The stock Miata fuel pump can only provide so much fuel (tends to max out around 85 psi). We have already discussed the potential of higher fuel pressures versus higher horsepower. So, you need to upgrade the pump if you plan to fuel past 70-80psi, usually due to running small injectors on an FMU setup. Raising the fuel pressure raises the size of the injectors. Since it requires an aftermarket fueling device to control +20/30% larger injectors, adding more fuel pressure is the logical solution.
Other than that, you don't need one. If you have an aftermarket ECU, the stock pump more than enough to support 300rwhp with the stock regulator. Changing the fuel pump does nothing but flow enough fuel…it doesn't change any other factors in the fuel entering the combustion chamber.
It is import you know what pressures your fuel pump is capable of. Follow these procedures to test your fuel pump pressures:
You’ll need to connect a fuel pressure gauge on your fuel lines to do this test. $20 at NAPA can get you what you need. Ask the counter clerk that you need a temporary fuel pressure gauge and length of 5/16” fuel hose, they can get you what you need.
Turn your Miata to ON but do not start it. Open the diagnositics box and put a paperclip on F/P and GND. That will run your fuel pump. With a pair of pliers, squeeze the fuel line shut that connects the FPR to the original fuel pressure regulator. When squeezed shut, the pump will be forced to maximum output. Make sure the pressure available is consistent with your intentions. In all cases, the pressure must show, in this idle test, to be about 10 psi higher than the desired fuel pressure, as the available pressure under real load conditions will be less than that measured at idle. This test does not actually prove the pump to be adequate under boost, but if it doesn’t pass this test, it is certainly a waste of time to continue with the same pump.
Once you determine how much fuel pressure you are capable of you can determine if you need to upgrade or not. As a rule, if you need to supply more than 85 psi of fuel, chances are you need to upgrade the fuel pump.
Fuel pumps are rated in LPH. LPH as in the 190 or 255 means “Liters Per Hour” and not fuel pressure available. For most boost levels discussed here in this forum, the stock pump supplies enough LPH, just not enough fuel pressure. The maximum pressure available from the stock fuel pump was between 78 to 85 PSI. That should be sufficient to run the 5-6psi of boost level.
The most popular model is a direct replacement of the in-tank OEM fuel pump. But keep in mind an overly large fuel pump is not always the way to go.
For the longest time the Walbro 255 lb/h fuel pump had been the accepted pump for all FI Miatas. On Miatas with a return line, the 255 flows more fuel at any giving pressure level than is needed. They tend to overload the OEM FPR and cause the system to run rich even in vacuum.
Instead of idling at 35psi and reaching 48psi at 0~hg, it will run closer to 40psi at idle and 55psi at 0~hg.
The 99-00 miata, with no return line, is noted as having fuel pressure drop towards redline from that factory. These miatas fuel at a constant 60psi of fuel regardless of pressure conditions inside the manifold, however it seems with forced induction, and even high powered N/A builds, the stock pump is not enough. The 255 HP model is the only model that has been noted to maintain the pressure at redline.
Regardless, there are still a few options in fuel pumps. The ideal pump is the Walbro 190 lb/h HP fuel pump. HP stands for high pressure. It is important you get a HP unit if you plan on raising the fuel pressure levels past 90psi. The HP models can supply fuel up to 130psi. The standard 190 lb/h fuel pump is maxed at 90psi and the standard 255 lb/h pump is maxed at 80psi. The 190 HP unit will not only supply four 700cc injectors with enough flow to maintain 40-50psi in the fuel rail (enough for 400hp), but out flows the 255 HP model after 100psi.
It is recommended for most applications to go with the 190LPH, 190HP if using a FMU.
Another option is the Pierburg auxiliary fuel pump. This pump is capable of producing in excess of 140 PSI of fuel pressure. (Systems the require more than 120psi of fuel on boost may need to consider fuel management). It does not replace your OEM fuel pump but in conjunction with it. Instead of replacing your existing fuel pump you add this pump in front of it and wire it separately to a power source.
Below is a comparison of fuel pump flow rates. It is easy to see how to 255 provides enough fuel for an F-15. The 190HP acts very close to a stock pump with more headroom for higher pressure systems.
To convert Gal. / Hr. to Lbs / Hr. - multiple by 6
Multiple that by .80 (duty cycle) and divide by .55 (BFSC) and see how much B.H.P. the pump is capable of flowing for.