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Discussion Starter #1
The EGR system on the Miata has four problems:

1. EGR comes off cylinder #4 on the header, so cylinder 3 (which is on the intake stroke when cylinder 4 is on exhaust) gets most of the recirculated gas. The EGR pipe needs an expansion chamber or muffler.

2. The EGR gases are intended to reduce the peak combustion temperature and it would be more effective if the gases were cooled before going into the intake manifold.

3. All kinds of debris comes out of the exhaust which will foul the EGR valve and coat the intake manifold, so there should be a trap to catch all the gunk.

4. Some manifolds do not line up properly with the EGR fitting.

Rerouting the EGR so it can cool off and adding an all-metal trap (for the temperatures involved) would solve all of these problems. I would do a stainless trap and pipe that goes around the front of the engine for cooling and the trap can sit just behind the radiator. There are a number of threads that indicate there is a problem when the EGR valve gets gunked into immobility and burnt oil is deposited on the intake manifold. There are also a number of headers that do not line up properly with the stock EGR pipe and this could be solved with a pipe that is long enough to bend to shape.
 

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I agree with your points above as I currently have my EGR pipe removed. Main reason why I deleted my EGR was because of the aftermarket manifold I bought. As you said, it did not line up right so I blocked the whole on the manifold and blocked the intake side.. I am trying to figure out a way to remove the CEL from coming on (OBDII) for my 96. Let me know your thoughts since it sounds like you've been hitting the books on the EGR system. :D
 

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Discussion Starter #3
If you have a 1996 (or any Miata from 1990 to 2000), the EGR pipe runs under the coil pack, roasting it to destruction and when it goes, it takes the cat with it. I would run the pipe around the front of the engine. One property of gases that not many people know is that the viscosity is proportional to absolute temperature - colder gas flows faster through a given pipe than hot gas, the opposite of liquids. By the time the exhaust cooled off, it would be cold enough to use a cast aluminum box for expansion and trapping duties and a Hammond 1550 metric or 1590 flanged series would be usable with maybe some stainless filtering mesh inside:

http://www.hammondmfg.com/1550.htm

http://www.hammondmfg.com/dwgf.htm

There is a supplier of stainless steel pipe in my area (west of Toronto) that sells 20-foot rolls of a suitable size for about $70, enough to do several cars. You would still have to attach the fittings on the ends.

My emissions tests usually see about 10% of the maximum CO, 50% of the maximum NOx and right at the limit with hydrocarbons.
 
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