Design of an accurate ‘tailpipe’ probe for Lambda sensors
Aim of this article:
After dealing with damaged sensors and struggling to get reliable idle calibrations (using one of the most commonly available probes) we decided we could easily do better without breaking the bank. Some quick ‘back to back’ testing soon showed the extent of the errors we had been experiencing.
We needed a probe that-
Why would you make one?
To get accurate readings and increased sensor life!
Poorly designed air/fuel ratio probes are common and lead to wildly inaccurate readings at low gas flows. This phenomenon is caused by exhaust gas pressure pulsations; it is nearly always overlooked by the tuner.
Besides incorrect readings a poorly designed probe will not allow the sensor to be positioned vertical due to the mounting clamp fowling with the vehicles bumper or spoiler. Being at the tailpipe and not vertical means large amounts of condensed liquid can flow into the sensor, this shortens sensor life and can cause ‘thermal shock’, basically a cracking of the ceramic element (it is approximately at 750ºC in a wide band sensor such as the Bosch LSU variety).
Features of an ideal probe:
Inaccurate, poorly designed probe (too small & fixed clamp position)!
Construction concept (guide only, use whatever material is at hand):
Good probe design
Approximate size of complete assembled probe.
Sliding probe clamp and mounted sensor into main tube.
Probe design undergoing testing.
Ideally a ‘bung’ welded into the manifold should be used for tunning, but tailpipe probes are common in the aftermarket tuning scene.
Don’t settle for compromised designs. A small amount of thought into your probe design can save you time in fitting to vehicles, money from damaging sensors and can lead to improved tuning results, especially at idle and low gas flows.
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