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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
Join Date: Jun 2008
Location: Detroit
Posts: 881
I think the location depends on whether you are using the sensor for control, or to just monitor AFR. For control the optimum location is as close to the cylinders as possible. This means mounting it before the turbo. The problem with this is that exhaust pressure throws off the reading, which is why it is usually suggested to mount it after.

For EFI with the capability for it, you can correct the AFR reading for the pressure effect using this formula:

AFR(corrected) = (AFR(measured) + B x P) / (1 + C x P)

where: AFR(corrected) = the AFR corrected for exhaust pressure.
AFR(measured) = the AFR output by the AFM.
B = 0.009140 for AFR < 14.57 (rich).
B = 0.012100 for AFR >= 14.57 (lean).
C = 0.000627 for AFR < 14.57 (rich).
C = 0.000830 for AFR >= 14.57 (lean).
P = the exhaust pressure in mmHg above the pressure at which the sensor
was calibrated (using the CAL POT on the AFM while the sensor is held in air).
Valid for -152 mmHg < P < 532 mmHg.
Old 11-15-2009, 12:14 PM
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