Air/Fuel Mixture Meters and Oxygen Sensors
Summary: Don't use low-cost narrow-band A/F meters for tuning your engine.Or, Why normal O2 sensors don't make good A/F meters?
A low cost A/F meter is not useful for fine tuning your car's performance. A wideband O2 sensor is required, and those typically run upwards of US$1000.
Why is a low cost A/F meter not useful for tuning?? These meters use standard O2 sensors to measure the mixture. These O2 sensors are extremely susceptable to temperature changes, a small change in exhaust temp alters their output significantly. The nature of these devices is that below 14.7:1 their voltage changes very little but it is low, and above 14.7:1 it changes very little but it is high (I may have the voltages backwards, but that doesn't matter for this). When the mixture crosses 14.7:1 the output voltage jumps dramatically from high to low or visa versa. This large transition is easy to detect and is unaffected by exhaust temperature, so you can always count on it being detectable (at least once the car has warmed up). All other changes are completely overwhelmed by the exhaust temp changes, and the exhaust temp is never stable. This means that no useful information can be gained from the sensor except whether the mixture is <14.71:1 or >14.7:1. Any (non-wideband) meter which is showing you how much richer or leaner than stoich you are running is making up this information and is misleading (and distracting). The graph depicted here is intended to give some idea of why standard O2 sensors aren't adequate:
O2 sensors are used in all modern cars for controlling idle and cruise in a closed loop because the best mixture to have for minimizing emissions and obtaining good fuel economy happens to be 14.7:1. If you run at that mixture under heavy load, especially with NOS or boost, you will blow up your engine -- it is far too lean. Maximum power is acheived at about 12.5:1.
Putting these facts together, it should be obvious that trying to use a device that only tells you what side of 14.7:1 you are on while tuning for mixtures that are quite a bit less than that is pretty much hopeless. It might be able to tell you that you have blown your engine, but it won't tell you that you're about to.
Now, if you are interested in whether your engine is operating properly in closed loop and idle or cruise, then an A/F meter is useful. You still can't really use it to tune, however, because the ECU is constantly fiddling with the mixture to keep it bouncing around 14.7:1 (it does this so that the catalytic works better, btw)... all you'll be able to do is fiddle around until it is working properly.
In the future wideband O2 sensors may become cheaper. Modern engines are heading towards "lean burn" designs which will run on mixtures like 19:1 or leaner... but doing so will obviously make the current O2 sensors less-than-useful for the same reasons as I outlined above, except on the lean side of things. Thus OEMs are likely to be working on better sensing technologies. I've also heard that the guys at http://www.autospeed.com are working on a relatively low cost wideband O2 meter... they can't do much about the cost of the sensing element, but they are trying to develop a cheaper box to attach it to.