If you are looking for the absolute best bang-for-your-buck modification for your car, installing a brz flex fuel kit is pretty much the gold standard for the platform. It is one of those rare upgrades where you actually feel the difference the second you pull out of the driveway after a tune. We aren't talking about a placebo effect from a noisy intake or a slightly deeper exhaust note; we are talking about a genuine transformation in how the FA20 engine delivers power.
Most people who buy a BRZ eventually run into the same realization: the car handles like a dream, but the engine can feel a bit sluggish in the middle of the rev range. That infamous torque dip is real, and while you can try to mask it with headers, nothing really wakes the car up like switching to E85. But since you probably don't want to be stranded if you can't find an ethanol pump, a flex fuel setup is the only way to go.
Why Ethanol is a Cheat Code for the BRZ
Let's talk about why everyone is obsessed with "corn juice." Ethanol has a much higher octane rating than the 91 or 93 octane gas you get at the local station. In simple terms, it allows your engine to run more aggressive timing and resist knock much better. On a naturally aspirated car like the BRZ, this translates to a much smoother power delivery and a significant boost in torque where you need it most.
When you install a brz flex fuel kit, you're giving your ECU the ability to see exactly what percentage of ethanol is in your fuel lines. This is the "flex" part. You can pull up to a pump, fill half the tank with 93 and the other half with E85, and the sensor tells the computer to adjust the tune on the fly. You don't have to stop, pull out a laptop, and reflash the car every time you change fuels. It's seamless, and honestly, it's how the car should have come from the factory.
What Actually Comes in the Kit?
If you're shopping around for a brz flex fuel kit, you might notice that most of them look pretty similar. That's because the technology is fairly straightforward, but the quality of the components really matters. Generally, a solid kit is going to include a few key pieces:
- The Ethanol Content Sensor: Usually a high-quality Continental sensor that sits in the fuel line.
- The Converter Box: This takes the raw data from the sensor and turns it into a signal your BRZ's computer can understand.
- Plug-and-Play Harness: You definitely want one that doesn't require you to cut into your factory wiring. Most modern kits just clip into existing sensors (like the EVAP or O2 sensor harness).
- Fuel Lines and Fittings: These need to be ethanol-compatible so they don't degrade over time.
Most enthusiasts prefer kits that are "plug-and-play" because, let's be honest, nobody wants to spend five hours squinting at a wiring diagram and hoping they didn't pin something wrong. A good kit can usually be installed in under an hour with basic hand tools.
The Installation Process
Installing a brz flex fuel kit isn't exactly rocket science, but you do need to be comfortable working around fuel. You'll be opening up the fuel lines, so there's going to be a bit of spray and some fumes. It's a good idea to do this in a well-ventilated area and maybe don't have a cigarette hanging out of your mouth while you're at it.
The sensor typically mounts near the firewall or tucked away by the fuel rails. Once the hardware is in place, you route the wiring harness to one of the factory connectors. The most common method involves "hijacking" the signal from the rear O2 sensor or the EVAP system. This allows the ECU to receive the ethanol percentage data on a channel it already knows how to read. It's a clever workaround that keeps the install clean and reversible if you ever decide to sell the car.
You Can't Forget the Tune
Here is the part where some people get tripped up: you cannot just install a brz flex fuel kit and start driving. Well, you could, but the car wouldn't know what to do with the information. The hardware is only half the battle. To actually use the kit, you need a custom tune, usually via EcuTek.
A tuner will take the data from your new sensor and create a map that scales based on the ethanol content. If the sensor sees 0% ethanol (straight gas), it runs a standard timing map. If it sees 85% ethanol, it cranks things up to maximize power. It's a beautiful thing to watch on a dyno. You can literally see the power curve smooth out as the ethanol percentage climbs.
What Does it Feel Like to Drive?
Driving a BRZ on E85 is a completely different experience. That "dead spot" in the middle of the RPM range—the torque dip we all hate—is significantly reduced. The car feels punchier when you're merging onto the highway, and it pulls much harder toward the redline.
Most people see gains of about 20 to 30 wheel horsepower with just a brz flex fuel kit and a tune. That might not sound like a huge number compared to a turbo kit, but on a car that weighs as little as the BRZ, you feel every bit of it. The throttle response is crisper, and the engine just sounds happier. It's hard to describe until you've experienced it, but it feels like the engine is finally breathing the way it was designed to.
The Practical Side (and the Downsides)
It's not all sunshine and rainbows, though. There are a few things you should know before diving in. First off, your fuel economy is going to take a hit. Ethanol is less energy-dense than gasoline, meaning you have to burn more of it to get the same result. Expect your MPG to drop by about 20-30% when you're running a full tank of E85. If you're using your BRZ as a long-distance commuter, you'll be stopping at the gas station a lot more often.
Secondly, you need to make sure E85 is actually available near you. There is nothing more frustrating than having a brz flex fuel kit and a spicy tune but being stuck on 91 octane because the nearest ethanol pump is three towns over. Luckily, because it's a flex fuel kit, you aren't stuck. If you can't find E85, you just pump regular gas and the car adjusts. You just won't have that extra power until the next time you find the good stuff.
Maintenance and Longevity
Some people worry that ethanol is "harsh" on engines. While it's true that ethanol can be corrosive to certain old-school rubbers and plastics, modern cars like the BRZ are built with materials that handle it just fine. That said, it's a good idea to stay on top of your maintenance.
Many people who run a brz flex fuel kit prefer to change their oil a bit more frequently. Ethanol can sometimes lead to more moisture buildup in the oil, especially if you do a lot of short trips where the engine doesn't fully warm up. A quick oil change every 3,000 to 5,000 miles is cheap insurance to keep everything internal looking pristine.
Is it Worth the Money?
When you add up the cost of the brz flex fuel kit, the EcuTek license, and the custom tune, you're looking at a decent chunk of change—usually somewhere around $1,000 to $1,500 depending on the brands you choose. Is it worth it?
In my opinion, absolutely. If you were to spend that same money on an intake and a cat-back exhaust, you'd get a lot of noise but almost zero actual performance gain. With flex fuel, you're getting a tangible increase in torque and horsepower that makes the car significantly more fun to drive every single day. It bridges the gap between the stock feel and the high-cost, high-complexity world of forced induction.
For many BRZ owners, this is the "final form" of their engine mods. It provides enough power to keep the car exciting on backroads and track days without compromising the reliability of the naturally aspirated setup. If you're tired of being out-accelerated by minivans at stoplights, it's time to look into a flex fuel setup. It really is the best way to let your Subie live its best life.