I noticed something earlier today that surprised me. I'm in the process of tuning my car, specs are in the sig. I had hooked up the vacuum reservoir in the hopes of gaining vacuum at idle for my power brakes (its fine now, but on stock tune they were bad at first). I disconnected that vacuum line today, and went ahead to keep working on it. I noticed that my map valus went up by about 5 or 7 kpa at idle, so in other words, it has slightly less vacuum with the reservoir not in use. Seems logical so far.

What surprised me was that my BLMs at idle went down from where they were yesterday by a significant amount. I thought about this for a bit, and it seems to make sense. Since there is slightly less vacuum at idle, volumetric efficiency drops a bit, and so less fuel is needed. What surprised me was how much the BLMs dropped by. I hooked it back up and it brought my BLMs back up. Does this make sense to anyone? I was surprised that this would make such a difference.

 

I'd avoid jumping to conclusions about volumetric efficiency.

You changed the MAP by 5 or 7 kPa, which moved you to a different part of the main fuel table, where apparantly you were slightly richer than before, so the ECM's reduced the BLMs accordingly.

Make sense?

 

I'm not sure what to say. I was idling at 800 rpm, between 40 and 45 kpa with the reservoir hooked up. With it disconnected, I'm idling at 800 rpm, between 45 and 50 kpa. The BLM difference was pretty significant, but the VE values within the table at those two points are very very close.

Don't the BLM cells overlap several VE cells? What I mean is.... don't several neighboring VE cells correspond to a single BLM cell? Where are the breakpoints for the BLM cells?

 

Just for kicks, you could burn yourself a chip where both of those VE cells, and the surrounding cells, are equal. You didn't post numbers for how much the BLMs changed, but sometimes a little goes a long way, or at least thats how it's seemed to me.

BLM cells do cover larger areas of the VE table -- you should be able to see the MAP and RPM boundaries with your editor. One of the stock boundaries (in AUJP/ANHT and probably several others) is at 50 KPa, so you may have crossed that boundary and changed BLM cells. Cell number is in the ALDL datastream, so this could be observed. Also, BLMs update fairly fast, so even if you stayed in the same BLM cell it'd still only be about 2-3 seconds before you saw a change in BLMs.

Also, I'm not saying there's not a difference in your engine's actual VE between those two MAP values, just that you have to account for the calibration, before drawing conclusions.

HTH,