When you replace your factory exhaust manifold with an Aftermarket header, you need to address the change in the O2 sensor bung locations, so your MX-5 runs correctly. The OEM manifold has a catalytic converter incorporated in it, with O2 sensor locations before and after the cat. Because a performance manifold removes that first cat, you'll need to move the O2 sensors down to before and after the secondary cat. The short 909-831 wire extension is perfect as you'll need one short 909-831 and one long 909-830 for a header install. The long O2 extension is for the rear bung and the short O2 bung is for the front bung.