Miata Turbo Forum - Boost cars, acquire cats. - How To

There are a couple different writeups/guides on how to add flex fuel to an NA/NB on here, but I still felt like I had to pick through some threads when I was doing the job so I figured I’d try my hand at a writeup.

I did this on an NB1 and my car has almost all of the emissions equipment removed, so you’ll have to adjust accordingly for your car/mods.

I'll do a writeup here and photos below just to try and keep things organized.

Prerequisites

I assume you’ve done some research if you’re here, but if not, one of the biggest things you’ll need to do before flex fuel is make sure your fueling system can handle the required flow. You’ll need to flow a lot more E85 than pump gas, so a larger fuel pump and injectors will be required. There are plenty of calculators around for sizing fuel pumps and injectors. For what it’s worth my DW100 was fine running about 10 PSI on a 2560r on ~E60-70 but ran out of flow pushing about 15 PSI.

Parts you’ll need

Flex Fuel Sensor - The continental/AcDelco ones seem to be the standard. Most people run them, you can pick them up for ~$60. Make sure to get the wiring pigtail for this as well.
Mounting Hardware - This is going to be specific per install, but the sensors are pretty friendly to being mounted on a piece of aluminum bar. I’ve also found some 3D printable brackets for these sensors around the internet, so if that’s available to you it’s worth looking into.
Fuel Line Adapter - On the NB we have the SAE Style fuel lines. On the NA I think you can get away with hose clamps directly on the fuel hard lines. Fuel lines are 5/16” for the Miata ( someone correct me if that’s incorrect) and 3/8” for the flex fuel sensor. For my install I used Dorman 800-081 to connect the hard lines on the car to the fuel hose and Dorman 800-091 to connect the lines to the flex fuel sensor. There are AN fittings/adapters as well if that’s desired.
Fuel hose - You’ll want to get something rated for fuel injection. I’m not an expert but SAE 30R9 seems to be the standard you want. SAE 30R7 seems to be designed for older carbureted vehicles. Depending on where you mount the sensor you’ll probably need between 2-5 feet of 5/16” hose. I used Evil Energy, but as long as it’s SAE 30R9 you should be fine.
Hose Clamps - Again, get the fuel injection style hose clamps. I was at the auto parts store and grabbed the MasterPro ones and they’ve been fine. You can probably get away with other styles, but I’d rather be safe than sorry when fuel is involved.
Extras - Some P Clamps or zip ties to help keep the fuel lines secure. Tessa (or similar) loom tape will help keep the wiring together and protected.

Wiring

The sensor needs +12V, (Sensor) Ground and a wire to a digital input from the ECU. I didn’t have a sensor ground direct to the ECU that was convenient so I used the ground on the intake manifold which seems to be working fine. I pulled +12V from what I think was the wiper pump plug via a spade connector. Not ideal, but it’s working. Other options would be to run +12V from the ECU or tap into the evap solenoid wiring. I’ve also heard of people tapping into the nearby fuse box as well.

Some ECU’s (specifically thinking about my old Speeduino) might have a specific “Flex” or “Flex fuel” pin. In that case you’ll want to wire that pin to the signal wire of the sensor. On the Link G4x I was able to use a generic digital input. Double check that whatever pin you’re going to use will support flex fuel before wiring.

Fuel lines

If you have an aftermarket ECU and can disable the fuel pump then you can get the fuel out of the lines by disabling the pump and running the car until it dies. If you can’t do that for some reason I think you can just pull the fuel pump fuse and do the same thing.
Disconnect the battery.
If you have the SAE fuel line connectors I’ve found it easiest to disconnect them by sliding a pair of 90* or hooked pick tools under the white locking tabs, prying the tabs outward and then pulling up. There are tools to disconnect them but they’ve never worked very well for me. On the NB there are 2 lines connected by a small U turn fuel line which is an easy place to intercept the fuel. I believe NA’s are the same, but there are differences in the fuel system (return vs returnless) so double check if you have an NA.
The new SAE connectors simply press onto the hard lines and will click into place. Make sure that the 3/8” adapters go to the flex fuel sensor and the 5/16” go to the hardlines. After that simply slide the fuel line onto the barb connectors and tighten the hose clamps.
If there is anything nearby that the fuel lines could rub on make sure to insulate them with some extra hose or use zip ties or P clamps to keep them out of the way.
Once the fuel lines are connected you should be able to prime the fuel pump (put the fuse back in if you pulled it) and fire the car up like normal. You can check to see if the sensor is working by looking for ethanol percentage in your tuning software. On pump gas you’ll likely see around 8%. As a bonus, you should also be able to see fuel temperature now.

Tuning

I will try to add tuning info when I’m back at my tuning laptop. Tuning will vary on each ECU, and I’m far from a good tuner, but there should be videos/documentation for most platforms. Basically just throw timing at the thing until it stops gaining power.

Now would be a good time to go to Lambda instead of AFR as it’ll allow you to run a single Lambda target table for both fuels.