Carb tuning w/O2 sensor

[mrmustang]

Originally Posted by Tom Kirkham

Please explain. I have never heard of the placement being so critical.

NGK says

http://www.ngksparkplugs.com/pdf/AFX...ual_REV_06.pdf

"The AFR sensor should be located between 12” and 48” from the engine, upstream of any catalyst device if so equipped. The closer the sensor is to the engine, the more likely it will be overheated, possibly shortening its life. The further it is from the engine, the more likely condensed water will get into the sensor and thermally shock it, again possibly shortening its life. The sensor should be mounted at least ten exhaust diameters upstream of the exhaust exit (ex. for a 3” exhaust pipe, that is 30”). If the sensor is mounted between one and ten exhaust diameters from the exhaust exit, the AFR measured will be leaner than the actual AFR by as much as 2 AFR at low engine speeds (i.e. less than 3000 rpm)."

Tom,

Placement up or down stream, just as placement in one header tube vs another will cause readings that are not accurate. NGK, Bosch, and several other sellers of these aftermarket "inexpensive" aftermarket O2 sensors will tell you differently, but having had access (while still living in NJ) to a computerized chassis dyno with an operator who was properly trained show me the error of my ways. Where we would want a air/fuel ratio of 12-14 in a reading, the O2 sensor (mounted in a bung in the proper place according to the manufacturer showed one reading, while the O2 sensor in the end of the header (actually 2 feet in just after the 4X4 collector dumped into the muffler assembly showed another. Tuning with the original O2 (bunged) showed me to be running rich, when in fact I was running lean. Again, being able to set an RPM or a speed on the chassis dyno so that we could tweak any carb I had installed (4 on one particular 427 side oiler engine one weekend) was the most beneficial thing that I could have happen. Trying to tune without that load on the engine (drivetrain load vs engine rpm only) turned out to be my downfall for one over the other.

Bill S.