Is there any settings in Tunercat for OBD I to look at how IAT modifies spark advance?

 

I've been wondering the same thing for a couple years because I can't find any reference table for IAT in TC or LT1 Edit. I guess you could fool the PCM easy enough, then tune for whatever you set it at, and just lose the IAT adjustment for cooler/hotter air. It's funny that they didn't find the table for it.

 

You got me looking at TC again.

The only thing that is valid is this:
"Inverse Temperature Correction Term Vs. MAT"

MAT = Manifold Absolute Temperature" (or so I surmise). However, I do not see what the "Counts" are for. What Counts is the computer modifying? Injector, in micro or milliseconds? MAF in hertz? These are not counts.

Now, air density is based (not accurately and wholly since humidity displaces the air and so reduces the accuracy) on pressure, volume and temperature (PV=nRT) so it makes sense the computer would modify the results of the MAF based on incoming temps.

However, this does not, in my mind, answer the complete question of intake temperature's role as a sensor. It only makes sense that intake air temp would impact an engine's ability to predenotate and thus GM compensate for it in our PCM's.

 

The internal calculations made in the MAF sensor, measuring heat loss to the incoming air, account for the changes in air density. By definition, "mass air" means they have accounted for density.

There is logic in adjusting timing based on inlet air temp, and I do that with my aftermarket ECU.

Often when GM speaks of "counts" they are refering to the binary representation of the analog signal. In the case of the IAT (or MAT) sensor, that would be the conversion from voltage to an 8-bit binary number.

 

The FSM does say that IAT is used for both fuel and spark. Presumably only used for fuel in SD mode.

Probst seems to support that presumption - says high IAT will retard spark, and then mentions that high IAT reduces fuel in SD mode.

 

This is old, however I cant find much about what "Inverse Temperature Correction counts vs MAT" actually does. Does anybody know? Does it actually change the sensitivity of the MAT sensor and what it modifies, or anything close to that?

I checked the stock PCM using a test tune (set all fuel and timing tables to constant values, such as 15 degrees of timing and 70 for the VE tables) to determine what affect a change in MAT had on both the Injector pulse and connected a drill to my spare opti connected to the harness...

Anyway, I found the MAT does change the injector pulsewidth but doenst change the spark in any way, the problem with the MAT and injector pulse is the amount it varies is constantly increasing or decreasing, and it's not completely repeatable, so not sure what else the MAT function does.

 

Sorry to drag up an old thread but, I just cant help myself.

Since a recent misunderstanding with a pro tuner I have been stuck on this and a few other items. During the conversation this tuner stated that there were a a few tables he had added to tuner cat with the tdf editor.

First this has nothing to do with timing changes only fueling strategy.

The first he talked about I believe is actually the table discussed above which everyone has if they're up to date with definitions. What he basically said was that throttle response and fueling in maf cars would depend on this most especially in ram air cars due to fresh incoming air. Basically he said that at the strip idling in the lanes and doing a burnout will drive the mat/iat high and make the car leave lean and richen up as time went on.

So I did some digging around and read some patents and engineering papers. I came to a conclusion after reading a few patents from gm about transient fueling in speed density systems that gm has. Now I believe that the inverse temp correction term vs mat has to do with transient fueling. What I think is that when you slam the throttle open the temperature of the mat (maf's internal iat) is going to take time to adapt to the steady state condition that will develope shortly after. I think the counts in this table help to predict what the final temperature that the mat will reach and how long it will take. They are not the whole story though. I think they are called counts because they are actually a characteristic time like used in engineering heat transfer problems.

This same tuner also mentioned that there was a maf vs coolant correction table, and a maf vs barometric pres table. These were other values also mentioned in these patents (along with actual airflow into the engine) that went into predicting the temp change and the time constant during transient conditions when sensor data would be inaccurate.

So has anyone tried modifying this particular table before or have any experience to share?

I hope I am not too far out of my tree here, been trying to piece this together for a few weeks since the guy that mentioned it to me basically refused to elaborate at all once my ears perked up.

 

I still don't have an answer so your read was interesting.

However, my thinking has changed on this subject in regard to how important it is to my situation. I would like someone to correct me if I am wrong, but for my use (75% road racing) transient fueling is not as important. Adhering to emissions is totally irrelevant to me as I don't have to smog (THANK YOU Arizona).

When you talk of going down the 1/4 and leaving lean and finishing rich, is this a whole point of variance or just a few tenths, referencing AFR? If just a few tenths, I don't really care but can see emissions being an issue in a mass produced vehicle.

