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 performance with reduced complexity compared to contemporary dual-cam designs. Emissions compliance was achieved via a three-way catalytic converter and oxygen sensor feedback system, enabling compliance with Euro 1-equivalent standards in European markets and US Federal/EPA Tier 0 regulations.
One documented concern is intake manifold vacuum port cracking due to thermal cycling and age-related plastic embrittlement, noted in Porsche Service Bulletin TSB‑928‑84‑12. Additionally, early L-Jetronic control units experienced intermittent grounding faults leading to erratic idling—a condition addressed by revised ECU grounding straps from 1985 onward.
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 M 28.21 delivers smooth, linear power well-suited to grand touring but requires attention to its vintage electronics. Bosch L-Jetronic systems are sensitive to ground integrity—corroded or loose grounds at the ECU or fuel rail cause idle instability. Use of 95 RON minimum (98 RON recommended) fuel prevents knocking under load. The plastic intake manifold’s vacuum ports are prone to cracking after 30+ years; inspect for hissing or lean codes. Oil changes every 7,500 km with Porsche A40–approved 15W-50 oil protect the timing chain and main bearings.
Oil Specs: Requires Porsche A40–spec 15W-50 mineral/semi-synthetic oil (Porsche SIB TSB‑928‑84‑12). Modern low-viscosity oils risk inadequate cam lubrication.
Emissions: Euro 1-equivalent certification applies to EU models (VCA Type Approval #VCA/EMS/2821). US models follow EPA Tier 0 with different catalyst calibration.
Power Ratings: Measured under DIN 70020 standards. Power output assumes 98 RON fuel and optimal ignition timing (Porsche TIS Doc. 928‑PERF‑09).
Porsche Technical Information System (TIS): Docs 928‑ENG‑03, 928‑FUEL‑05, TSB‑928‑84‑12
VCA Type Approval Database (VCA/EMS/2821)
DIN 70020 Engine Power Certification Standard
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 engine type is stamped on the left side of the block near the oil filter housing (Porsche TIS 928‑ID‑07). The 10th VIN digit indicates model year; 928 VINs begin with WP0ZZZ for US or WP0AB0 for EU. Early M 28.21 engines (1982–1984) feature a black plastic intake manifold with vacuum tree near the throttle body; 1985–1986 units have revised vacuum routing and reinforced ECU ground points per TSB‑928‑84‑12. Do not confuse with M 28/40 (3.3L) by displacement alone—M 28.21 retains 84.0 mm bore.
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.
Find answers to most commonly asked questions about PORSCHE M-28-21.
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