Minimum/Maximum BLM Values
11-17-2014, 01:15 AM
#1
Instructor
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Minimum/Maximum BLM Values
Has anyone tried changing their PROM's minimum/maximum BLM values and/or minimum/maximum integrator values? I'm wondering what the effect(s) of this would be.
If I increased the BLM range from 52 (108-160) to, say, 64 (102-166), would that extend the operational range by which the ECM can richen or lean out the mixture? Given that there's a static number of 16 BLM cells, I'm guessing the answer's no, but I'm not sure.
If not, then what would this do? Absolutely nothing? Since BLM cells can neither be created or deleted, nor can their contents be modified, that's how it would appear to me.. But I welcome any input or corrections..
If I increased the BLM range from 52 (108-160) to, say, 64 (102-166), would that extend the operational range by which the ECM can richen or lean out the mixture? Given that there's a static number of 16 BLM cells, I'm guessing the answer's no, but I'm not sure.
If not, then what would this do? Absolutely nothing? Since BLM cells can neither be created or deleted, nor can their contents be modified, that's how it would appear to me.. But I welcome any input or corrections..
11-17-2014, 01:22 AM
#2
Race Director
The limits are put on the BLM cells so the car will still run in a reasonable way if you hit the limit. Extending the limits would allow the AFR to go so rich or so lean that the car wouldn't run (assuming something caused the BLMs to go that far).
The ECM uses the BLM value as the basis for calculating the injector pulse width. It then takes into account various factors (like coolant temperature) to adjust the injector pulse width to match the conditions the ECM is seeing.
The BLMs are calculated from the O2 sensor readings. If you hit the limit then something is wrong.
The ECM uses the BLM value as the basis for calculating the injector pulse width. It then takes into account various factors (like coolant temperature) to adjust the injector pulse width to match the conditions the ECM is seeing.
The BLMs are calculated from the O2 sensor readings. If you hit the limit then something is wrong.
11-17-2014, 11:36 AM
#3
Drifting
BLM minimum and maximums could be changed to 0 and 255 if you should chose to. However, most are set to 108 and 160, because a properly tuned engine will not run outside this parameter. I've seen other GM BCC's that use different BLM's than 108 and 160, but all are fairly close. You want to tune to as close to 128 as possible, or a tad bit richer. Using Datamaster will give you BLM averages that allow you to tune to keep from ever reaching these max, or minimums making modification a moot point. Also, tuning reduces the wild BLM swings which brings smoother operation.
11-18-2014, 04:46 AM
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I appreciate your input, guys..
It just makes me wonder what GM had in mind when they made those constants adjustable. I'm sure it wasn't a tinkering fool like me asking questions about it, though.
It just makes me wonder what GM had in mind when they made those constants adjustable. I'm sure it wasn't a tinkering fool like me asking questions about it, though.
11-19-2014, 03:03 AM
#5
Race Director
Back in the '80s those values were not known outside of GM. As far as I know, the very first "hack" of the GM ECM was done in the mid '90s by Ward Spoonemore. Everything since then has been based on what he figured out. I think he did have help from people with inside knowledge, either current (at the time) or previous employees of GM.