A/F gauge questions.

[BlkBird]

I just completed the installation of a new voltmeter and an A/F gauge. Everything works well but the A/F gauge seems not quite as useful as I'd hoped. I understand that the signal used to measure the ratio comes from the O2 sensor (another expensive part) and that the gauge is inaccurate until the engine is up to operating temperature.

1. But why does the gauge fluctuate so much at idle?
2. Will reducing the mixture to "Stoich" help performance or does it just put my pistons at risk of a meltdown?
3. Has anyone trusted their A/F gauge enough to set the mixture, or is this just a nice-to-look-at addition?



Appreciate your thoughts..

[Brant]

its a gross tool if its narrow band
mostly a nice to look at.

If it were wideband then it would be very accurate and could be trusted enough to dial the mixture in.

brant

[kuehl]

Smart move putting real time AFR in the cockpit.

But why does the gauge fluctuate so much at idle?
Probably what you are seeing, at idle, is the DME adjusting the AFR to maintain the designer's "perfect" ratio for the system.

Since there is only one 02 sensor, you are seeing the combination of 6 firings or pulses that become mixed to some degree in the exhaust gas stream. The AFR system, and O2 sensor, at some point is taking a reading, depending upon the type of AFR monitor you have, every so many times per minute or second. And, if you had six AFR monitors, one for each exhaust outlet (hmmm) you would probably see all six vary to some degree as well.
(you can also have some fun with six independent temperature sensors).

If you had a digital rather than analog display, and depending upon the number of characters displayed less than a whole number, ie. 14.X, 14.XX, 14.XXX, the variation in the 10ths, 100ths or 1000ths tends to be more obvious, meaning a display that was one decimal place, 14.X, would vary less than one that is 14.XX.

Assuming, based on your console picture, you have a 86-89, at idle and through the point where the system is not under wide open throttle (WOT), the engine's fuel and ignition system, DME, is running in "closed loop" (using the 02 sensor) trying to maintain the perfect AFR based on the designer's criteria, by adjusting timing or pulse time of the injectors. Typically the engine management system is based on a clock cycle which is in increments rather than a smooth line naturally. The 02 sensor see's the AFR and sends the reading to the DME, the DME looks at the AFR (in voltage terms) and makes an adjustment, if necessary, for the next firing pulse.

When the throtte position goes to wide open "WOT", the O2 sensor is taken out of the "loop" and you probably see AFR's near 12.5. At this point the DME is working on a set plan, programmed in the "chip" which says at xxxx rpms there will be so much advance in timing and the injector's open-time will be so many milli seconds; the MAP plan.


Will reducing the mixture to "Stoich" help performance or does it just put my pistons at risk of a meltdown?

The stoichiometric ("stoikheion + metriā") measure of air and fuel for gasoline combustion is in "theory" 14.7 parts of air to one part of gas. If it was a perfect world you would want zero parts of fuel left after the combustion process. However in such a case you have various other unfriendly environmental gases as well in addition to high temperatures. So the auto industry finds a point or ratio that keeps the tax dollar mungers, the engineer and the performance lover somewhat happy but never satisfied.
For a quick answer that is not qualified in terms of satisfaction to anyone, let us just say that a number when running in the "closed loop" (not WOT) that is less than 14.5 will not in theory melt your pistons, however a number that is greater than 15.0 "won't help the situation". WOT is another topic.


Has anyone trusted their A/F gauge enough to set the mixture, or is this just a nice-to-look-at addition?

I'm sure some have. However if we step to the side of a general answer and ask the provocative questions such as:
What type of AFR monitor, narrow band or wide band?
While in the loop or at WOT?
Driving up a hill (load)?
What octane?
At what ambient temperature and relative humidity?
With what stock or innovative engine cooling components?
ETC... yada yada yada.

There are plenty of hard copy reference sources to explore AFR, such as turbo or supercharging publications, and naturally what you will find here in the forums. Do, by all means, explore it!

However, it is best to BUMP this up to those who have less scraped knuckles (the experienced ones).

[RarlyL8]

I run a narrow band A/F meter in my car. Rely on it religiously. When I tune the mixture (with the Gunson) I can watch the A/F change with each nudge of the wrench. The guage is accurate enough that it warned me of a DEVELOPING vacuum leak long before it affected the running of the engine. For several months I watched as the cruizing A/F ratio began to inch up about 0.2 units every couple of weeks or so. I thought the O2 sensor was going out. About 3 months later the car started having starting difficulties. After much looking I found a vacuum hose that was being abraded by the throttle lincage. Changed the hose and whala - engine ran perfect and A/F ratio showed back to normal.
The unit I use is not as accurate as a wide band (and not near as expensive) but is a good tool once you are used to how it works and what it is showing you.

Modulation at idle is due to a continuous reading taken at a slow rpm exhaust velocity.

[89turbocabmike]

IMHO

A narrow band a/f gauge is not accurate enough to give you a false sense of security.

Innovate Motorsports has some great info on w/b and tuning articles and there are many other w/b products on the market. A turbo motor is an expensive thing to rebuild.

http://www.innovatemotorsports.com