Does the CO2 sensor need to be in front of the header collector or can it be behind the collector in the exhaust pipe. I have aset of new headers that are coated and I need to add a bung for the CO2 sensor.

It should be located in a position that permits the sensor to see a combination of the combined exhaust flows and not so close to the front of the collector that it tends to sense/sample just one or so of the individual runners input.
If you have a fairly short collector I would recommend you locate the sensor in the adapter or pipe after the flange.
If you have a fairly long collector you can mount the sensor towards the rear of the collector assembly itself before the adapter.
Locate the sensor bung towards the inside of the collector/piping to keep wiring runs out of debris area behind the tires.

The only other consideration is the servicing of the system.
On competition vehicles (that require exhaust in the class) it is often easier to mount the sensor in the adapter/pipe vs. the collector, as the sensor does not need to be removed if headers/collectors may need to be removed during/for repairs or service and/or the collectors switched for at track tuning ...

Behind the collector is OK. Just mount it so it's above horizontal so condensate and other stuff can't collect in the tip. Don't know about a big block but one tuner/tech told me that SBC's will sometimes read a little richer on the passenger side due to the firing order or something so put it in the driver side if using it for the EFI autotuning. My gauge is in the passenger side and sure enough it reads a little richer than the EFI system feedback in the driver side.


http://farm2.staticflickr.com/1389/4592586379_a88d9c438d_z.jpg

Shouldn't this read O2 instead of CO2 ?

Shouldn't this read O2 instead of CO2 ?

:yes: but I took a WAG - figuring he couldn't really have a 'Soda Fountain' in his car ;)

Yea give me a break. It is a O2 sensor. I had a brain fart.