it doesnt matter! Stock o2 sensors arent accurate enough to accurately set the A/F ratio at WOT

 

[Got Incon?]

But you realize that before you go wot, thats going to affect something. I don't know what exactly or to what extent because I didn't do the programming for Ford, but I would bet it would. You don't just hit wot imediately after taking it out.


I am a worry wart though, and little stuff that could cause a problem bugs me. If 5 people showed me matching dyno charts one where the disconnected an o2 and one where they welded in a new bung and they were exactly the same then I would feel comfortable doing it that way. But until then I wouldn't.

 

[Got Incon?]

Originally posted by NetworkingGuru
Slow Lane,

How sure are you that it's a wide band? That would be great since the last post I saw on it said a WB O2 sensor was like $700 each (OUCH!).

The sensor would be 150$, the stuff to read it accurately is about 450$ (Unless you build it yourself)

 

[mineral03]

Originally posted by Got Incon?
But you realize that before you go wot, thats going to affect something. I don't know what exactly or to what extent because I didn't do the programming for Ford, but I would bet it would. You don't just hit wot imediately after taking it out.


I am a worry wart though, and little stuff that could cause a problem bugs me. If 5 people showed me matching dyno charts one where the disconnected an o2 and one where they welded in a new bung and they were exactly the same then I would feel comfortable doing it that way. But until then I wouldn't.

true, but even after you start the car, it doesnt start to read the o2 sensors until a few mins afterwards,because it has to wait for the sensors to warm up. typical dyno pull would be:
-yank the sensors
-start the car, might be running it for maybe a minute before you start doing the pull. ( 1 minute)
-start accelerating gradually up to pull speed(30 sec)
-then your actual WOT dyno pull
This only gives you 1-2 mins from the time the sensor was yanked, to the time youve started your pull. This usually isnt enough time for the computer to read the 02 sensors, since they arent completely warm yet...
When i did my baseline dyno, they just yanked one of my forward o2's, and this is the way ive seen it done at all the dynos around here.