Hey guys sorry if I posted this in the wrong area, my first time posting on forums. I have a 2000 1.8L NB that is a California Emission model and I just put together a second hand FM Stage 1 Turbo Kit. I had a question about the FM down pipe and O2 sensors. The Cali Emission Miata's have an upstream O2 sensor(O2 #1) connected to the exhaust manifold and a second one(O2 #2) connected on the downpipe right after the first CAT which i assume is the downstream. The FM downpipe only has one bung for the factory O2 #2 and another bung for a wideband (FM part 5 installation video).

My question is where would I put the upstream sensor(O2 #1) on the FM downpipe? Would I need to drill and weld my own bung onto the FM pipe for the O2 #1 or do I not run it? Can the car run with only 1 out of the 2 factory sensors plugged in? Did I miss something in the video that said where the 1st sensor goes?
Any insight would be a big help, THANKS!

FM Downpipe

Factory system

 

I think you're over thinking it. You have 2 sensors, and 2 holes.

If you're going to a standalone ecu, I believe sensor 1, is getting replaced with a real wideband and gauge wired into the ecu, and the second sensor wouldn't be used.

 

I agree with Shamrock. 2 sensors and 2 holes. Considering there isn't a cat between the two I don't think you're going to see a difference in the readings.

If you're standalone then just throw a wideband in, connect it to the correct connector on the wiring harness for your ECU and you should be good to go.
If you're doing a piggyback then it might matter which sensor gets plugged in where.

"Upstream" = Upstream of the catalytic converter
"Downstream" = Downstream of the catalytic converter

Usually these 2 would be compared against each other to see how well the cat was working. IE, how much of the residual particulate in the exhaust is getting burnt up by the cat. To be safe I'd just run the frontmost bung on the exhaust to the front connector in the engine bay (should be towards the front of the valve cover) and and the rear bung to the rear connector. Again, for standalone I'd just plug whichever one doesn't get the wideband.

 

An ECU needs two O2 sensors before and after a cat. As long as you are using the stock ECU and don’t want a CEL, you just place a the sensors around ANY catalytic converter.

By adding the Turbo, you’ve eliminated the CA pre-cat, and the O2 bungs.

so you need to add a bung after the stock cat. Then move the O2 sensors before and after the main cat.

Technically the only true, CARB legal, CA spec kit was built 25 years ago with a custom exhaust manifold to re-use the OEM pre-cat. Extremely rare, and generally useless.

 

I have the FM turbo kit installed (albeit NB2) don't know if your ECU will throw a code missing the 1st cat that is eliminated by the kit.
It's simple in order going from the turbo down to the exhaust, the order is:
Turbo--> Downpipe:1st hole O2 #1 upstream, 2nd hole Wideband --> Midpipe CAT --> 3rd hole O2 #2 downstream.
Keeping a depowered sensor in the stream will destroy the sensor, so remove what you're not using and plug the hole.