ECM Tuners... Please help...

[Mark B]

Hey everyone,
I am just starting to enter into the world of tuning & chip burning. I made my first datalog last night. I did a 4th gear full throttle pull from 1900 to 4600rpm. This is what it spit out. Please help me understand what this is telling me. I am getting most of my knock during my shifts. I see that my TPS never hit 100% I guess I need to adjust it. When my foot is off the gas, it registers at -1.8%. Let me know what you see & how you saw it.

Thanks in Advance,
Mark B




One more thing. I know what MAP is, but why is it a value in just about every chart? I dont understand how it relates to other values in the ecm & how I can use it to enhance performance.

[Injuneer]

Think of MAP (manifold absolute pressure) as the opposite of vacuum. It is simply the pressure in the intake manifold, referenced to "absoute" 0 pressure - a total vacuum. Most pressure gauges are reference to standard atmospheric pressure, so a pressure of "0" on a typical pressure gauge is actually at a pressure of 14.7psi (assuming you are at sea level) in "absolute" terms.

MAP is in metric units in your data log - kiloPascals (kPa). 100 kPa is a standrad metric atmosphere (which to complicate things even further is 14.5psi).

Your scanner should also be able to read "barometer", which will be the ambient barometric pressure where your car is located. The difference between barometric pressure and MAP is vacuum. For example, when your car is idling with a stock cam, you might see a MAP of 35 kPa. If the barometric pressure was 100 kPa, your vacuum would be 100-35 = 65kPa vacuum = ~19.2"Hg.

MAP is important to the ECM because it indicates the "load" on the engine. As noted, an idle MAP (no load) might be 35 kPa. At WOT, as you can see from your data, the MAP is 100+ kPa. That means your engine was under maximum load, because full atmospheric pressure is present in the intake manifold to force the air against the dropping piston. This provides the maximum density of air in the cylinder, and a high volumetric efficiency.

As load increases, the A/F mixture has to richen. As load increases, you want to back off a little on spark advance. So the MAP value is used as one axis of the tables that control fuel (volumetric efficiency in a speed-density system) and spark.

On the spark tables, the other axis is RPM. Obvisously, the faster the engine is turning, the more advance you need to allow the mixture to ignite and reach maximum cylinder pressure at ~ 15deg after TDC.

As far as your data, it appears the A/F mixture starts off a bit lean, and richens as RPM increases. Its always risky to estimate WOT A/F ratio from the O2 milliVolts, since the sensors are not all that accurate at the rich A/F ratios (12.8-13.2:1) used for peak torque and HP. Nothing unusual in the rest of the dats.

[Kevin - Blown 95 TA]

Hi - I wrote a complete answer to this earlier but got booted off the net and the post was lost. Looks like Fred did a good job of explaining the MAP sensor. He did it almost word-for-word as my post was written and hit all the major points. Just remember that the ambient pressure at sea level is around 14.7 psi which = 1 atmosphere of pressure which = 100KPA of pressure (approx). At WOT, you will see close to atmospheric pressure in the manifold (14.7 psi or 100 KPA) because the throttle blades are open and the manifold is at the same pressure as the outside air and at part throttle with the butterflies partially closed, the engine still sucking air - you have less than atmospheric pressure (we consider it vacuum) - it is closer to 0 psi and 0 KPA at idle and so on.

Anyway, regarding the TPS problem, my advice is to move the throttle by hand to WOT and see if the scanner reports 100%. If not, then you may need to try to adjust the position of the TPS or slot it and turn it slightly. If it does report 100%, then check the throttle cable to see why the gas pedal doesn't fully open the throttle.

[Injuneer]

The TPS sensor on his 91 is already slotted, and needs to be adjusted to a specific value, unlike the LT1 TPS.

[Kevin - Blown 95 TA]

Well that makes it easy