Re: Question of Air/Fuel ratio's and my Air/Fuel meter

[Dan Bubb <jdbubb@ix.netcom.com>]

First, adjusting the mixture on your CIS is basically an idle
adjustment. The greater the airflow into the engine the less effect the
adjustment has. Set the thing for good idle and starting. It has a
negligible effect on full throttle A/F ratio.
What you should see in real basic terms is light load or cruise at 14.7
or higher. Full throttle should be 13.5 or lower. If you have an O2
sensor the car "should" be in closed loop at idle and cruise conditions,
so the A/F ratio should oscillate around 14.7.
Inexpensive A/F gauges are way optimisitic about their ability to read
ratios away from stoichiometric! The standard O2 sensor is almost a
switch in that it changes from almost 0mv to almost 1000mv over a very
narrow range of ratios. (see attached jpeg) It's output curve also
changes depending on O2 sensor temperature.
As an example most gauges will assume that;
400mv =15.2
500mv =14.7
600mv =14.2
700mv =13.7 and so on.
A quick look at the sensor output curve indicates this is not close.
So, I wouldn't get too worked up about the gauge output unless it cost
~$1000 since it isn't going to have any accuracy anyway and should only
be used to indicate basic trends.
If you car really isn't running right I'd get the CIS fuel pressures
checked, make sure your injectors have a decent spray pattern and make
sure you have no vacuum leaks.
Now for all you guys that are going to chime in and tell me I'm full of
shit about O2 sensors, just let me say O2 sensor output CAN be used to
tune a car and that is exactly what I'm using to tune my Haltech. But,
it has to be a heated sensor to have any repeatability and you have to
be looking at the outut voltage and know what specific voltage to be
tuning for for various engine operating conditions.
Assuming the output voltage to A/F ratio curve is linear, as all cheapy
gauges do, is wildly inaccurate.
Dan

Randy B wrote:
> 
> Ok, I have had a Westach analog Air/fuel meter in the car for about 1.5yrs
> now.....not hooked up.
> I *finially* hooked it up last week and am a little baffled as to what I am
> getting with the readings.
> The gauge is analog, 1/4 sweep needle, range 12:1 - 17:1 with 14.7 marked as
> stoich.
> 
> When I initially hooked it up (with an old oxy sensor from my old Scirocco
> that worked back then) I was getting reading of mid 16:1 under load and high
> 15:1 to low 16:1 while cruising (maintaining speed and load). After reading
> Probst's Bentley Fuel injection book I attempted to adjust the air/fuel (by
> means of adjusting the mixture next to the fuel distributor with the allen
> tool) so that it would be at or around 12.6:1. According to him this is
> maximum power with 15.4:1 being best fuel economy. As I adjusted it richer
> and richer I did notice more and more power.
> 
> The problem is however that when the car is started cold it runs like a top
> fuel funny car, missing and spitting and stumbling.
> It used to start perfectly, and always ran very well (with a slight loss of
> power).
> 
> And to confuse me even more...
> I leaned it out a little and drove it tonight, after barely trying to keep
> it running cold, and now it's reading ~16.2:1 under load and ~13:0:1 while
> cruising. WTF???
> Everytime I nail the throttle the gauge hard pegs at 17.0+:1 and slowly
> comes back to 13.0:1 but if I go up a hill it will peg at 17.0:1+ and slowly
> come back to ~16.2:1.
> 
> When I got home, I pulled the plugs (Beru Silverstones @ 28, indexed, with
> Nology wires and coil amp) and all 4 had white powder on the arm and
> surrounding area and black soot on the base where the arm comes up from -
> sorry I don't know what these parts are called but I imagine you can
> visualize it. The Oxy sensor also had white powder on it. This means it is
> running lean right???
> 
> I switched the Oxy sensor for the one that used to be in the car and wired
> it to the gauge. (Haven't fired up the car yet)
> BTW, Westach says in their instructions that the signal from the oxy sensor
> will not be affected as long as the provided size of wire is used....that's
> what I used. A friend of mine and I bought these gauges from Ed Valvo (some
> might know him) so I am positive it is a quality gauge and should work
> right.
> 
> I guess my question (trying to figure this out) is what range should a guage
> like this run in?
> Is Mr Probst (fuel injection book) right that 12.8:1 is the magic #?....too
> many variables to tell?
> How do I get the system to run lean, seems the richer I make it the worse it
> starts and doesn't seem to make much of a difference on the gauge.?
> How do I get the gauge, ie.. system, to run a little more stable and give me
> good cold start?
> 
> Sorry for such a confusing post but I am quite a bit confused. :(
> 
> Thanks for any insight,
> 
> Randy
> 81 Scirocco S (X2)
> "The Twins"
> 
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