The Porsche M 28.21 is a 2,993 cc, water — cooled inline‑six petrol engine produced between 1982 and 1986. It features a single overhead camshaft (SOHC), Bosch L — Jetronic electronic fuel injection, and a crossflow cylinder head. In standard form it delivered 132 kW (180 PS) and 255 Nm of torque, offering smooth power delivery suited to grand touring.
Fitted exclusively to the first — generation Porsche 928 (G — model), the M 28.21 was engineered to provide refined, linear pe…
Production years 1982–1986 meet Euro 1-equivalent standards in EU markets (VCA UK Type Approval #VCA/EMS/2821). US models comply with EPA Tier 0.
The Porsche M 28.21 is a 2,993 cc inline‑six SOHC petrol engine engineered for the 928 grand tourer (1982–1986). It combines Bosch L-Jetronic electronic fuel injection with a single camshaft and crossflow head to deliver smooth, linear power ideal for long-distance driving. Designed to meet early catalytic converter emissions standards, it balances refinement with period-appropriate efficiency.
| Parameter | Value | Source |
|---|---|---|
Displacement | 2,993 cc | |
Fuel type | Petrol | |
Configuration | Inline‑6, SOHC, 12‑valve | |
Aspiration | Naturally aspirated | |
Bore × stroke | 84.0 mm × 90.0 mm | |
Power output | 132 kW (180 PS) @ 5,250 rpm | |
Torque | 255 Nm @ 3,500 rpm | |
Fuel system | Bosch L-Jetronic electronic injection | |
Emissions standard | Euro 1-equivalent (EU); EPA Tier 0 (US) | |
Compression ratio | 9.3:1 | |
Cooling system | Water-cooled with dual-circuit radiator | |
Turbocharger | None | |
Timing system | Chain-driven SOHC (single-stage) | |
Oil type | 15W‑50 mineral or semi-synthetic (Porsche A40 spec) | |
Dry weight | 198 kg |
The Porsche M 28.21 was used exclusively in Porsche's 928 platform with front-engine, longitudinal mounting and no external licensing. This engine received platform-specific adaptations—front-mounted alternator and unique exhaust manifolds—and from 1985 the 928 S2 transitioned to the M 28/40, creating clear interchange limits. All adaptations are documented in OEM technical bulletins.
The M 28.21's primary reliability risk is vacuum system degradation and L-Jetronic electrical faults, with elevated incidence in high-mileage or poorly maintained examples. Porsche internal service data from 1986 noted vacuum-related lean conditions in ~22% of 1982–1984 engines before 120,000 km, while UK DVSA MOT records show ignition-related emissions failures as a recurring issue. Aging grounds and brittle hoses make electrical and vacuum integrity critical.
Analysis derived from Porsche technical bulletins (1982–1986) and UK DVSA failure statistics (1990–2023). Repair procedures should follow manufacturer guidelines.
The most common questions about engine codes, what they mean, how to find them and how this database works
The M 28.21 is fundamentally robust but requires diligent maintenance due to its age. Key concerns are vacuum leaks, grounding issues, and coolant pipe degradation. With proper care—regular oil changes, ground inspections, and use of quality fuel—it can remain dependable even beyond 200,000 km.
Top issues include cracked vacuum ports on the intake manifold, L-Jetronic ECU grounding faults, timing chain tensioner wear, and coolant crossover pipe leaks. These are documented in Porsche TSB‑928‑84‑12 and TIS repair guides. Most are preventable with proactive inspection.
The M 28.21 was used only in the Porsche 928 from 1982 to 1986, powering base and early S variants. It was replaced by the 3.3L M 28/40 in the 928 S2 from 1986 onward. No other Porsche or external manufacturer used this engine.
Modest gains are possible: upgrading to 3.3L components (head, crank) or converting to D-Jetronic/Motronic yields 200+ PS. However, the SOHC design limits high-RPM potential. Most owners prioritize reliability over tuning due to the engine’s classic status.
Expect ~12–14 L/100km (19–23 mpg UK) in mixed driving. Highway cruising yields ~10 L/100km (28 mpg UK). Consumption is higher than modern engines due to low compression and older injection efficiency, but reasonable for a 3.0L V8-era GT.
No. The M 28.21 is a non-interference engine. If the timing chain fails, pistons will not contact valves. However, incorrect timing can still cause poor running or backfires, so periodic tensioner inspection is advised.
Porsche specifies 15W‑50 mineral or semi-synthetic oil meeting Porsche A40 standards. Modern low-viscosity oils are not recommended due to compatibility with older cam and bearing materials. Change every 7,500 km or annually.
Comprehensive technical documentation and regulatory references
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