The flat-4 used to be one of the most popular engine layouts, thanks to Volkswagen’s extensive use of it, but nowadays, it’s a layout that’s most associated with Subaru... but they’re not the only ones using it. There’s four flat-four engine families sold in the US market today, and one of them isn’t Subaru’s.
Subaru FB20, FB25
The Subaru FB20 is Subaru’s lowest-end engine in their current US lineup, a 2.0 liter naturally aspirated engine making 148 hp @ 6200, 145 lb-ft @ 4200. It’s being replaced by a tune of the FA20, but the Crosstrek hasn’t been updated yet, and therefore still uses it, getting 23 mpg city, 30 mpg highway with a manual, 26 mpg city, 33 mpg highway with a CVT.
An updated version with direct injectionis used in the 2017 Impreza, making 150 hp @ 6000, and 148 lb-ft @ 4000 - slight improvements over the original tune. However, the addition of Subaru’s direct injection system has allowed it to gain a bit of efficiency along with the power and torque bump - a high of 28 city, 38 highway is possible in the base Impreza sedan with a CVT, compared to 28/37 for last year’s model.
However, there’s also a 2.5 liter variant used as the base engine in the Forester and the Legacy/Outback - the FB25, making 170 hp @ 5800, and 174 lb-ft @ 4100. Fuel economy ranges from a high of 25 city, 34 highway in the Legacy with a CVT, to a low of 22 city, 28 highway in the Forester with a manual.
Subaru FA20 (aka Toyota 4U)
The FA20 is a derivative of the FB20, optimized for lighter weight and with the addition of direct injection.
The FA20D, also known as the 4U-GSE in Toyota applications, was the original variant of the engine, designed for the BRZ and what we now know as the Toyota 86. Because of the joint development, this engine uses Toyota’s D-4S dual injection system, combining both direct and port injection. For model year 2017, manuals got a power upgrade, and are making 205 hp @ 7000, 156 lb-ft @ 6400-6800 - this one definitely wants to be kept high in the rev range. It gets 21 city, and either 28 or 29 highway depending on which brand you get (interestingly, they weren’t tested together). Automatics are still on the original tune, though, making 200 hp @ 7000, 151 lb-ft @ 6400-6600... but unless you had to, this isn’t a car you buy with the automatic. Yes, the automatic does get better fuel economy - 24 city, 32 or 33 highway - but that’s through taller gearing, and a high-revving engine and tall gearing doesn’t go well together.
There’s also the FA20F, the turbocharged variant of the engine. It’s available in two tunes.
The Forester XT uses the milder of the two, making 250 hp @ 5600, 258 lb-ft @ 2000-4800. It gets 23 city, 27 highway with its mandatory CVT.
The other tune is used in the WRX - still based on the previous-generation Impreza - making 268 hp @ 5600, and with the same torque peak broadened to 5200 RPM. Fuel consumption is 20 city, 27 highway with a manual... and it gets quite dire with the CVT - most likely it has shorter gearing to optimize power, whereas the other CVT-equipped Subarus are trying to maximize economy with long gearing - at 18 city, 24 highway. But, then, you’re not buying a WRX to save on gas...
The EJ25's old enough to drink (having been used in the 1996 Legacy, in naturally-aspirated form) - and drink it does, at 17 city, 23 highway, on premium - but it’s still serving as Subaru’s top-end flat 4, in the WRX STi. 2.5 liters, turbocharged, making 305 hp @ 6000, 290 lb-ft @ 4000.
While the Mitsubishi Lancer Evolution is dead, Subaru has new competition in the form of the Focus RS, which thoroughly outclasses it in terms of power, while somehow being more efficient. Hopefully Subaru’s able to update the STi to compete.
Porsche 718 engine
So, I’m not actually 100% sure of the designation here - I think it’s a member of the A91 family - so I’ll just go with “718 engine”, as that’s the car family it’s found in. (But, that car family would actually be the 981.2, not actually the 718, because the 718 is a 1950s-1960s race car. But I digress...)
This one’s... controversial, because it signifies the death of both natural aspiration and the flat-6 in the Boxster and Cayman. Many, many lines of print (both digital and physical) have been created, debating whether this is a good or a bad thing - it means less weight, more power, a lot more torque, and better rated fuel economy, but it also changes the character of the car.
Base models are 2.0 liters, making 300 hp @ 6500, 280 lb-ft @ 1950-4500, and you’re looking at either 22 city, 29 highway for the PDK, or 21 city, 28 highway for the manual.
S models are 2.5 liters, making 350 hp @ 6500, 309 lb-ft @ 1900-4500. 21 city, 28 highway for the PDK here, or 20 city, 26 highway for the manual.
Next up will be the logical progression from the flat-4 - the flat-6. It’ll be the same manufacturers again, though...