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Old 03-23-2009, 12:49 AM   #1
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Touchy subject - BKT, Max Allowed, MBT spark tables

So, I'm really getting tired of searching for hours on end and having this information obscured. Maybe its some kind of secret of the trade? I dunno. But I need to know, and so does everyone else who tunes their car. So we'd all be really grateful if someone in a high place could shed some light on the topic.

I've heard many many things about each of these tables. But these three "opinions" seem to come up most often:


case 1) The PCM takes the lowest of the three values and then performs adders/retarders

case 2) MBT does absolutely nothing. The PCM takes the lowest of either 1) BKT +/- adders/retarders or 2) Max Allowed without any adders but with retarders.

case 3) MBT does "something" that has been obscured from the public. The PCM applies adders/retarders and then does "something" with the three tables to pick which one to use, a process that has also been obscured.

Is any of these correct???? If it is case 3..... PLEASE tell us what "something" is!!


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Old 03-23-2009, 08:22 AM   #2
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Im interested in this as well. Ill be hooking up the logger an wideband on thursday ro friday so if you like ill play around with the timing as well.

Right now im having an issue tunign my 70mm maf correctly, load scaling in wrong so im loosing timing up top. If i figure anythign out ill let you know for sure.
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Old 03-23-2009, 08:45 AM   #3
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Spark is a very complicated calculation. To into all of it would go even over my head.

Essentially, every vehicle/calibration is different. You will typically have your BKT table, MBT then sometimes max allowed. You will then have your adders/retard tables for ECT, ACT, CHT, IMRC, knock sensor, EGR, combustion noise, torque considerations and other temperature factors.

As I said, not every vehicle will have all of those tables or they may be there but are calibrated out (not used.)

For the most part, MBT can be thought of as acting as a max allowed spark table. In a lot of calibrations there is no max allowed spark table and the MBT table takes that place. Even in cases where there are both, the MBT is typically higher than the max allowed table.

MBT is not involved in the actual spark calculation. That does not mean though that it is not important for the reason I have listed above.
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Old 03-23-2009, 10:01 AM   #4
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So here is what I inferred from Mike's post and other things i've seen (for the focus):

MBT is important mostly because it is involved with torque source and torque-based calculations in the tune. It doesn't do anything directly to the actual spark advance value, but your torque modulators might have a hand in it (especially during atx shifts).

The Max Allowed table is simply the maximum allowable spark, if the PCM calculates more than this it will clip down to this level.

BKT is the limit of spark before detonation at 200* ECT and 70* ACT.


So overall, just upping BKT will not necessarily change your spark value the motor sees, since if the adders put it over the max allowed it will just stay at max allowed.
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Old 03-23-2009, 10:11 AM   #5
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So overall, just upping BKT will not necessarily change your spark value the motor sees, since if the adders put it over the max allowed it will just stay at max allowed.
This is definitely true. If you jack your BKT table up to 50 degrees yet your max allowed (if there) or MBT are only at 15 and you datalog, you will see nowhere close to 50 degrees. You will only see roughly 15 at the most.
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Old 03-23-2009, 10:12 AM   #6
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The Max Allowed table is simply the maximum allowable spark, if the PCM calculates more than this it will clip down to this level.
The MBT can also act as a max allowed table when there is no max allowed table present.
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Old 03-23-2009, 10:24 AM   #7
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I assume you are doing all this with the PRP? Both fuel and spark tables are a pain, and the are so many varibles that can come into play. Not sure about sct software tuning, but we still have to modify our Motec tune files that we've had for...crap, longer then the five years ive worked at my job just about every day or two. Good luck with your journey.
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Old 03-23-2009, 10:30 AM   #8
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That's the beauty of the stock computer, and the reason the tables are a pain, the PCM adds/subtracts based on environment conditions so you dont have to change them every day.
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Old 03-23-2009, 10:39 AM   #9
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Thats the main problem as some of our dynos arent climate controlled. But fuel differences (10% ethonal makes a difference), and even engine deteriation.... Stock ecu's self learn/adaptability is quite amazing.
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Old 03-23-2009, 10:12 PM   #10
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Adaptive should be turned off , with boost you will find to deal with the borderline timing tables and set the others higher , turn off the adders and subtracters and make the computer use the borderline timing tables as well as IAT and ECT and you will find the timing holds very stable and does not go all over the place with boost

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