I'll admit that I haven't started even gathering the parts to start burning my own chips (I'm lurking here to learn as much as I can) but the current chilly weather conditions here in Houston have raised a question. How do you take into account different weather conditions while gathering data and tuning your chips? Do you only gather data on "mild" weather days? Is there some way to normalize logged data vs. weather conditions? I know cold air is more dense than hot air therefore the engine needs more fuel and the ECM compensates for that to some level but doesn't the tuner also have to take that into account somehow?

 

With MAF, the MAF sensor itself detects the cooler temperatures by showing higher flow. MAF cars do not use the MAT sensor to make fuel corrections. It's there primarily for enabling the EGR temp on a MAF car.

SD cars are different because the MAT sensor DOES alter the fuel trim. If you have relocated the MAT sensor, you will need to alter the "MAT Inverse Lookup Delta Multiplier" table to have the ECM calculate the correct fuel trim as GM has set this table for the MAT to be in the plenum.

 

This is interesting to me....so what happens on a MAF car if you remove the MAT? I would assume a code, but does it actually control anything else?

I was thinking about doing the MAT sensor relocation when I install the SR I have....From what you are saying, it probably wouldn't matter then...

Oh yeah, I am !EGR, !AIR tubes, and basically !emissions...

 

Here's a wake up call, due to the increasein back pressure, with a *normal* car you lean the car out do to the increase of Back Pressure due to high barometeric reading. Mind you not a huge amount but that is the way the compensation goes.

On a MAP system the sesnsor is an Manifold Absolute Pressure senor so it's *seen* by the sensor.

When moving sensors around you may be upsetting a given kludge factor, and unless you really understand what your doing should be avoided.

 

On some GM ECMs, the ECM does a test of the CTS and MAY use the MAT sensor if the ECM thinks the CTS is in error. This is a reason why some people do not advocate relocating the MAT even on SD cars.

I cannot say for sure that MAF cars does this MAT/CTS "switching" as I have not spent the time reading/documenting the MAF ($6E) soure code like I have on my SD car. But on SD cars there is code in the eprom that does this test and the ECM may chose to use the MAT instead of the CTS in some circumstances.

 

All the above information is good but it's not really the answer(s) to the question I thought I asked.

Maybe I didn't ask the proper question. Here's another angle...Let's say you record a particular BLM is 128 with an outside air temp of 90*f, humidity of 95% and the Barometer around 30.00 (aka a mild summer day in Houston, TX). If I do all my data recording during the summer and tune my chip to that data, it is not going to be correct on a day like today (temps in the high 20 to low 30 range, no humidity and high baromoter). I know the ECM will make some corrections but itsn't the engine still going to be way lean in that BLM?

If I'm way off base here, someone tell me that flat out and I'll shut up.

 

The BL is the final adjustment to the fuel trim. It's a reflection of what the O2 is sensing. the better the calibration the less it will deviate. 128 is not a number written in stone, it just allows the max allowance for the ecm to move rich or lean to acheive an average of 14.7: AFR.

 

I would tune your cars PROM during your most common weather conditions that you run the car in. Once you get the cal fine tuned, the ECM should be able to adapt to other, "rare" conditions. After all, GM, and the others, build production cars to operate in a wide range of atmospheric conditions.

Now to Glen's comment on the infamous MAT sensor. I have mine relocated to the plastic air intake, and I am running the MAF system. I moved my sensor BEFORE I started burning chips, since that was the "trendy" thing to do. But, in TC's $6E cal, there are no tables or constants referring to the MAT sensor. That kind of bothered me, but sort of confirms that the ECM does not use this sensor for fuel trim. But I also don't see any tables relating the MAT to enabling the EGR. (just the TPS) There could be tables or constants referring to the MAT, just that TC hasn't pulled them out yet.

But common sense does tell me that the air temp entering the motor would be more accurately measured further upstream of the plenum. I have seen the MAT rise quite a bit, ie "heat soaking", after I shut down the motor for a few minutes. It took a few miles of 55mph driving to get the temp back down to where I though it should be. But after I relocated the MAT, the recorded temps seem ed to be more realistic.

Sorry to get off the main topic, just getting in my 2cents!

 

Mike, look at the "EGR Enable Temp" (location $C247 in the $6E calibration). This is the field the controls whether the EGR will operate or not. That temp is based on the MAT, not the CTS.

It should be defined in the $6E TDF calibration by TunerCat. I had mentioned it them about 6-8 months ago.

 

There are two very different matters here.

In my car I run a relocated MAT, it's installed right up by a runner to get the most accurate temp for the air/fuel entering the chamber, but my calibration also uses a timing and fuel correction using MAT.
Might add this is a turbo application so it's easy to see where end gas temps are so important.
Now on some calibrations they use IAT, and that's basically used for figuring out the weight of the fuel so there are no tables to atler that calculation. Then in other applications IAT/MAT are used for enabling parameters for EGR.

 

Let's say you record a particular BLM is 128 with an outside air temp of 90*f, humidity of 95% and the Barometer around 30.00 (aka a mild summer day in Houston, TX). If I do all my data recording during the summer and tune my chip to that data, it is not going to be correct on a day like today (temps in the high 20 to low 30 range, no humidity and high baromoter). I know the ECM will make some corrections but itsn't the engine still going to be way lean in that BLM?

Another way to address your concern:
The ECM picks the fuel and spark values based on what it sees as the incoming air mass flow rate. So, in your case of a MAF system, it measures this mass flow rate of air directly, compensating for temperature and pressure. The BLM only makes corrections when the oxygen sensor has reported a deviation from the ECM desired "tune".
Now, the MAF does not know whether the incoming air is humid or dry, only how heavy is is. So in your first exanple where the air is loaded with water, it will be realtively low in O2, resultijng in a richer mixture than the second case where the O2 content will be about 2.5% higher than the MAF sees, and the mixture will be relitively leaner. The BLM will change to correct that deviation.
The botom line, as I see it, is that the BLM (say 128) that you achieve will remain fairly constant with changes in temp and pressure, but will vary with humidity changes.

 

Chevero, you are very correct. Bruce (and others) have mentioned in the past that humidity is a problem with the MAF system. Hopefully Bruce will expand up this more.

I will say that while I have not found humidty to have a great effect on my SD car, I would also add that where I live is quite dry and the humidity is quite consistent. But SD does not have a "direct way" to measure humidity either. If the MAP sensor reading is affected by humidity, thus allowing the ECM to make a correction, I have never noticed it.

I DO see the effects of air pressure and elevation on the MAP sensor. At sea level, I record the same baro reading as the local weather station. And I have also seen the effects of elevation on my MAP sensor too. Both with the engine on and off.

I should mention that I use Kpas to monitor the MAP sensor as this has more resolution than using In. Hg. with my scan tool. I have not "decimal precision" with In. Hg.

 

This is good info, fellas. I am beginning to realize that the MAT sensor is not that vital to the running of my motor. If your have a very unique engine combo, IE Grumpy, then the sensor location is critical. Where I live, humidity is NOT much of a factor how my motor runs.

I think the best setup would be to use all 3 sensors, MAT, MAP, and MAF, to control fuel management. I think the newer F-bodies and vettes use this seup. Kind of the best of both worlds. Of course these setups have a much more different kind of programming. Has anyone updated there TPI motor to a newer engine management system?