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NB2 to NB1 exhaust manifold project

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Joined: 1 year ago
Posts: 11
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Has anyone made this mod to their NB1?  My car came to me with a CARB compliant NB1 manifold.  I'm looking to be a bit more flexible in future exhaust choices and create some better flow thru.  While the NB2 to NB1 manifold exchange appears to be straightforward, the CARB compliance complicates things a bit.

I'm reading elsewhere on the web some conflicting information.  The CARB installation places an O2 sensor at the 1st cat.  One school says that i need to move this to a new bung placed after the second standard cat.  their are kits on the market to do this move.

Another school says that a movement of the 1st O2 sensor is not necessary.  To this school, i ask "what does one do with an O2 sensor hanging in the breeze?"

I'm looking for help in sorting out this little detail as i don't want to drive around with a CEL all the time, if indeed one would be thrown.  And i don't want to cause premature standard cat decomposition, if indeed this could happen.

Has anyone enjoyed this project in the past?

Ken Hill
LVMOC Governor Moderator
Joined: 4 years ago
Posts: 96

Your NB1 Miata uses 2 catalytic converters in states that complied with California's emissions standards.  Pennsylvania is one of them, along with NY, NJ, CT.  One catalytic converter (cat) was mounted in the exhaust manifold and the second was mounted under the car, about under your seat.  The first catalytic converted was used to clean up cold start emissions.  The computer running the engine used (2) O2 sensors to monitor performance of the catalytic converter in the exhaust manifold.  One before the cat and one after the cat.  When the delta in hydrocarbon readings got to close to each other, this indicated the cat was loosing its effectiveness.

So if I understand your question correctly, you want to install a manifold onto your NB1 that does not have a catalytic converter in it.  That is easy.  Buy a Racing Beat Header for your car.  The RB header does not have a cat.  Since the O2 sensors are in the engine bay to monitor the cat in the manifold, the wire to them are short.  The RB header comes with 2 wire extension harnesses that allow the O2 sensors to move underneath the car to monitor the second cat.  The factory computer will not know it's monitoring the lower, second cat.  The only additional mod you need to do is weld a fitting for the second O2 sensor behind the cat under the car.  Any one who can weld can do this for you.  


Ken Hill
2002 HZ Yellow
with a few Modifications