DIY Dual Feed Fuel Rail
The stock fuel rail on a high HP Miata can lead to #4 failure. Fuel is distributed unevenly and injectors #1 and #2 tend to get more fuel than #3 and #4. There have been many cases on the miata.net where not enough reached the #4 injector and the piston burns up leading to engine failure. (Also largely due to cooling system)
One simple cost effective "solution" is making a dual feed fuel rail. It's acutally very simple and require little tools. On the stock rail fuel is fed into one side and out between the #3 and #4 injector. To modiy the rail you simple feed the fuel to both sides of the rail making the fuel distributed more evenly throughout the injectors.
What you need:
21/64" Drill bit
1/8" NPT tap
1/8" to 5/16" brass ftting
5/16" Brass Tee
2 - 1' sections of 5/16" fuel injection line
5/16" Hose Clamps
First you need to remove the fuel rail. Start by unhooking the harnesses to the injectors. Then unbolt it (2 12mm bolts). Don't lose the spacers. Open your fuel lid to help depressurize the fuel lines a little. Stuff rags under the rail. Disconnect the fuel lines - note - they will spill fuel, try to get it on the rags. Remove the injectors and keep them aside.
With the fuel rail removed you can start modifying. Grease up you drill bit and drill into the far end of the fuel rail. The grease helps collect any metal shavings. Tap the hole with your 1/8" tap. Throughly clean the rail. Make sure there are NO
metal shavings left in the rail. Screw the brass ftting in place. JB Weld the fitting to the rail to make for a good seal. Cure for 24 hours.
In the engine, attach the brass tee to the fuel line attach to the hard line. Attach the two sections of fuel hose to the tee. Reinstall the injectors to the fuel rail. Get the rail into postion and attach the two sections of hose to the existing fitting and new fitting on the rail. Reinstall the rail - harness - gas cap - battery terminal.