Grim Reaper

I have made a similar post in the TPI Board regarding “Relocated MAT…” but I have included it here as what I am discussing is getting into the PROM and feel it might be better discussed here. My real objective in this discussion centers around the MAT and WOT operation. Specifically, I am curious whether a relocated MAT does ANYTHING @ WOT, or just a fancy thermometer to tell you the incoming air temperature. I don't think there is a question that the MAT is needed at part-throttle for the EGR to function.

I realize TunerCat does not allow you to make any adjustments for the incoming air temperature. And in reviewing the Tables of the hack for the 8d, there does not appear to be any tables that can be modified to adjust for the MAT (PW or Spark). However, I have plenty of WOT runs I've done at various MAT readings (many on the same day, done with the stock MAT, relocated MAT and 4.7K resistor to make the ecm THINK I had a specific air temperature.

And I can tell you for a fact, that a relocated MAT DOES affect the injector pulse width @ WOT (on my SD car anyways). It does not appear to do anything to the spark advance. I offer the following Diacom+ data I have taken from a number of runs I did while doing these tests.

MAT*F CTS*F InjPW
108.....179.....10.8ms
93.......163.....11.1ms
75.......172.....11.4ms
64.......163.....11.8ms
55.......176.....11.8ms

I realize the CTS also has a great effect on PW and spark, but my CTS readings are fairly close in range. Also my CTS readings varies both Up and Down...but regardless of the CTS reading, a lower MAT ALWAYS results in a longer Injector PW. So I have to conclude that GM has some algorithm within the 8d code for the SD cars that adjusts the PW based on a given MAT.

All of these tests were done at the same location within a few days of each other and with relatively the same ambient air temperature. The only major variable was the MAT reading. Also, the order that I performed these tests are not in the order I gave the data (I sorted in in descending sequence on MAT*F).

And for those that may doubt this data, I have a conclusive test that I would like to offer. (For SD cars and users of Diacom, I cannot say whether MAF cars will get similar results...but it would be interesting none the less).

Get 3 resistors, 1.5K (105*F), 4.7K (55*F) and 16.2K (14*F)...I've included the MAT temperature reading those resistors should equate to. Do back to back runs, using each of the 3 resistors...do it twice...do it thrice for each resistor if you wish...the more the better. I am willing to bet that you will have a higher PW reading for the higher resistance resistors (lower MAT temp).

I plan to do this very test in the spring with my Diacom and I would be interested if others would be willing to participate to get independent confirmation. You must have a fan switch or some means of maintaining a constant CTS for the test to be truly valid as a higher CTS will result in shorter PWs.

Of course, if others do this test, get the same observations (longer PWs with lower MAT readings), that still does not solve the problem of where GM embedded this algorithm in the PROM and how to use this information to our advantage.

Further analysis that I am working on, indicates that the change in the PW is too long for the given change in temperature. As the MAT temp goes down, the ecm adds too much fuel. Until the algorithm can be modified to give a better correction factor, it still would appear the best solution would be to burn a specific PROM for a given range of ambient air temperatures to obtain optimum performance @ WOT at a specific temperature.

Again, I am only saying this with SD cars. For MAF cars, a relocated MAT may only be a fancy thermometer of your incoming air temperature...unless some MAF person does a similar test with Diacom.

Also, for those people curious about the tests I’ve done, the test of the 108*F MAT (stock location), 75*F MAT (relocated) and 55*F (4.7K resistor) were all done on the same day within 30 minutes of each other. The other test at 93*F (stock) and 64*F (relocated) was done a few days later at a slightly lower air temperature due to testing earlier in the day at the same

Grim Reaper

I will also add that my Injector PW is my maximum PW obtained from 3,200 to 3,600 rpm. The PW reading was flat in that rpm range AND the same in 1st, 2nd and 3rd gear.

Grumpy

Only way to rule out different variables is doing testing like this on an ecm bench.
.x MAP and the way you have the VE tables set can account for variables.

If you go over to the GMECM area and look around you can see a pic of what mine looks like, and a narative on building one.

Some of the ecm stuff get real convoluted, and can appear to follow theory and then just crumble.

Grim Reaper

I don't know if I found something, but I have been review the 8d hack and found an algorithm at address LCC34. This is remarked as a "Calculation of Speed Density" and definitely the MAT is component of this calculation. Another remark at LCC40 says "VE Calc = ((MAP - EGR part press) * Cyl Vol/((MAT + 233) * 128))". There is even a compensation factor should the EGR part pressure exceed the MAP (forces it to 0).

I also noticed that if the calculation results in calculated air flow exceeds 256 Gm/sec that it fixes it at 255.996 Gm/sec. This is at location LCC6B. There is also a reference to "Ideal Gas Law".

Also there is a Subroutine at address LD7DA to LD810 that references a table dealing with some calc comparing the coolant temperature to the MAT. LD7FC again does some manipulation to the MAT forcing it to ((MAT^.5)+233)*128.

Though I have worked with assembly language (many years ago) on some different processors and I can "fumble" through most of the instructions, I really need to get a good book on the actual instruction set to truly understand what this is exactly doing.

TRAXION

Glenn,

I don't understand how I missed that before. It is right there. I am baffled how I missed that. IMHO, you are 100% correct.

The calculation of BPW after address LCC75 uses the grams air / cyl calculation (ideal gas law) from LCC75. Good work!

Tim

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TRAXION's 1990 IROC-Z
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