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Pre vs post cat wideband O2 readings


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Hi all,

I currently have my Bosch LSU 4.9 wideband attached to one of my exhaust manifold bungs (there are 3 on my engine, 2 in the exhaust manifold and 1 post cat) so I am getting readings from a combination of cylinders 2 and 4 which is giving me a stable reading.

I have a full race manifold to go in which doesn't have any O2 bungs in it, would it be ok to use the post cat bung location or should I see if I can get a bung welded into the pre cat pipework just before the cat? I am guessing that the AFR would be slightly different post cat, however I know that a lot of dynos use an exhaust sniffer in the backbox which would be post cat for tuning.

The engine is currently NA but will be running a turbo at some point in the future (with a bung in the downpipe pre cat) - was just wondering if I would be ok to run the wideband sensor in the post cat position until I swap to the turbo setup - I guess I could log the AFR in the current location and then again in the post cat location to see how much it might be different - guessing something like 0.5 AFR points perhaps.

Anyone have experience on this matter? My current fuel table is pretty close to the target AFRs I am aiming for and am currently using the auto fuel correction option to adjust where needed - not wanting to run the engine lean if its trying to fuel differently to the current table.

Thanks in advance!

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