Originally Posted by Michael Guzzo

IAT - Intake air temperature: there is a thermistor in the intake tube which measures the temperature of the incoming air. Cooler air is more dense than hot air. IAT is needed to calculate mass air flow (see below)

MAP - Manifold absolute pressure: the air pressure in the manifold measured in inches of mercury (inHg). Normal atmospheric air pressure is about 29 to 30 inHg. A MAP reading of less than 29 is vacuum, more than 29 is boost. to calculate boost, subtract the barometric pressure (this is available from ODBII) from the MAP and divide by 2.036

MAF - Mass air flow: The MAF sensor is a constant current wire. The incoming air cools the wire which changes the voltage across it. This is proportional to the mass of air coming into the intake at the intake air temperature. The MAF sensor then outputs a frequency proportional to the voltage. The maximum MAF frequency for a stock GP MAF is 11,500 Hz. In general
up to 3000 hz - idle
4000 to 6000 - cruise
7000 hz to 11500 hz - heavy to wide open throttle
The bigger the MAF frequency, the more air is coming in and the PCM adds more fuel by increasing the IPW.

O2 - Oxygen sensor: The GP front O2 sensor measures A/F ratio of the exhaust gases by outputting a voltage from 0 to 1 volt. At a A/F of 14.7:1 the exhaust should be completely burned and the sensor outputs .450 volts. At less than .450 volts there is more air than stoich (lean) and and more than .450 there are unburned hydrocarbons (rich). The front O2 sensor (B1S1) changes quite rapidly. OBDII performs a diagnostic to verify the O2 sensor is changing properly.

IPW - injector pulse width: The PCM uses the MAF and engine load (gear, RPM) to calculate a base injector pulse width which is the time each injector is open to squirt fuel into the engine. The base IPW is recorded in tables in the PCM as a starting point. The PCM then gets smart and uses fuel trims to "learn" and optimize the fuel delivery for performance and emissions. At a IPW of 21 or 22 milliseconds a stock injector is "static" (fully open and flowing as much as it can).

STFT - Short term fuel trim: The PCM makes small adjustments to the base IPW using the O2 as feedback. If it sees the O2 sensor lean, it increases the IPW a little bit. If the O2 is rich, it subtracts from the IPW a little. The STFT can range from +16.4% (increasing IPW by 16.4% to add fuel for a lean condition) to negative 22.3% (decrease IPW to take away fuel for a rich condition).

LTFT - long term fuel trim: As the STFT instantaneously adjusts IPW, it will reach limits. Then the LTFT will be adjusted to re-zero the STFT. The LTFT is a better average of how the engine is performing as a system. LTFT can range from +16.4% (increasing IPW by 16.4% to add fuel for a lean condition) to negative 22.3% (decrease IPW to take away fuel for a rich condition).

Closed Loop: The state where the PCM is using feedback from the O2 sensor to maintain a 14.7:1 A/F ratio

WOT: Wide open throttle. At WOT, the PCM does not use closed loop mode. It goes "open loop" and uses preprogrammed tables to command IPWs.

Now as you tune, you want a couple things;
* LTFTs as close to zero as possible because when you go WOT, the PCM "locks in" the last LTFT value to bias the IPW during WOT.
* WOT O2 values as lean as possible without causing KR

Here'* my approach to tuning - I tune idle and cruise for LTFT using my AFC 2.0
* Cruise around town and record general driving cycles. Note your LTFTs. Use your ICCU or mini AFC to adjust MAF to get the LTFTs close to zero (within 5%). I have mine set so that up to 4Khz my MAF is 100% (PCM sees actual MAF freq)
* To change the WOT O2 values, use the AFC or ICCU to increase or decrease MAF frequency.

Example: I use my AFC to tune cruising LTFTs to within 5% of zero. During a WOT run, I notice that my O2s start at .940 but drop off to .850 as I approach redline and I get some KR. I graphed O2 vs MAF frequency and determine a curve to dump more fuel as the MAF increases. I also notice that at a MAF of 7000 Hz I get a spike of KR (burst knock). So at 7Khz I add a little bit of fuel there too.