How much collector will I need to ensure accurate O2 readings? At the moment collector is 7 inches long, too short I fear.
TIA

We get more accurate readings with it in a tube about 18" from exhaust port.

8-10" is acceptable any closer and you get false readings.

I would add extensions to your collectors you could
use them anyways then put them in about 4" - 6" from
the end of the primaries... about 10 o'clock or 3 o'clock
so you dont get water in them.... if you have a evac
make sure you put them in before the evac... the evac
will give you a false reading

From the NGK AFX manual

AFX Installation
Below are recommended guidelines for installing the AFR sensor. Some exhaust configurations may make it difficult to meet each of the recommendations exactly, and some compromise in mounting may be required. The sensor does not necessarily have to precisely meet every mounting guideline below to operate, but please understand that the better you conform to these rules, the longer the sensor will last and the more accurate the results will be.
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).
Make sure there are no leaks in the exhaust system, as this will create an artificially lean (higher) AFR reading. Also, install the sensor upstream of any factory air-injection if so equipped, as this too will cause a false lean reading.

The biggest problem with a wideband O2 sensor in the collector without exhaust pipes is at idle and low throttle positions. I think you will find that 7" from the outlet of the open collector will be useless at low/idle throttle positions. Around 15" should be fine, the more the better.

Mine is around 10" from the outlet, and it works fine at WOT going down the track, and also footbraking at 2500 rpm at the starting line. I use collector extensions or my bullet mufflers for idle and low throttle numbers.

No magic number, just rough guidance.

my extensions would nt fit on the dyno.
only the short adaptor at the end of the collector.about 3.5" extends into the dyno exh tube.

Thanks for the reply's guys I'll fab up some 18 inch extensions, trickey 'cause of the 'box crossmember. Mainly used for WOT ratio's at the track. The rest I can set / monitor on the street as I have a full system.
Thanks again