Powertrain genealogy · provisional coverage

One layout, four families, one outlier.

Subaru didn't invent the horizontally-opposed "boxer" engine — Karl Benz did, in 1887. What Subaru did was build a brand identity, and nearly its entire modern lineup, around it. The EJ20 stayed in JDM STI production a full decade after the US had already moved on. The SVX's EG33 flat-six is the one modern Subaru that isn't a boxer four at all.

Horizontally-opposed four-cylinder (boxer four) · 1966–early 1990s · Pre-Legacy Subaru ownership — Subaru 1000, Leone, Brat, and early Loyale-era cars.

EA-series boxer four

The EA family was Subaru's first boxer engine, introduced in the Subaru 1000 in 1966 and marking the start of the horizontally-opposed layout the brand would build its entire identity around. Subaru marked the design's 50th anniversary with a 2016 press release, though this dossier hasn't independently re-fetched that release's full text. One engineer, Hideshige Gomi, is credited in a single, uncorroborated source with completing US-emissions federalization of the EA boxer by 1975.

Output Subaru 1000 debut engine (1966): roughly 1.0L, ~55 hp. Later EA-family displacements and outputs across the Leone/Loyale/Justy-adjacent era are not comprehensively researched in the corpus.

“None recorded”

2.0L horizontally-opposed four, 16-valve, SOHC/DOHC, naturally aspirated and turbocharged variants (EJ20T/EJ20G/EJ20K/EJ205/EJ207 among others) · 1989–2020 (JDM WRX STI production continued on EJ20 through the 2020 Final Edition, well after most other Subaru lines had moved to FA/FB) · JDM WRX/STI ownership — grey-market imports and enthusiast collectors of Japan-only Version 1 through S208 cars, plus the US EJ205/EJ255-equipped 2002–2005-era WRX.

EJ20

The EJ20 is the signature engine of the WRX/STI lineage and the reason Japan and the US ran different WRX/STI displacements for most of the model's history: Japan's road-tax bands and an informal manufacturers' horsepower cap kept JDM cars on the 2.0L EJ20 family all the way to the 2017/2018 S208, the final catalog JDM STI built on it, while the US ran the larger 2.5L EJ25 family from the WRX STI's 2004 introduction. The EJ20's turbo variants powered every Group A/N WRC-homologated Impreza and JDM STI special edition from the 1994 Version 1 onward.

Output Reported outputs ranging from 94 to 341 hp across the full EJ-family spread (Wikipedia's Subaru EJ engine article); the JDM WRX's launch tune was rated around 240 PS (1992) with roughly 224 lb-ft (304 N·m) of torque at 5,000 rpm; the S206 (2011/2012, EJ207) was rated 320 hp with approximately 318 lb-ft (431 N·m) of torque; the related 22B STI (EJ22G, a 2.2L sibling built for the 22B specifically) was officially rated 280 PS and 268 lb-ft (363 N·m) of torque at 3,200 rpm.

“None recorded”

2.5L horizontally-opposed four, 16-valve, SOHC/DOHC, naturally aspirated and turbocharged variants (EJ251/EJ252/EJ253/EJ255/EJ257 among others) · 1996–present in some derivative form; the naturally aspirated/turbo EJ25 line was Subaru's mainstream 2.5L engine through the 2000s and 2010s before FA/FB-family replacement · US Legacy, Outback, Forester, and Impreza ownership (1996–2010s); a known used-buyer concern given reported head-gasket failure patterns.

EJ25

The EJ25 was the mainstream 2.5L engine across US-market Legacy, Outback, Forester, and Impreza applications, and became the US WRX STI's engine from 2004 onward in EJ257 turbo form. Aftermarket-service-industry sources (not a manufacturer statement or independent engineering study) describe two distinct head-gasket failure patterns: internal coolant leaks on 1996–1999 first-generation EJ25D engines, and external leaks around the 100,000-mile mark on later EJ251/EJ252/EJ253 variants — a pattern those same sources say improved with post-2012 revisions and the FA/FB-family replacement, though this remains a service-blog-level claim rather than a rigorously verified one.

Output US-market WRX STI (EJ257): standard 2018 Type RA donor car rated approximately 305 hp, with the adjacent MY2019–2021 export tune at 290 lb-ft (393 N·m) of torque; the 2019 US-only S209 (EJ257) was rated 341 hp / 330 lb-ft — notably the only S-series STI ever built on the 2.5L EJ25 family rather than JDM's 2.0L EJ20. The turbocharged EJ255 (2004–2009 Legacy GT / Outback 2.5XT) was rated around 243–250 hp depending on trim and transmission, with roughly 241 lb-ft (327 N·m) of torque at 3,600 rpm. Broader naturally aspirated EJ25 output across Legacy/Outback/Forester/Impreza applications is not comprehensively researched in the corpus.

“None recorded”

2.0L horizontally-opposed four, direct-injection capable, 86mm × 86mm square bore/stroke (naturally aspirated FA20D; turbocharged FA20F/FA20DIT) · 2012–present · Modern BRZ, Forester XT, and 2015+ WRX ownership (2012–present daily drivers).

