Toyota did an excellent job designing the MR2's fuel system, with most parts supporting double or triple the factory power output. However, there is room for improvement, and often need for repair, at every step. Most MR2 fuel systems don't need every upgrade listed here, but I hope to help you understand what components will become a restriction sooner or later.
Tank
The stock fuel tank is well baffled, and keeps weight very low and central. It is however a pain to service. We keep Fuel Pump Hanger Gaskets and Fuel Fill Hoses in stock. These aren't always necessary, but they often break during service.
The fuel tanks are steel and many MR2s have been left to sit for a long time at some point in their life. We often see badly rusted tanks that need to be replaced. As many people do, we have these refurbished at a local radiator shop. This can get complicated, as badly rusted tanks require several weeks of soaking in acid, and are often handled poorly.
The fuel pump sits in a small reservoir, or bowl. This keeps the sock submerged under acceleration and cornering, especially when the fuel level is low. This bowl is made of plastic, and they are often degraded. After sitting in a tank and being pressure washed at a radiator shop, the bowl is often destroyed. This will lead to fuel starving at lower fuel levels.
To address the rust and deteriorated bowl issue, we offer a Refurbished Fuel Tank. We cut a patch out of the bottom of the tank, and weld in a new stainless steel fuel pump bowl. We then send the tanks to have any rust removed and a sealant applied to prevent future corrosion.
The fuel pump bowl has a small inlet at the bottom. On the MR2 Spyder, this hole is not large enough for more than about 350whp. Whenever the fuel level is below the lip of the bowl, the engine will consume fuel faster than the bowl will fill, and it can be sucked dry. The hole just needs to be drilled larger. The AW11 and SW20 do not have this issue.
Sock
The fuel pump sock is the first component in the fuel flow path, and the only one under suction. If it does not flow enough, it can collapse and greatly limit fuel flow. Our Walbro 255 kit includes a sock similar in size to stock. Our Walbro 525 kit includes an Oversized Fuel Pump Sock, the largest that will fit in the MR2 bowl. This is unnecessary for a 255, but can help prevent clogging on cars with loose rust in the tank.
Pump
On pump gas, the stock AW11, NA SW20, or Spyder fuel pump is capable of about 220whp. The stock turbo pump is capable of right around 300whp. E85 requires about 30% more flow, meaning a turbo pump only flows enough for about 230whp on E85.
The Walbro 255 is our go-to fuel pump upgrade. It's affordable, reliable, works with stock wiring, and flows enough for 500whp on E85. This is enough for 95% of MR2 owners and is the limit of the stock fuel pressure regulator. This pump does get noisy when hot. Keeping the tank above half full prevents this noise. AW11s are particularly prone to this noise, as the coolant pipes run beside the tank and heat the fuel more than an SW20.
The Walbro 525, i.e. Hellcat pump is a great choice for those wanting more. This requires an aftermarket fuel pressure regulator, and upgraded wiring. This is capable of about 800whp on E85. In our experience this pump is quieter than the 255.
The DeatschWerks 810 brushless pump is the ultimate choice a single pump setup. We have not found the limit of this pump, but have seen several people in other circles running it well over 1000whp on E85. It's a complicated install, but has a dual speed controller to avoid overwhelming the system before the extra flow is needed.
The fuel pump must be installed with submersible fuel hose, and all above options require wiring changes. We include these parts in our fuel pump kits.
None of these work for a Spyder. We have had good luck with an AEM 340 paired with an aftermarket regulator in high fuel demand Spyder applications.
Pump Wiring
The stock MR2 fuel pump wiring is likely 30ft long with multiple junctions, providing lots of opportunities for voltage drop to the pump. When installing anything larger than a Walbro 255, we add a Fuel Pump Relay Upgrade in the frunk to send voltage straight from the battery to the pump. Be wary of cheap relays and fuse holders, as they often cause issues down the road. Resistance makes heat, which makes more resistance. This melts things.
Plumbing - Pump to Filter
Stick with the stock fuel pump hanger. Anything aftermarket causes more issues than it solves. The stock pressure hardline is capable of tons of flow.
The stock hose from the hanger to the filter flows plenty for a Walbro 255, but we always upgrade this when we go to anything bigger. We sell a Fuel Filter Hose upgrade that includes a Fuel Feed AN Adapter Fitting.
Filter
The OEM fuel filter is awesome, even up to 800whp. Aftermarket replacement fuel filters have a paper filter element that flows a lot less and clogs easily on cars with rusty tanks. Since Toyota discontinued the OEM filter, we've been installing a DeatschWerks 6AN Fuel Filter with a stainless steel 5 micron filter element. This fits into the stock filter bracket.
Plumbing - Filter to Rail
Our Braided AN Fuel Line for 3SGTE includes the necessary fittings to connect the filter to the passenger side of a stock or ATS top feed rail.
