I have a 260/260 soild roller cam in my 383 ci. My problem is the BLM boundaries are incorrect ( it has hte stock talbes still) The car idles about 70 KPA in neutral and 85 KPA in gear. I know I can adjust the BLM boundaries in LT1 edit to not add so much fuel at an idle.

MY questions are

-THE BLM boundaries vs RPM/MAP has 3 lines. the 1st one is for idle right ? and does that mean if I set the 1st line to 1200 RPMS and 70 KPA ...the car will not try and add/subtract fuel as long as the car is below 1200 RPMs and above 70 KPA ?

If this is true, what are the next 2 lines for ? PArt throttle?

I know I need to adjust these tables but im not sure how they work in conjunction with eachother.

MADz28 !!! Do you any idea? I know this table is what I need to adjsut, I jsut need to know the rules that it follows so I set it correctly.

 

RPM
(00-900)(900-1200)(1200-2000)(2000-redline)
0 -----------------------------------------------------------------
| | | | |
| 0 | 1 | 2 | 3 |
| | | | |
30----------------------------------------------------------------
| | | | |
| 4 | 5 | 6 | 7 |
M | | | |
A 50 ------------------------------------------------------------
P | | | | |
| 8 | 9 | 10 | 11 |
| | | | |
80 ----------------------------------------------------------------
| | | | |
| 12 | 13 | 14 | 15 |
| | | | |
100 -----------------------------------------------------------

EDIT** When I post this message all my boxes get messed up on my drawing. Just put each cell number under each rpm range.


Here is a messy representation on the cells. With stock boundries 900,1200, and 2000 RPM and 30, 50, and 80 MAP. The numbers in the boxes just represent the cell number. They would have your BLM values in them normally.

With that sort of vaccum loss at idle you are most likely not even using cells 0-7 at all because your MAP is so high. To make the cells usable again and give your car more "choices" on the A/F ratio I would change the boundries of your MAP from 30, 50, and 80 to something like 75, 85, 95. That would make all cells usable again.

For the RPM; the car may not be using cells 0, 4, 8, and 12 if you idle is above 900 rpm. Move the RPM boundries up to something like 1200, 1600, and 2200.

This should get all your cells back.
The car is going to add/subtract fuel in every cell. What you want to do is make the cells so that cover the entire rpm range and MAP values the motor actually sees anyway. Why waste a cell that can store a BLM number with a motor that will never see that range. Use all the cells you can. The computer doesn't have to search as much.

I bet you are only using cells 13,14,15 (Closed loop-learning that is) when you are driving around. Their is no way the car will run good with just 3 cells that big.

You can also look at cell 16. Supposely used only for idle.

Jeff D.

 

This discussion is specific to OBD1 PCM's for LT1 engines, but the premise is sound for others.

I noticed that within BLM cells 0 - 15 in my 95 Formula essentially only 8 of 16 cell were being used with stock programming while cruising and WOT running. Stock programming had my cells bound at 900, 1200 and 2000 RPM as well as 30, 50, and 80 MAP giving a total of 16 cells for long term fuel trims to develop within. Not many of us run our cars below 1200 RPM while driving and idle is managed by cell 16 and thus cannot be considered.

I wondered why GM had decided that the majority of our part throttle fuel trims could be best maintained within 8 of 16 useable cells. Being that I am not a GM engineer, I couldn't come up with a good answer. I concluded that I would like full use of all 16 cells in order to give my car the greatest capability to transition its fuel needs from cell to cell. The best way to do this is to give it more cell blocks to work within thus achieving less fueling fluctuation.

What I did was log with Datamaster about 30 minutes of driving around town with WOT runs for good measure. I then looked at my data and determined where each 1/4 of my data landed in terms of RPM and MAP. I found my data boundaries at 1500, 2200 and 2800 RPM. Amazingly, the stock MAP boundaries seemed pretty accurate but fine tuning is always something I can do in the future. Just remember that we are trying to get full use of all cells, not necessarily get all of them being used exactly 1/16th of the time. This will never happen.

To confirm, I took a few logged drives to see if I use all 16 cells. It worked and now both my fueling and air demand changes are much easier to analyze and tune.

Good luck with your results.


Note: I got this from another board I posted this on. And on a similar note, by modifying your boundaries, you can drill down in areas that are fueling issues.


Hope it helps.
Ben

 

That exactly what I just said.

Jeff D.

 

Thankyou for your help guys. This helps tremendously. I know Im am not unlocking hte full potential of my power until i get hte tuning perfecT!