https://datazap.me/u/tequilaboy/feli...0.00&tmax=0.92

This car is running a little rich in open loop about 8% AFR error (7.514% according to my histogram) without any correction. Once in closed loop the action of the BLM and Integrator (not shown) almost immediately resolve the AFR error. Eventually the BLM settles to around 121. Wish the log was a bit longer but don't think much would change. One can never have too much data.

The data as shown has been trimmed to only show BLM cell 0 in order to make it easier to see the effect with less clutter/noise.

On the next start up, the Cell 0 BLM should be initialized to around 121 (if the coolant temp gets hot enough: in this case 190.85 °F), so it will be even better.

Note: 1 BLM count = 1.28 0.78% correction. So 128 - 121 = 7. 7 counts * 0.78% = 5.46% correction.

From histogram data: the running average AFR error% in closed loop was -0.825% (ever so slightly rich) and the running average WBO2 was 14.605:1 (with 14.73:1 target). The next tune will be 3-4% leaner which should bring the BLM up to around 125, but this is largely academic.

 

Same data showing the combined effect of the Integrator and BLM working together: https://datazap.me/u/tequilaboy/feli...0.00&tmax=0.92

 

I never learned enough about the integrator...though I definitely see the BLM change (in addition to AFR) as the engine goes into closed loop. Your recent help to another 383 owner (with Blueprint engine) was enlightening -- though I may come back and pose a question/two after making sure I spend enough time trying to understand/learn on my own -- about load and our apparently ability to recalibrate it. I'm sure I should see what's on the net about the integrator 15yrs after my initial tune. Back then, no one seemed to point me to that very much. Thanks!

 

Off topic but what's up with 26 degrees BTDC spark at idle? Is that accurate?

 

what size can?
do you have any issues with split BLM’s?

ive always had that issue on my 1993 even with smaller cams like hotcam.

A new engine, new cam, new heated o2’s, even swapping ecm to a spare one, has never resolved the split blm issue

ive been told its just a fact of life with speed density and a cam - as a result i run an open loop tune (which is still real fun to drive, but fuel econ seems terrible)

 

Some info on the car: Its a 1989 Iroc-z convertible with a 5 speed and presently a 355 ZZ4 based engine with Procharger P600B and small intercooler. This is a MAF car with only one O2 sensor (besides the wideband O2), so no split blm possibilities here.

We've been through many tuning iterations with this car since it was an iron head 305 originally. Some of the tuning has remnants of the the old 305 f-body bin since that is where we started.

The owner seems to like the idle at 900 rpm, so 26 degrees advance is about right for the rpm and load (6 degrees initial advance + 20 degrees relative to reference pulse (provided by ESC/EST) = 26 degrees relative to tdc).

For reference, the C4 tpi tunes run 20 degrees advance at idle but that can be as low as 500-550 rpm for an auto trans car and as high as 700-750 rpm for a manual. APYP for example, would run about 32-33 degrees at 900 rpm and 75 load but would be quite unstable with minor load and rpm variation due to the spark table being quite dynamic in this region. Flat 26 degrees (below 1,000 rpm and 120 load here) is a good compromise for idle stability, vacuum and responsiveness down low with a mild cam. I don't have the cam specs handy, but think its around 224/230 duration.

Note: the load range has been extended by 150% (which can be confusing) so you can compare the two load signals if desired. 42 lb/hr injectors. 200% MAF range. 3500 kg/hr sensor.

Update info:

Old engine
305
Powermax 2032 cam
stock lb9 heads
stock tpi intake, tb and runners

New engine
355
Comp cams 08-305 cam
AFR 195cc heads
TPIS Big mouth intake, accel runners and lingenfelter tb

 

That's not unusual... mid-90's LT1's were running into the mid-20's at idle. I run about 25 with my Miniram too.

 

Not a function of speed density.. It's a function of the LT1 manifold and batch fire injection. Low rpm A/F distribution in these manifolds is pretty terrible, and with no way to adjust individual injector pulse widths, you're kinda stuck.

I thought GM had cast in idle-air passages in the LT1 intakes to correct for that, but it may be that it's insufficient if you start modifying the engine.

After learning about that, I copied the concept onto my Miniram and it actually fixed the issue for the most part.

https://www.thirdgen.org/forums/diy-...ram-split.html (the pictures are very old and show the original prototype phase with the ugly external air intake and all that... I've since refiined it to where you can't even tell there's anything done from the outside).