Rail
The stock Gen2 3SGTE side feed fuel rail has small passages that can allow pressure drop between injectors, leading to uneven fuel injection. We offer a Ported 3SGTE Side Feed Fuel Rail to address this issue. This is a great option for owners who will be content with about 300whp. Above this level, E85 and a top feed fuel system will be necessary.
The stock Gen3 rail is also side feed, but with larger passages. Again, any owner with ambitions above about 300whp will want to upgrade to a top feed system.
The Gen4/5 fuel rail is top feed from the factory, but is rather small. We do not recommend running this above 400whp.
Our Top Feed Fuel Rail features 8AN o-ring ports at both ends, and accommodates 14mm injectors. We include port adapters for Bosch EV14 style injectors, discussed below.
Regulator & Return
Regulator and return flow are just as important as pump flow. If there is more pump flow than return flow, pressure will not be properly regulated.
The stock 3S fuel pressure regulator flows about 250 lph efficiently. This means a Walbro 255 is right on the edge of proper fuel pressure control. Some cars will be a few psi above the set pressure at idle. Doing the fuel pump relay upgrade can unlock some extra flow on a 255, allowing more power, but it can also be the difference between overwhelming a stock regulator.
Upgrading to an Adjustable Fuel Pressure Regulator takes care of the regulator part of the equation, restoring proper fuel pressure control with increased fuel flow. We normally mount this directly to the top of the fuel filter, running the rail as a dead head. This removes the need for a bracket and makes really simple return plumbing.
The SW20 fuel return line flows about 500 lph efficiently. Again, this means a Walbro 525 is right on the edge of overwhelming it, and some cars will be a few psi above the set pressure at idle. This is one of the benefits of the DeatschWerks 810 brushless pump setup. Its dual speed controller allows a low speed mode that does not overwhelm the fuel pressure regulator or return line, with more flow available on demand.
The AW11 fuel return line flows less than the SW20, and they seem to vary depending what hanger is installed. The Spyder has a dead head system factory with a regulator built into the pump assembly. We have only plugged this and run a new return line from an adjustable regulator, so I am not familiar with the capability of the stock unit. If you choose to upgrade your return line, be sure it returns into the fuel pump bowl, or the pump will constantly be emptying it.
Fuel Pressure Sensor
If you've read this far, you surely have gotten the impression that there are several reasons you could have fuel pressure issues. Losing fuel pressure while driving can lead to a catastrophic lean condition and engine failure. For this reason, we install a Fuel Pressure Sensor on almost every car with a standalone ECU. This allows us to address issues before and during tuning, and set up a safety feature that cuts power before any damage can be done.
Do not mount a fuel pressure sensor to the engine, as vibration will shorten the lifespan significantly. We install these at the filter outlet.
High fuel pressure from inadequate fuel return flow isn't a big deal. Once injector flow matches the difference in available pump and return flow, the pressure will be properly regulated. Inadequate return flow will cause slightly unpredictable fueling in situations where injector flow is changing quickly around that point, typically at idle and cruising. Popular aftermarket ECUs have compensation built in when a fuel pressure sensor is used, which makes this a non-issue in most cases.
Injectors
Stock Gen2 440cc injectors are good for about 300whp on pump gas at 3.5 bar base fuel pressure. On most setups, octane becomes an issue around this point, and E85 is needed. Because E85 requires about 30% more fuel flow, the same injector will be capable of substantially less power on E85.
1000cc Bosch EV14 Injectors are our standard recommendation to pair with our top feed fuel rail. These support up to 500whp on E85, with great idle quality and consistent performance out of the box.
Balanced 1300cc Bosch EV14 Injectors are a great choice for those pushing for more power. These are the same body as the 980cc, but with the tips modified. We flow test these in house and match sets within 1%. These will support up to 700whp, while maintaining good idle quality.
For those wanting unmatched control at low injector fuel demand, we offer Injector Dynamics ID1050, ID1300, and ID1750 injectors. We have excellent results with the cheaper options, but this is often a decision made by tuner preference.
All discussed options require an EV14 Injector Connector Kit or EV14 Injector Plug and Play Adapters.
These injectors require the injector resistor pack to be deleted. The stock MR2 injectors are low impedance, almost all newer injectors are high impedance. Simply cut all of the wires at the resistor pack, and connect them together.
If you're sticking with the side feed fuel rail, it is still wise to get your injectors cleaned checked. These are 30+ years old now. You want to be sure they are flowing evenly, so that you don't have one cylinder running much leaner than the rest. We offer Flow Matched Stock 440cc Injectors. We also have Flow Matched Side Feed 550cc Injectors, as supply allows. These are stock Gen3 injectors and hard to find. Stay away from any aftermarket side feed injectors.
Flex Fuel - E85
We use standard AN hose for all of our fuel hoses. In our experience, these may lose some flexibility from prolonged E85 use, but do not shed material into the lines, break or burst.
We install a Flex Fuel Sensor in the return line. With the regulator is mounted to the top of the filter, this makes a very clean install with no extra brackets and short hose runs.