I think obsessing over blm's is highly overrated in many cases. Here is something I wrote the other day for a guy with a TT chip who keeps worrying about blm's and always wants to change the fuel pressure to "make them right".
BLM's- blm's work at idle and part throttle. Depending upon the maf reading and the rpm, there are 16 possible blm settings. Most of these are in effect between idle and 1500 rpm. The last cell is from 1500 rpm and up but it DOES NOT control wide open throttle. This is controlled by a programmed fuel curve in the chip.
BLM's that are programmed into the ecm can be adjusted thru a learning mode if the blm's are not consistently 128 and are being changed based upon input from the O2 sensor. The ecm can adjust fueling if the blm's are in a range from 105 to 150. Therefore there is no great concern if, at any given time, the blm's are not 128. Typically we say that 128 +/- 10 is a good range. If the blm at a given cell based upon maf flow and rpm is always 120 and you take out enuf fuel pressure to make the blm read 128, then you are simply doing what the ecm is doing internally but you cannot see it do it...in theory, the car should run exactly the same whether you adjusted the pressure or not. The problem is that you affected the entire fuel curve and not just the blm block that you were looking at when you decided your blm's were bad. The ecm is a lot smarter than you are.
Look at this link-it spells it out in better detail and notice the rpm bands where the blm's are controlling the fueling.
http://www.gnttype.org/techarea/faq/BLMINT.htmlthe point I am trying to make is that obsessing over blms is largely a futile exercise. Now, if blms are grossly out of whack from what you would expect on a modern chip, then you can start looking for a vacuum leak, a header leak (which fools the O2 sensor so that it sends erroneous info to the ecm which makes it adjust the fueling when it should not), or a fueling problem that may exist.
Note that the last cell covers a much larger rpm band but again, not wide open throttle. The ecm slips into wide open throttle mode largely based upon Load (LV8). Contrary to folk lore, tps voltage does not have much to do with it.
Now, on your chip (TT), you have four parameters that you can adjust.
Parameter 1 controls wide open fueling in all four gears.
Parameter 2 controls wide open fueling in FIRST gear and it is a trim factor based off Parameter 1. In other words you first set Parameter 1, then you look at your low gear fueling. If you want to lean it down some to improve turbo spool and get a quicker launch from a standing start, you can. If you have too much wheel spin from too much turbo spool, you could make it richer and cool down the charge a bit to slow down the spool a bit. It really depends on how good the traction is and how much boost you are trying to leave on.
Parameter 3 is the timing in first and second. Normally you can run more timing in first and second at the strip if you can hook the tires because there is less load on the engine and combustion chamber temps are not as high. This can be a bit more dangerous on the street when the engine is hot because excess timing might cause detonation in that case where it may not on the strip because the engine is cooler.
Parameter 4 is timing for 3rd gear and 4 gear. Again on the strip you can usually get by with additional timing unless you are hot lapping the car and the engine is not cooled down between runs.
As a rule, I am much more conservative on the street because engine conditions are usually not optimum-particularly if you have been driving in traffic or pounding on the car at boost multiple times.
So what about fuel pressure. When you adjust fuel pressure you adjust every facet of fueling from idle to wide open throttle. A change in fuel pressure affects flow by taking the square root of the (new fuel pressure divided by the old fuel pressure). Therefore a change in fuel pressure is a relative small change in the actual delivery but it is more obvious at idle and low rpm than it is at higher rpms because the percentage of change compared to the fueling is larger due to the small amount of fuel used at idle and low rpm.
This is what I do, I set the fueling to 43 and I drive the car until it is fully warmed up and I have done some stop and go driving, then I stop and look at the O2s in gear idling. If they are 780-800, I call it good and forget it. Note that the O2s will change when you stop and the O2 sensor starts cooling down so your 780 might increase to 810 in a short period of time. Don't be confused because this is a function of the stock o2 sensor not a change in fueling.
If, when I first stop, the o2s read something like 820, then I reduce the fuel pressure 1-2 psi and try it again, until my first readings upon stopping are in that 780-800 range. Remember that the TT chips are open loop at idle so that first blm cell is not adjusting the fueling like the factory chip. It does have some learning capability, however, so that is what I like to drive the car for a bit before beginning to play with the adjustments.
If everything is right, the O2s should be pretty stable wherever you set them and they should not be jumping around at idle due to the open loop feature. That is why the idle is normally smoother and better.
When you come off idle, then the chip goes to closed loop an the o2s will jump around a lot more (at part throttle) as the ecm takes charge.
In essence, you can use the fuel pressure to set up your idle and low speed blms but I only look at the idle O2s first when I stop and the sensor is good and hot.
Now, remember that the fuel pressure affects the entire fueling range so the wot fueling will be a little leaner than what Eric put into his default if you reduced fuel pressure at idle with the HOSE OFF so you need to look at the wot fueling to see if it needs adjusting. Normally, it will probably not be noticeable. Be sure to put the hose back on the regulator before you drive the car."
