Here is a snippit of a data log I made.

LogFile

Notice when I go WOT the spark advance is like 40 degrees. I do not understand why . I checked the LT1 edit file and the most is should have at WOT is 35 degrees. Any ideas? Also, the Throttle position is only reading 10% but I am definitely at WOT. Are these readings caused by a data logger problem? Ignore the speedo reading as it is not calibrated correctly. If you want a copy of the PCM file, let me know. I will need to email that directly. Another think I noticed is the KPA reading is a tad off of 100 at WOT (as low as 94). Seems I read a few days ago that this may be caused by a leak in the intake leak or air induction leak.

Allen J. Brotze

 

All I see is a jumble of numbers, no labels, no columns, etc. You need to post as a .csv or .xls file so people without AutoTap can open it.

There is nothing sacred about MAP being 100kPa at WOT. MAP will typically be a few kPa less than barometric pressure (BAR on the typical log). If you live in Denver, you will see MAP in the 80's, because the barometric pressure is so low. Look at the difference between MAP and BAR to determine if you have a major obstruction in the air inlet track. A leak in the system is going to show up as a higher MAP, not a lower one.

 

I've had MAP in the 104 range, I dont remember if I ever saw 105 or higher though. This was I think around 3400 RPM and above.

I remember alot of people on the LT1 Edit list talking about mystery timing showing up, I think 2º or so added to anything that was put in the PCM tables, dont remember details though. Tune it till it runs well and I guess I dont care much about the quirks that may be in the system, however strange they may be.

I use Freescan and it should dump all data very nicely, if you open the CSV with Excel, if you trimed some data though obviously leave ROW1 alone since that has the listings of what each column is. I often just get rid of, or hide the columns I dont need to look at, and save the rest...

 

I have never established to my satisfaction where the "mystery timing": comes from. It refers to timing reported with various scanning programs being 2-4 degrees greater than the timing called for in the PCM tables. I'm sure a GM engineer could explian it, but I can't. IOW: is your "real" timing 40 degrees or the value called for in the PCM table?

If it runs good, like Mudge said, don't worrry about it. My timing table calls for 29 degrees max at WOT/high load. The scanner reads 33 degrees.

Rich Krause

 

Just checked the TPS voltage and it is 5 volts at WOT. No problem.
I was just concerned about the timing advance at 40 degrees (real or otherwise) because I was going to spray. I do not think that 40 degrees of timing would be good on nitrous. kaboom!

If anybody has more info on this mystery timing please post it.

 

I was just thinking of another source for the added timing. Could the process of degreeing a cam shaft result in the added timing
??????

 

The advance value you see in your data log is what the PCM is trying to set, based on the tables, less retard and possibly +/- some hidden timing offsets. The PCM has no way of knowing what you did to the cam... it seea some pulses coming out of the Opti, and assumes they were generated by the cam in the stock orientation.

So, if you have advanced your cam, you have advanced the Otpi signals, and this might explain why the PCM needs to include what appear to be greater than expected values in the timing tables.

If I were tuning your engine on nitrous, I don't think I would start with 40deg advance, no matter what the explanation. Back it off, depending on how much you are planning to spray, and then play with the advance until you see incipient signs of detonation on the plugs (I let the experts do that.... ). Back off a couple of degrees and play with mixture, octane and minor timing tweeks to optimze power.

 

That would be true IF the pcm has reference to TDC. Does it?

 

TDC info comes only from the Opti. There is no other postion input.

I'm not saying that advancing the cam would tell the PCM the cam was advanced.... it would only tell the PCM that things were at TDC when in fact it would be a few more degrees before that was a fact. So the cam advance (relative to stock) would have to be manually added to the timing tables to get the timing where you really want it.... e.g. if cam is 4deg advanced, enter "40" into the timing table, when you want "36".

Or did I miss the point of your question... I get confused sometimes.... .

Fred

 

Expecting that answer ahead of time, I agree with the above. However, to me, it appeared to not be aligned with portion of previous post:

So, if you have advanced your cam, you have advanced the Otpi signals, and this might explain why the PCM needs to include what appear to be greater than expected values in the timing tables.

My first post was basically not agreeing with the above statement, that is why I posed the ? originally.

Fred, maybe I'm the one confused. Afterall, even I confuse me occasionally.

 

If the opti was responsible at all though, it would do it everywhere, not just at WOT.

 

I guess I'm just confusing things by trying to cover all the cases.

He has 35deg in the program for that particular (WOT) combo of MAP and RPM .

(Brotze - did you verify by looking at the specific value for MAP and RPM, and recognizing that the PCM probably interpolates between the adjacent cells?).

But he sees 40deg as the PCM timing output under those conditions. The hidden offsets would be the only contribution I could think of. That is not the result of programming.

(And I'm just assuming there are hidden offsets, since the problem of a few degrees extra showing up in the actual PCM ouput vs. timing map has been noted elsewhere in several posts - but I don't even use the stock PCM, so I have no rteason to go searching for the cause - whewwww hard to explain).

And none of the above would be affected by advancing the cam physically. The PCM would remain "blind" to that action.

But, if someone was trying to do a mail order tune, and wanted to net a particular value of advance, and was told the cam would be installed "X" degrees advanced, they would have to use a number "X" degrees larger in each entry of the timing map to get the timing they wanted.

Also not sure how we know that it doesn't do the same thing everywhere, only at WOT. Did I miss this somewhere? Remember, I haven't been able to read the log, since all I see is a jumble of numbers when I open the file.

Fred

 

Take another look at the logfile link. I corrected the format. Also, I set the timing at 90,95 kpa and 100kpa to 35 degrees for all RPMs via LT1 edit.

 

This response to the mystery timing question is from Christian Millard

Hi Payam and everyone,

....
The extra spark is real, the scantools get the true commanded advance, the
LT1edit and Tunercat software don't see/ affect the values.

Just keep it in mind that 3 -5 degrees will be added for higher rpm
(actually adds 1-2 below as well) when adjusting timing. I always make
changes in timing proportional to what it has now, no major jumps in timing
unless the car just got a blower.

There are a few smart people that confirmed this in the disassembly, wish I
could take credit for finding this, but it's their hard work that confirmed
it.

Hope this helps,
-Christian

 

As aBrotze mentioned above, there a few very knowledgeable ecm/pcm members on other lists that have verified what you considered to be the case.

Fred, regarding my quoting of your previous posts, it took several (re)readings to realize I was misinterpreting what you stated.

So, if you have advanced your cam, you have advanced the Otpi signals, and this might explain why the PCM needs to include what appear to be greater than expected values in the timing tables.

The key phrase that I MISinterpreted is 'PCM needs to include'. I interpreted that to mean PCM can include this extra timing. This obviously is NOT the way you meant it. MY mistake! Sometimes reading something 5 times is not enuf for me.