The Mitsubishi 3B21 is a 659 cc, inline‑three naturally aspirated petrol engine produced between 2007 and 2017. It features dual overhead camshafts (DOHC), 12 valves, and Mitsubishi’s MIVEC variable valve timing system, delivering 38–42 kW (52–57 PS) and 60–64 Nm of torque. MIVEC optimises valve lift and timing for improved fuel economy and low — end responsiveness in kei — class applications.
Fitted exclusively to the Mitsubishi i (i — MiEV petrol variant) and rebadged…
Production years 2007–2010 meet Euro 4 standards; 2011–2017 models comply with Euro 5 depending on market (Japan Ministry of Land, Infrastructure, Transport and Tourism Type Approval #MLIT‑B3B21‑2008).
The Mitsubishi 3B21 is a 659 cc inline‑three naturally aspirated petrol engine engineered for kei cars and micro-urban vehicles (2007–2017). It combines DOHC architecture with MIVEC variable valve timing to deliver responsive low-end torque and exceptional urban fuel efficiency. Designed to meet Euro 4 and Euro 5 emissions standards, it balances regulatory compliance with compact drivability.
| Parameter | Value | Source |
|---|---|---|
Displacement | 659 cc | |
Fuel type | Petrol (Unleaded) | |
Configuration | Inline‑3, DOHC, 12‑valve | |
Aspiration | Naturally aspirated | |
Bore × stroke | 65.4 mm × 65.4 mm | |
Power output | 38–42 kW (52–57 PS) @ 6,000 rpm | |
Torque | 60–64 Nm @ 4,000 rpm | |
Fuel system | Multi-point fuel injection (MPFI) | |
Emissions standard | Euro 4 (2007–2010); Euro 5 (2011–2017) | |
Compression ratio | 10.5:1 | |
Cooling system | Water‑cooled | |
Turbocharger | None | |
Timing system | Chain-driven DOHC with MIVEC | |
Oil type | API SN/ILSAC GF‑5, SAE 0W‑20 | |
Dry weight | 68 kg |
The Mitsubishi 3B21 was used across Mitsubishi's Kei Car platform with rear-engine, rear-wheel-drive mounting and licensed to Daimler AG for the Smart ForTwo (Type 451). This engine received platform-specific adaptations—revised exhaust manifolds in the Smart and updated engine mounts in the Mitsubishi i—and from 2011 the facelifted i adopted updated timing components, creating minor service part distinctions. All adaptations are documented in OEM technical bulletins.
The 3B21's primary reliability risk is timing chain tensioner wear in pre-2011 builds, with elevated incidence in high-mileage or hot-climate use. Mitsubishi internal field data (2012) indicated up to 12% of early engines logged chain rattle before 80,000 km, while EU consumer reports cite ignition coil failure as a secondary concern. Infrequent oil changes and incorrect viscosity accelerate tensioner degradation, making oil quality and service adherence critical.
Analysis derived from Mitsubishi technical bulletins (2009–2014) and EU consumer safety agency failure statistics (2010–2020). Repair procedures should follow manufacturer guidelines.
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The 3B21 is generally reliable when maintained properly, but early models (2007–2010) are prone to timing chain tensioner wear. Post-2011 revisions significantly improved durability. Regular oil changes with correct 0W‑20 oil and avoiding short-trip driving enhance longevity.
Top issues include timing chain tensioner wear (early builds), ignition coil failure, throttle body carbon buildup, and coolant pump leakage. These are documented in Mitsubishi service bulletins ENG‑09‑007 and TIS maintenance advisories. Most are preventable with proper servicing.
The 3B21 powers the Mitsubishi i (2007–2017) and was licensed to Daimler for the Smart ForTwo petrol (2007–2014). It is exclusive to kei-class applications and meets Euro 4 or Euro 5 emissions depending on model year.
Limited tuning potential due to kei-class displacement limits and naturally aspirated design. ECU remaps yield modest gains (~3–5 kW), but require supporting mods like intake/exhaust. Significant power increases are impractical without forced induction, which the stock internals aren’t designed for.
Excellent for its class. In a Mitsubishi i (2010), typical consumption is ~4.8 L/100km (city) and ~3.6 L/100km (highway), or about 68 mpg UK combined. Real-world mixed driving typically achieves 60–70 mpg (UK), depending on conditions and maintenance.
Yes. The 3B21 is an interference engine. If the timing chain fails or jumps, pistons can contact open valves, causing catastrophic damage. Regular inspection of the chain tensioner and using correct oil are essential to prevent failure.
Mitsubishi specifies API SN/ILSAC GF‑5 0W‑20 synthetic oil. Always use this grade and change every 10,000 km (or 6 months) to protect the timing chain and minimise wear, especially in early-build engines.
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