 

The lean to rich thing is very short term, like a lean spike and then hitting your target a/f (think a few ms here). For what its worth these same adjustments as per the patents I have read help with swinging rich on a sudden throttle closing as well.

Its not ultra important to me either, I have great 60ft times, but I know the response and perhaps the performance can be improved. I just know there are things that have not been worked out about the stock pcm and it really bugs me. I know there are plenty of cars out there that could use some improvement on tip in.

 

This might add something to your thoughts, not sure but this is from my other post a while ago.. deals with the IAT and it's measured affect on pulsewidth:

...I see a slight increase in AFR between shifts last time I was at the track, by the beginning to the end I leaned out about 0.5-0.75 to 1 on the AFR scale, I also noticed my IAT numbers were increasing, so I tested it in the garage using a new and improved benchtop simulator (really just a 12v motor attached to an old opti with the PCM installed in the car).

My first try didnt look good for some reason, maybe cause I was holding a drill trying to heat and cool the IAT sensor, but the second try showed a big drop in Injector Pulse as IAT temps went up, but the pattern is a bit confusing: here's the results:

IAT------Injector Pulse (ms)
(Injector going down)
45.1-----8.58
89.6-----6.85

(Injector Going Up)
154------6.88
120------8.44

(Injector Going Down)
64.0-----8.53
96.4-----6.94


So, as IAT goes up, Injector Pulse Goes Down, and vise Versa, but not at the same points.

If you graph it, it looks like the injector pulse direction matches the change in direction of the pressure closer then it matches the actual switch points, if that makes sense (when IAT changes direction, so does the Injector Pulse width)

Also, changing the values in the "Inverse Temp Correction Counts vs MAT Temp" to a constant value, the Injector Pulse stays exactly the same.

One other thing I noticed, I have all fuel tables including VE set to the same value, PE also all 0'd out, and still I get a jump in Injector Pulse from 6.27ms to 7.42ms, just from twisting the throttle in and out of PE mode. ???? ? That doesnt make sense either unless there's a section I didnt know about.

using the 2 Bar tune I think magnifies this x2 in the higher and lower numbers, and is maybe why it's so noticable, so I started around normal driving temps in the "Inverse Temp..." section and removed up to 2x at the highest and lower numbers, and blended the numbers in between, centering around "normal" summer temps which is around 80F since it seems to run very consistent there.

 

I am reading your post with interest but you need to restate this paragraph. I cannot get my head around it and therefore cannot focus on what you came up with clearly.

Sorry maybe I am just not getting what you mean by pressure.

Thank you,
Ben

 

Sorry, I left alot out and probably used pressure instead of IAT. If you plot the stock IAT vs Pulswidth 5 times with everything else consistent like I did then you see the same basic trend but the switch points (from ramping up to down) are not the same every time and the ramp rates are not the same, unless you 0 the IAT table then there is no change in pulse width.

I understand the need to change pulsewidth with different IAT but I only understand it if it's the same every time.

 

Now that this has affected me greatly in my tuning, I see where you were coming from and can respond.

Temperature affects pressure differently at different temps. It is no linear and that is your confusion I think.

 

My confusion comes from the results of the IAT calculation on injector pulse width, nothing else. Specifically, why the Injector BPW modification amount does not always match the "Counts" in the columns. It seems the "switching" or change is occuring as listed at the right temp, but the amount the injector is modified is inconsistent.
The attached picture/graph shows these facts:
1) IAT only affects FUELING
2) IAT does not affect timing
3) As IAT adjustment value (counts) increases, the Injector pulse width decreases (fuel mixture leans out).
4) GM slowly reduces fuel from -40c all the way to max IAT temp, so this is why I was seeing a leaning occuring as my IAT temps increased.
5) The relationship between IAT and the modification it applies to the Injector pulsewidth is not constant. It is close, but not constant, there appears to be some type of hystresis or other happening. In addition, the IAT "Counts" flat line does not match the charted Injector Pulse width flat line. Easiest way to stop the leaning while under boost and increased IAT temps, is freeze the value at above say 100F.

6) For all of those that have problems with Speed Density and open loop and having the adjust fueling between summer and winter, this should all you need to fix it. If it's OK in summer since that's how you tuned it, and too lean in the winter, then all you need to do is lower the values in the "counts" column from the colder temperatures that have the problem. Lower values increase fuel, higher numbers decrease fuel.



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