Reading Air/Fuel Ratio
A well-equipped dyno is capable of measuring the engine's air/fuel ratio either with a wideband oxygen sensor or a tailpipe probe. This allows very accurate tuning of the fuel curve either by rejetting the carb or reprogramming the computer in an EFI car, and it's one of the most important benefits of dyno tuning.
Unlike narrow-band sensors, which basically just tell you if the engine is rich or lean, a wideband sensor can tell you exactly what the air/fuel ratio is in real time, making them far more useful for tuning and calibration.
You can use a tailpipe probe (shown) similar to that used by smog-testing stations, but sometimes the air/fuel readings aren't as accurate because the probe is affected by air from outside getting pulled up the tailpipe and there may be a lag in the readings.
Ideally, have a standard O2 sensor bung welded into the header collector about 4-6 inches from where the primary pipes enter the collector. This should take about 15 minutes at any muffler shop and costs about 20 bucks. After the session you can plug the hole with a pipe plug.
Types of Dynos
Two basic types of chassis dynos commonly used by performance tuners: inertia dynos and load-cell dynos.
Inertia dynos are based on the law of physics that states force equals mass times acceleration, and in practice calculate horsepower by measuring the time it takes the rear wheels to accelerate a known mass (in this case the weight of the dyno roller itself) to a given speed. Inertia dynos are very good for evaluating before-and-after performance gains from engine bolt-ons, and for tuning at wide-open throttle (WOT). They don't work well for tuning at part-throttle because most can't hold the car at a steady part-throttle load. One of the most popular inertia dynos is the Dynojet 248.
A load cell dynamometer uses a power absorber, usually an electromagnetically controlled brake, to create resistance against the dyno roller or rollers, allowing it to simulate the rolling resistance of an actual road surface at various speeds. This allows for part-throttle or steady-state operation. Load-cell dynos are particularly useful for calibrating electronic fuel injection (EFI) systems or for smog testing under various engine loads. Some popular load-cell dynamometers include the Superflow Auto-dyno SF-830 and SF-840 and the Mustang MD-250 and MD-1750. Load-cell dynos usually operate as inertia dynos for full-throttle testing.