FA20

The FA20 is the shared engine at the center of the Subaru-Toyota BRZ/86 joint-development program — Subaru supplied the core engine design (on its own Impreza-derived chassis), and Toyota added its D-4S injection system, with the resulting unit carrying two names for one physical engine: FA20 (Subaru) and 4U-GSE (Toyota). Its 86mm-by-86mm bore and stroke is the literal source of the 86 nameplate. A separate turbocharged branch, FA20F/FA20DIT, became Subaru's mainstream direct-injection turbo four across the Legacy GT, Forester XT, and WRX.

Output FA20D (BRZ/86/FR-S, using Toyota's D-4S dual direct/port injection): 197–237 hp depending on market and tune, with the first-generation BRZ's 200–207 PS tune rated at 151 lb-ft (205 N·m) at 6,400 rpm. FA20F/FA20DIT turbo: up to approximately 300 PS; introduced in the JDM Legacy GT (late 2012), reached the US in the 2014 Forester 2.0XT (reported at 250 hp for 2012–2015 production and 268 hp for 2014–2018 production, an overlapping and unresolved boundary, both tunes sharing a reported 258 lb-ft/350 N·m of torque at 2,000–4,800 rpm), then the 2015+ WRX on a further-revised tune.

“None recorded”

2.4L horizontally-opposed four, direct-injection capable (turbocharged FA24DIT/FA24F; naturally aspirated FA24D), bore increased to 94mm from the FA20DIT's 86mm · 2019–present · Current Ascent, WRX, and second-generation BRZ/GR86 ownership (2019–present).

FA24

The FA24 headlines the Ascent, Subaru's modern three-row, and represents a deliberate departure from the larger flat-six engines (the SVX's EG33, the Tribeca's own H6 units) Subaru had previously reached for in flagship or large-vehicle applications — pairing turbocharging with Subaru's more typical four-cylinder boxer displacement instead. A naturally aspirated FA24D variant separately powers the second-generation BRZ and Toyota's GR86.

Output Ascent FA24 turbo (2019 debut): 260 hp, 277 lb-ft (376 N·m) of torque at 2,000–4,800 rpm. The FA24 turbo succeeds the FA20DIT and is reported at 260–271 hp depending on application, with the same Outback XT tune sharing that 277 lb-ft figure. The naturally aspirated FA24D (BRZ ZD8/GR86, second generation): 228–232 hp, 184 lb-ft at 3,700 rpm.

“None recorded”

2.0L–2.5L horizontally-opposed four, direct-injection capable (FB16/FB20/FB25 and derivatives, including e-Boxer hybrid variants) · 2010s–present · Modern Legacy, Outback, Forester, and Impreza ownership (2010s–present daily drivers).

FB-series boxer four

The FB family, developed alongside the FA family starting around 2012, replaced the EJ engines across most non-STI Subaru applications and is the direct-injection-capable four-cylinder underpinning the bulk of the modern lineup — Legacy, Outback, Forester, Impreza, and the Levorg (not sold in the US). The Forester's SL generation extends the family into e-Boxer mild- and full-hybrid form, Subaru's first US hybrid powertrains for that model.

Output FB25 (Legacy/Outback/Forester): reported at 170–182 hp depending on generation and tune (e.g., 175 PS/174 lb-ft in the sixth-generation Legacy, 182 hp/176 lb-ft direct-injection in the SK-generation Forester and sixth-generation Outback). FB20D e-Boxer mild hybrid (Forester SL): a 12.3 kW electric motor paired with a modified gasoline engine, the motor alone contributing roughly 48 lb-ft (65 N·m) of torque; combined output not researched. FB25D e-Boxer full hybrid (Forester SL): an 88 kW AC synchronous motor paired with the FB25, the motor alone contributing roughly 199 lb-ft (270 N·m) of torque per Subaru's equivalent Crosstrek Hybrid spec sheet for the same motor architecture; combined output and combined torque are not published by Subaru for its hybrids. The turbocharged FB16DIT shares only its crankshaft with the naturally aspirated FB16 despite the shared displacement code.

“None recorded”

3.3L horizontally-opposed six (flat-six/boxer-six), naturally aspirated, automatic-transmission-only applications · 1991–1996 (SVX/Alcyone SVX only) · SVX ownership; a used-market curiosity given the model's automatic-only drivetrain and its status as Subaru's only production flat-six outside this one car.

EG33

The EG33 was the SVX's sole engine throughout its 1991–1996 run — a deliberate departure from Subaru's near-universal four-cylinder boxer norm, though still fundamentally a boxer layout in the literal engineering sense, just with six cylinders instead of four. It was reportedly offered only with an automatic transmission because Subaru had no manual gearbox rated for its torque output at the time, a claim repeated by secondary sources but not yet independently verified against a period road test or Subaru engineering account.

Output 231 hp (172 kW) at 5,400 rpm and 228 lb-ft (309 N·m) of torque at 4,400 rpm, per Wikipedia's Subaru Alcyone SVX article — the only horsepower/torque figures currently sourced for this engine in the corpus.

“None recorded”