A/F Ratio Meter

[gedmeyer]

I would like to put an A/F ratio meter in my 81. Not a permanant mod to the dash or anything, something I just can use to help verify I'm running where I should to be.

It is my understanding that the meter just monitors the voltage from the O2 sensor and uses pretty lights to display the results... Can I just pigtail my current O2 sensor with a multimeter or with the Summit Gage below to monitor my ratio? What is the voltage that would correspond to a "good" A/F ratio? I've read 0.45V?

http://store.summitracing.com/partdetail.asp?Ntt=air+fuel+&Nao=10&N=4294822093&autoview=sku&part=SUM%2DG2986&Ntk=KeywordSearch

Thanks! Not long before Roxy is back on the road and I want her running right!

-gedmeyer

 

[WayneLBurnham]

The range IS 0-1V, but not what corresponds to

11:1, 12:1, 13:1, etc. The voltage drop may NOT be linear....

I'm also not sure which way is richer/leaner (if no voltage is pegged lean or pegged rich, etc.)

As to just splicing on a second device OR the multimeter, I'd be worried about the multiple devices causing voltage drops and thus giving you a false reading on both devices.

Welding on a second O2 bung an getting a second O2 sensor is really not that much - a muffler shop would do the whole thing for $30 or so. You could use your current O2 sensor for this function and use this as an excuse to replace the one you have for your FI system needs - which really should be replaced every 10K miles or even less anyhow.

That was one of the first things I found out about having two A/F meters (I have the pretty ROUND display ones! :W ) on my car that doesn't need them except for my information - the sensors (of several brands) are sensitive, go bad frequently and misperform for engine hours at a time. They seem inexplicably sensitive to events OUTSIDE of the exhausts like coolant and especially oil leaks. Even running through water several times takes one out for a while. I'm sure some of that is just them cooling down to not work, but I think more is going on than that.

I am so very glad my carbureted system and the high end FI system that will one day go in on the real engine will not rely on such erratic data sources to make such crucial moment to moment decisions in fuel management.

 

[UKPaul]

In my spare time I've been trying to find out about the '81 ECU (namely, how to change the spark advance tables). So far I've not got far, but I came across this: http://yarchive.net/car/engine_control.html
To save reading through the whole lot, here's the bit that's of interest:

Unless you have a wide range EGA (if you paid less than $1000 new
for it, it is not a WREGA.) These low cost "EGA" sold by Summit
Racing and the like that read a standard oxygen sensor are really
frauds. The standard oxygen sensor only has a range of lambda = 1
+_ perhaps a tenth. Lambda=1 is stoichiometric. The sensor is
essentially a toggle device, indicating either rich or lean. The
transfer function looks like a "Z" with the leg being almost
vertical. The ECU toggles the mix back and forth across lambda=1
because a) it is the easiest control modality and b) oxygen storage
cats work better that way.


I assume that EGA = Exhaust Gas Analyser & that it refers to the meters that indicate lean/rich which are being talked about. There's some quite interesting stuff in the remainder of the post (written by John De Armond), especially regarding pre-ignition. Something that caught my eye was:

You'll never read about this just looking at the popular hotrod
press written by knuckle dragging Neanderthals who think multiple
carburetors and tunnel ram manifolds and open exhausts still have a
place on the street.

Wondered why my knuckles have been smarting lately


Justin,
If you're reading this, he also goes on (and on, and on....) to say:

Fast burn rates, as induced by modern
high swirl combustion chamber designs result in LOWER octane
requirements and less spark lead. If the swirl makes the combustion
proceed faster, it needs less "head start" (advance) to get the peak
pressure at the right point.

Has your 406 got a high swirl chamber design? If so, this could mean that the burn is faster so the stock advance is too much? Maybe retarding the base timing (what he calls the broad axe approach to tuning!) will not hit performance as much as we'd expect? I was going to suggest that maybe a trip to the Dyno would throw a lot of light on this, but the guy above admits to tuning dynos to give false readings.....
cheers

 

[gedmeyer]

Paul,
I've heard similar things where the wide range sensors are the ones that are really accurate. I don't quite understand how the oxygen sensor can be like a toggle device. That would imply that it is digital. It was my understanding that its output varies over a range of values (therefore analog).
I think the problem with the cheap gauges is that they only display the value of the here and now. Since the readings are nearly continuously updated, there would be significant fluctuation in the reading. I believe the wide range gauges would give you a more consistant value (an average of sorts). I bet an oscillioscope would work equally well in measuring the output/response.
Talk about transfer functions is just bringing back bad memories of a Modeling and Control class I took. YIIIKES.

-gedmeyer

 

[UKPaul]

Fully agree, but even if the result is only an instantaneos snapshot then at least, over time, it should give an indication of whether the A/F ratio is in the right ballpark (or be more accurate than doing it by "feel").
Modeling & control? You don't men control theory and differential integrals do you? I use to write assembler code to do that 20 years ago. Couldn't do it now (wouldn't even know where to start!) and even at the time it was a headache
:cheers:

 

[WayneLBurnham]

But I like the pretty LIIIIIGGGHHHHTTTTTSSSS!!!!!



Seriously, some of that quoted text is simply not correct, at least as regards the output from generic O2 sensors going to A/F gauges - I've never used a specific EGA meter such as from Summit.

I hooked up a meter to the leads from mine once and watched that thing vary all over the place - oddly, it exceeded 1.0V to about 1.2 as I recall. It changed in the tiniest of increments constantly. It is certainly not a binary signal. If it's not analog at all, then those increments are tiny and I didn't see a chip on any of the sensors I replaced.

To me, the wide range detection was of less criticality in an engine that was probably running half way decently (unlike something in a shop with a problem) than how fast it responded to changes. Less "does this thing cover 7:1 to 17:1 instead of just 10:1 to 14:1?" than "does it respond in 0.1msec or 0.01?" For engine control information that information is more useful as you really don't expect it to ever be so rich or so lean it's outside of functional.

Thus I wanted "fast read" more than "wide range" - like you might find on a dyno (which actually is probably both if I'd take a guess...)

I notice when I get a low fuel pressure event I see it in the A/F meters BEFORE the FP gauge, which always amazes me. It also seems fast enough to flash only a moment after hitting the throttle, right in time with the vacuum meter.

I also note when it shows reasonably lean for very long at all sure enough my temperature, especially oil, goes up.

Most of the rest of that article rang true, so maybe it's just somewhat dated in information. The head chamber design in not supporting preignition goes along with what I've heard before (although "swirl designs" are not the only design or condition that effect this.)