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Hey Guys. I have a mild 1641 with a mexi efi setup. I have factory heater boxes and factory muffler. I get horrible reversion on the afr probe with this setup. I want to keep my heater boxes but change my muffler to get better aft readings. Can you recommend an exhaust to me. Would a tri-mil hotdog work better or do I need to move to a header. Would like to keep it clean looking out the back. Not fond of the big headers and muffler sticking out the back but if I have to do it I will. Let me know your thoughts. Thanks!
You San see where my current probe is in this picture.

Here is where I placed mine on the Tri-mil Euro exhaust. I tried centering it as much as possible. Good luck. Here it is with a narrow band o2 sensor, just installed to verify clearances.

AFR signal in the muffler is going to be a muddy mess. Exhaust pulses are going to be bouncing around in there and may create a signal lag. Not to be a Debby-Downer, but it's not advised to have the sensor at that angle. May cause water to accumulate in sensor probe.

If your EFI is batch fired then you can mount sensor in one of the primaries. Best bet out be cylinders 1 or 3 about 12" from cylinder head. Yes, you'll only be reading AFR from that one cylinder but if your PCM is batch firing injectors then each cylinder will be the same AFR anyways.

-Nick-

nbuscemi wrote: ↑Mon Apr 05, 2021 5:42 am If your EFI is batch fired then you can mount sensor in one of the primaries. Best bet out be cylinders 1 or 3 about 12" from cylinder head. Yes, you'll only be reading AFR from that one cylinder but if your PCM is batch firing injectors then each cylinder will be the same AFR anyways.

-Nick-

Not completely true. Many more factors than simply the injector will effect the afr in each cylinder.

Chip Birks wrote: ↑Mon Apr 05, 2021 2:33 pm

nbuscemi wrote: ↑Mon Apr 05, 2021 5:42 am If your EFI is batch fired then you can mount sensor in one of the primaries. Best bet out be cylinders 1 or 3 about 12" from cylinder head. Yes, you'll only be reading AFR from that one cylinder but if your PCM is batch firing injectors then each cylinder will be the same AFR anyways.

-Nick-

Not completely true. Many more factors than simply the injector will effect the afr in each cylinder.

Assuming that the OP is using the sensor for engine management feedback and not for tuning purposes I still think placing the probe in the muffler is not ideal. I also noticed the sensor in the response pictures is a single wire... which is useless for tuning any how. If the OP is planning on using a wideband than it needs to be placed in the system with linear flow and the only spot that would take place in a system like that would be in a primary. Correct me if I'm wrong or if I'm over thinking this.

-Nick-

nbuscemi wrote: ↑Tue Apr 06, 2021 6:40 am I also noticed the sensor in the response pictures is a single wire... which is useless for tuning any how.

xzener wrote: ↑Wed Mar 03, 2021 1:00 pm Here it is with a narrow band o2 sensor, just installed to verify clearances.

The narrow band sensor was used to verify clearances. And yes, the tuning proved to be difficult in that muffler. I have since bought a Sidewinder exhaust and installed the sensor horizontal at the merged header where it connects to the muffler.

That's what I get for not reading the actual post. How do you like the Sidewinder? I really want one of those to use as a side-mount turbo header. What size primary did you use?

-Nick-