I just got a Megan racing exhaust manifold and I noticed it only has 1 o2 sensor hole downstream... does a b16 only have 1 o2 sensor or 2? And if so is their a way around it or do I need to have a bung welded in and if so where would I weld it at? Or can I just put the one upstream in the downstream spot so I can keep my MPG cause I'm already catless

 

The early (OBD-0) B16A has 2 single wire O2 sensors... both close to the cylinder head... one in the #2 cylinder and the other in the #3 cylinder. In OBD-1 and later versions of the B16A, there is only one O2 sensor and it is found behind the block and is a 4-wire heated sensor. Heated sensors can be further away from the cylinder head because they do not require the heat from the cylinder head to warm them to proper operating temperature. The early single wires do not have this assistance, and thus, MUST be close to the cylinder head to operate properly.

You did not state which application you have. If it is the earlier version, you will need to weld in two bungs for the single wire sensors in the locations I specified above. If you have an OBD-1+ version, you only need the one primary O2 sensor and it can be mounted in the location found on your header. If you have an OBD-2A/B application, your secondary O2 sensor after cat. will have to be bypassed if you use an OBD-2A/B ECU. It doesn't even have to be plugged in if you use an OBD-1 ECU.

 

I have the OBD2 setup out of a 00 EM1. So I'll be fine with 1 o2 sensor or I'm going to need to 2nd one? Sorry man I'm a noob to all this first motor swap

 

well its the 00 EM1 motor going in a 99 ek hatch

 

Ahh that makes more sense. The upper sensor in your stock hatch manifold is the upstream (primary, pre catalyst) sensor. This is the one right up in the middle between runners 2 & 3. The second O2 sensor in your hatch is the downstream (secondary, post catalyst) sensor, and is located at the bottom of the stock converter. After the swap, if done correctly, it will now be under the car at the back of the new converter. The upstream/primary/pre catalyst one will be located one will also be located under the car, at the end of the header, just ahead of the new converter. So, the bung in your new header is actually the upstream sensor. Make sense?

 

Yes that actually does lol so the o2 bung at the bottom of the headers is actually for the upstream o2 sensor which control air/fuel ratio. And since I don't have a cat I can just bypass the downstream one correct?

 

Depends. What ecu are you running? You should really run a cat if it's a street car.

 

think it's P2T ecu and its a street car just not a daily driver.

 

Then you will need the secondary sensor to keep the ecu happy, unless you are really handy with electronics.

 

Alright I appreciate your help. Thank you

 

Alright I got everything I need for welding the bung in. How far past the upstream one should I put this bung for the secondary o2 sensor

 

Back of the catalyst.

What you have, stock, on your hatch is in the top/left half of the parts diagram - what you are trying to get to is in the bottom/right half:



Primary sensor is part #40, secondary is part #27. Catalyst is #4.

This is your stock manifold with closed coupled catalyst:



Primary sensor is #8, secondary is #9.

 

. The front sensor is under the car at the end of the collector infront of the cat on the b16

 

If you intend to use an OBD-2 ECU, you will need both O2 sensors... mount the primary O2 in the collector of the header as delivered (like in the pic above) and then secondary O2 in the cat... or in whatever you use between the back end of the header and the front flange of the cat-back exhaust system. You can purchase plug-n-play O2 sensor wire extensions (my tuner has them on the shelf, PM me if you need a reference). If you run cat-less, you will get a CEL for the secondary O2 sensor unless you get lucky with using a fouler... or some type of electronic bypass/trick.