The stock exhaust manifold is a 4-1 design with a built in catalytic converter. Where the four exhaust pipes join to become one there is a restrictive pinch point that is less than ideal.
Fitting an aftermarket exhaust header without this pinch point and without the catalytic converter will allow the engine to breathe easier creating more sound and releasing some power. The problem with this approach is that you need some sort of catalytic converter for your car to pass the emissions test part of an MOT and thus be street legal. Fortunately the NC has a pair of cats, one in the manifold (primary) and one in the middle pipe (secondary). This gives us a number of options and possible exhaust configurations. The stock manifold has an air/fuel sensor before the primary cat and an oxygen sensor after the cat. Fitting an aftermarket manifold may require you to relocate the oxygen sensor to a position after the secondary cat depending on your setup. Here is how the stock exhaust system looks...
One option we have is to fit a cat-back system, these keep both cats but replace the rear half of the middle pipe with a stainless steel pipe and muffler. This will improve exhaust flow and give a nice sound.
The next step from there is to fit a full de-cat centre pipe, but keeping the primary cat in the manifold. This will be louder than the previous setup as you would be removing restriction from the exhaust system.
Alternatively we could keep the secondary cat and add a de-cat manifold. This is probably the most common setup. Fitting a de-cat manifold may require you to relocate a sensor from the stock manifold to prevent the PCM getting confused and producing an error code and dash warning light. There are wiring extension kits for this job.
If we now put the de-cat centre pipe back in, we get maximum power and the least restrictive setup. This is not legal for a road car, but for a trackday build it could be a useful option providing it stays within circuit noise limits.
Another option to keep things legal but still use an aftermarket manifold is to fit a centre pipe that includes a high flowing sports catalytic converter. These will allow you to pass emissions tests and are less restrictive than a standard cat.
Alternatively you can keep the de-cat centre pipe and fit an aftermarket manifold that includes a high flow sports cat.
Ok, so those are the options for exhaust configurations. Let's take a look at some of the off-the-shelf systems available from aftermarket vendors.
Power Enterprise De-cat Manifold
Trust GReddy De-cat Manifold & De-cat Middle Pipe
Trust GReddy Sports Cat Manifold & Sports Cat Middle Pipe
HKS De-cat Manifold & Sports Cat Middle Pipe
IL Motorsport De-cat Manifold
Integral Kobe Jet's Sports Cat Manifold
Nopro De-cat Manifold
Pitcrew Racing De-cat Manifold
Power Craft De-cat Manifold
Racing Beat De-cat Manifold
RE-Amemiya De-cat Manifold
RS Aizawa De-cat Manifold & Sports Cat Middle Pipe
SARD Sports Cat Manifold & De-cat Middle Pipe
Tomei De-cat Manifold & De-cat Middle Pipe
Goodwin Racing RoadsterSport De-cat Manifold
Goodwin Racing RoadsterSport Sports Cat Manifold
Maxim Works De-cat Manifold
PPE Engineering Ceramic Coated Sports Cat Manifold