Axis Extends

Wednesday, December 28, 2016 - 09:00

Axis Extending the Smoke Limit map is the secret weapon of the new school remap.

When a value being looked up exceeds the maximum value for the axis the ECU uses the last value of the axis. In the case of an EU2 Auto the Airmass axis has a maximum value of 850mg/stroke. Any Airmass values above 850mg/stroke use this column to calculate the smoke limit IQ. This has the effect of gently rolling off the maximum AFR as Airmass climbs beyond the header value.

Looking at the stock IQ's of 850mg air, 48mg IQ at 2000rpm we can see this gives a maximum AFR of 17.7 to 1.
As Airmass increases, say to 1200mg/stroke, the same 48mg IQ limit applies giving a maximum AFR of 25:1.
Bumping the Max IQ to 57mg gives a AFR of 14.9:1 at 850mg/stroke, and 21:1 1200mg/stroke.
Due to the restriction of the airmass in the final column it becomes difficult to increase IQ without making the smoke.

The stock maps appear to select a final column value near the Airmass required at peak torque so the roll off in AFR occurs after this point.
Peak torque seems to occur around 2000-2500rpm so worth checking what Airmass you are making at full throttle at this point and then select the column header based on the airmass at peak torque.

As an example of how this works I've taken the last column of an EU2 Auto smoke map and calculated the existing AFR.
This is simply:

AFR = ( column header value /10) / (table value / 100)

The new values are calculated from the AFR and replacement column header value.

IQ = (New column header / AFR ) * 10

RPM 8500 9500 AFR
650 4000 4000 21.25 / 23.75
700 3000 3353 28.33
1000 3200 3576 26.56
1200 3600 4023 23.61
1500 4800 5365 17.71
1800 4800 5365 17.71
2000 4800 5365 17.71
2700 4800 5365 17.71
3000 4800 5365 17.71
3500 4400 4918 19.32
4200 4400 4918 19.32
4900 3600 4024 23.61

The main tables that need this kind of treatment are the Smoke maps as the higher airmass values in the axis headers allow increased fuel inject amounts while avoiding setting AFR to a level that results in excessive smoke.