billc

OK, so I’ve been reading and experimenting and making some progress. Maybe it will help some others who are just starting out to hear about what I’ve been doing. My progress has been slow because I have been out of town on business and deprived of my cars, but I’ve gained some fundamental understanding and successfully corrected my AFR at idle.

First, I got the Nissan Datascan Map Trace Function working. I set it for the fuel map, rather than the timing map, because that’s what I need to be working with. I used the inputs that Calum provided here for the B13:

http://www.sr20forum.com/calumsult/1...cing-cool.html

OK, so the Map Trace showed me that when the car is idling, it is pulling the fuel correction factor from fuel map cell 1000 RPMx20 on the load scale. For this bin file, the location is the top row, third column from the left. I also got a good idea of where the ECU reads from for light throttle application (upper few rows left of center) and WOT (totally in the far right column).

For the “idle” cell (1000x20), the value on the map is “3.” I found a posting on another forum that explained what this means. The value is a correction factor that is applied to the basic pulse width calculated for an AFR of 14.7 (which is called “stoich” -- the correct ratio for complete combustion of gasoline based on the chemistry of the fuel). To calculate the basic injector pulse for “stoich” the ECU uses K – a parameter that is entered into the bin based on injector flow, and the MAF voltage converted into a flow quotient estimate using the VQ map for the MAF that is being used. The VQ map relates the MAF voltage to the air flow to generate the column headings that we see in ROM Editor (i.e., 8,16,20, 24…80 – it works for me to think of these as % flow based on the actual MAF voltage reading and the maximum MAF voltage reading possible).

OK, so I said the value in the fuel map was “3” for the idle cell. What does this mean? If I reduce it to “1” do I cut the fuel by two thirds? What about if I enter “0,” will I get any fuel at all? The posting that I mentioned above from the other forum gave me a way to noodle through these questions. Here is how to calculate the injector pulse duration correction factor (multiplier) based on the value in the fuel map:

If the MAP VALUE is less than 128, then MULTIPLIER = (DATA + 128)/128
If the MAP VALUE is greater than 128, then MULTIPLIER = (DATA – 64)/128

I set up an excel spreadsheet to calculate the MULTIPLIERS for me based on the MAP VALUES. For a map value of “0” we can calculate that the multiplier would be “1.” For a map value of 3 we see that the multiplier is 1.02. To get multipliers less than “1” we need to enter values greater than “128.” For example, a value of 160 gives a multiplier of 0.75, and for 192, the multiplier is “1” again. Values higher than “192” have multipliers greater than “1.” Kind of weird, but workable.

Now I was ready to change the fuel map to fix my rich idle. After warm up, my idle was about 12.5 AFR with a map value of “3.” (multiplier of 1.02). I wanted to move it to 14.7, or 2.2 ratio units. To figure out how much to reduce the fuel flow multiplier, I divided 2.2 by 12.5 to get a 17% decrease in fuel delivery, or 83% of the original multiplier. 0.83 x 1.02 = 0.85, the new multiplier that I would want to use for this cell. Using the formula above, a map value of 173 gives us a multiplier of 0.85.

So I entered “173” in the idle cell. I also entered the same value in adjacent cells to smooth things out, and then “0” in the next rank of cells to transition back to the value of “3.”

I burned my first 2 chips with the new program and installed them in the ECU. The ZIF sockets are really nice – I just pulled the ECU over near the accelerator pedal, removed the top, and swapped the chips, making sure to put them in the correct orientation.

Now, the moment of truth: Turn the ignition key. The fuel pump hummed, the car started and ran fine. After warm up, the idle was corrected to a nice oscillation around 14.7. Just changing these cells and increasing the Feedback Control in the Global Map from FO (240dec) to F5 (245dec) not only corrected the idle, but also put the light throttle AFR under 3000 RPM into the closed loop area and it is no longer rich, but running at 14.7. After a week of driving, I have experienced no drivabilty issues and still got 30 MPH in a lot of stop-and-go rush hour driving.

I plan to use Map Tracer to determine how many more cells to give this treatment to get a more complete resolution of the light throttle richness. And finally, I will address the richness at WOT and high RPM. Then I will be ready for the dyno. I plan to play with the timing map a little and then do a back to back comparison of the OEM tune with the Calum tune.

Any advice that you can give me will be appreciated.