Ok, i had read some where about this guy making his own lean/rich indicator, he said he just got an oxygen senor and had a bung welded into his header and read the voltage with a digital multi meter. Anyone else tried this, and if so, how do you knwo what the voltage reading is equivalent to stoichiometric what ever. I'm really intrested cause my carb is a b---- to tune and it sounds like a really cost effective alternative to the $150 msd lean rich indicator.

 

The O2 sensor is designed to produce 450mV at 14.7:1. There are curves available that attempt to show mV output vs. lambda (lambda = 1 means stoichiometric = 14.7:1). The OEM sensors are very accurate at 14.7:1 and relatively insensitive to operating temperature at that one point. So if all you are interested in is checking to see when your engine is operating at 14.7:1 (data of questionable value), you could easily devise a circuit that would measure the output voltage. I do think the PCM applies a 450mV bias voltage as part of the measurement circuit, since you will see 450mV at startup, when the O2 sensor is cold and not functioning. Your measurement circuit would have to include this feature if you are using a "stand alone" sensor.

Using a voltmeter becomes problematic, because to measure the voltage, the meter can pass a small current through whatever it is measuring, and so you need a very high impedance meter to insure the current applied by the meter does not destroy the sensor element. Even then, I have seen references in the literature that indicate that measuring the sensor output voltage with a 10Megohm DVM can still produce voltages that are up to 130mV different than what the PCM reports.

Then there is the issue of hysterisis. The voltage produced by the sensor at stoichiometric is different, depending on whether the exhaust gas is swinging from rich to lean or lean to rich. I guess if you averaged the readings over a period of time, or dampened the measurement circuit, you cold eliminate this problem.

And finally, you need to recognize that the 14.7:1 stochiometric value is of primary importance only for emissions... that is the point at which the combined values of CO, HC and NOx are minimized, and at which the catalytic convertor is designed to operate. It is also the point where fuel economy is "good" but not "great"..... you would get better economy at even leaner A/F ratios, but you could produce too much NOx.

Most people want to see the A/F ratio at max power or max torque, and at those levels, the OEM narrow-band sensor becomes almost useless. The curve of mV vs. A/F ratio becomes so flat that a very large change in A/F ratio only produces a small change in mV output, and the sensitivity of the sensor to exhaust gas temperature is so great that a small change in exhaust gas temperature can produce a greater change in mV's than a large change in A/F ratio.

What are you planning to use the meter for?

 

well, i want to be able to tune my carb in a few wot runs, because when the temp changes my performance drastically changes. Like if i tune it in the after noon before i go to work, and it runs good, when i get off work , it runs not so good.

 

I don't know anything about building a rich/lean indicator other than the fact that alot of guys use them. The only problem with that is the fact that guys have learned to rely too much on them. O2 sensors, EGT sensors.... they can give you alot of useful information but they can also be a bit misleading. Not every engine runs the same, has the same a/f requirements or "optimum" exhaust gas temp. There's no such thing.

Bottom line, learn to read a spark plug. It's a lost artform but it will tell you just about everything you need to know. Keep a log. Note air temperature, humidity, bar pressure, everything. Get the tune right (read the sparkplugs) and keep a log of that tune. You'll build up a good "database" of information that way and that will keep you on top of the tune with varying weather conditions. I carry my laptop for this stuff.... no data acquisition for the el cheapo motors I like to race.

-Mindgame

 

Your far better of with an EGT gauge and reading your plugs..The thing thta sucks about the plugs is that to get the best reading you have to either be towed back to the pits or take your chances reading them after you have driven back..The latter isnt the way to go..A buddie of mine who had a mustang shop next door used to do 2 gear burnouts at the front door to check his tunes.He used my perometer to catch the header temp and read the plugs..I guess it works the same..Just shutting it down under load really abuses the converter in auto cars.

I used to use my Fluke 88 and Fluke meter before myself when i did diagnosis like you plan to use for tuning..I wouldnt say go ahead since when compared to my vantage meter the respose is the same but hard to read since its real time..Import racer Eddie bello used to have one in his LC2 300zx set up like that but I remember him saying that its hard to keep an eye on during a run.If you must use a meter then the ceapest one to use would be a fluke 73 wich cost the same as an EGT or A/F guage..

Its best to start with the basics and if your at the track sometime ask the guys there for help..Most will be willing to give you guidance as long as they arent to busy.