One of the threads in the rumor mill within the MiataPower circles suggested that the stock fuel rail was suspect to be inadequate when running high levels of boost (12+psi). The idea is that the pressure distribution along the rail (and thus, each fuel injector) varied a significant amount to cause a significant difference in air/fuel ratios from one cylinder to the next. The result could be a rich cylinder #1 with a lean #4... and worse, a meltdown on #4.
The stock rail feeds gasoline into the #1 end of the rail from a full-on fuel pump (full pump pressure). Fuel is bled off of the rail near the #4 end via a pressure regulator and excess fuel is dumped back into the tank. The volume of gasoline inside the rail is suspect to be low. It is assumed that the size of the rail is adequate for stock, naturally aspirated applications. However, with the increased amounts of air crammed into the engine while under boost, and the larger fuel injectors (opening up like barn doors compared to the peep hole stock injectors), the potential for fuel pressure variations from one cylinder to the next is there. A way to check this would be to install a thermocouple at the exit of each exhaust port. Temperature variations would suggest a variation in fuel mixtures, possibly related to varying fuel pressures. I did not perform this test but went with the collective opinion that a larger rail with more fuel capacity would smooth out pressure variations and certainly wouldn't hurt to have.
I found two aftermarket rails for the miata. The first was made by Vishnu Performance. Certainly a work of art from the pictures, though I have not seen one in person. The second was the Jim Bobowski rail. The JB rail is similar to the Vishnu rail in that it has a larger volume, but also has a couple other features.
The JB rail feeds fuel into the rail at two points... between #1 and #2, and between #3 and #4. The pressure is regulated and fuel bled off between #2 and #3. Furthermore, a hose is attached from #1 to #4 to act as a fuel accumulator. This feature helps smooth out pressure differences from the #1 end to the #4 end while adding more volume to the rail.
Another good thing about the JB rail is the price. At $120, it's very affordable. The Vishnu product is considerably more expensive (in the $300 range).
A downside to the rail is installation. With the number of hoses and size of the rail, it's a bit of a pain to install compared to the stock rail. It is apparent that Jim is still in a continual development role for this rail as the one I received benefited from trails before hand. The rail I received did not fit very well as first but treating it as something I would make, I pulled out my Dremel tool and made some modifications for clearance. The first attempt to install the rail resulted in a fuel leak but after some continual efforts of reseating the injectors, the leak stopped.
Please see the TWIN page as there are pics of the same rail on his car.