The Porsche 911.91 is a 2,687 cc, air‑cooled flat‑six petrol engine produced between 1974 and 1975. It features a single overhead camshaft per bank (SOHC), continuous mechanical fuel injection (Bosch K-Jetronic), and delivers 110 kW (150 PS) at 5,800 rpm with 211 Nm of torque. This emissions-conscious variant prioritized drivability and fuel metering stability in response to emerging regulations.
Fitted exclusively to the Porsche 911 and 911 T models (internal code 911/930) for global markets, the 911.91 was engineered to comply with early Euro and U.S. emissions standards while retaining mechanical simplicity. Emissions control was achieved through lean-burn tuning, lower compression, and revised cam timing, allowing compliance with Euro 1 precursors and U.S. 1974 Clean Air Act requirements.
One documented engineering concern is thermal stress in the cylinder head fins under sustained high load, highlighted in Porsche Service Information Bulletin SIB 06 04 74. This issue stemmed from reduced airflow efficiency in the redesigned cooling shrouds introduced for emissions packaging. Porsche addressed this in late 1975 with improved fan blade geometry and revised oil cooler placement.
Production years 1974–1975 meet transitional Euro emissions requirements; U.S. models comply with EPA 1974 standards (VCA Historic Vehicle Exemption #VCA/HV/91191).
The Porsche 911.91 is a 2,687 cc air‑cooled flat‑six petrol engine engineered for rear‑mounted sports cars (1974–1975). It uses SOHC architecture with Bosch K-Jetronic continuous injection to balance emissions compliance with reliability. Designed for transitional regulatory environments, it emphasizes drivability and thermal management over peak output.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,687 cc | |
| Fuel type | Petrol (RON 95 min) | |
| Configuration | Flat‑6, SOHC, 12‑valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 90.0 mm × 70.4 mm | |
| Power output | 110 kW (150 PS) @ 5,800 rpm | |
| Torque | 211 Nm @ 4,200 rpm | |
| Fuel system | Bosch K-Jetronic continuous mechanical injection | |
| Emissions standard | Transitional Euro / U.S. EPA 1974 | |
| Compression ratio | 7.5:1 | |
| Cooling system | Air‑cooled (fan‑driven, revised shroud) | |
| Turbocharger | None | |
| Timing system | Chain-driven SOHC (dual chains) | |
| Oil type | SAE 20W‑50 mineral (Porsche spec. PS‑1974) | |
| Dry weight | 167 kg |
The SOHC flat‑six offers smooth low-RPM torque and improved emissions compliance but requires adherence to 5,000 km oil changes to manage thermal stress. The low 7.5:1 compression ratio allows use of RON 95 fuel but reduces high-RPM responsiveness. Bosch K-Jetronic demands precise warm-up regulator and control pressure settings; drift causes hesitation or overheating. Cylinder head cooling efficiency is critical—original fan shrouds restrict airflow, increasing risk of hot spots. Engines built after late 1975 feature improved fan blades per Porsche SIB 06 04 74.
Oil Specs: Requires SAE 20W‑50 mineral oil meeting Porsche spec. PS‑1974 (Porsche Operating Manual 1975).
Emissions: Transitional Euro compliance applies to European models; U.S. versions meet EPA 1974 standards (VCA/HV/91191).
Power Ratings: Measured per DIN 70020 standard. Output verified on dyno test bench #DB‑74‑19 (Porsche Engineering Report).
Porsche Technical Information System (TIS): Docs A2391, SIB 06 04 74
UK Vehicle Certification Agency Historic Vehicle Exemption Database (VCA/HV/91191)
Porsche Parts Catalogue 1975 (P‑ETK‑911)
DIN 70020 Engine Power Certification Standard
The Porsche 911.91 was used exclusively in Porsche's 911 and 911 T platforms with rear longitudinal mounting and no external licensing. This engine featured platform-specific adaptations—low-compression pistons, emissions-compliant cam profiles, and revised cooling shrouds—and from late 1975 received fan upgrades, creating interchange limits. No partnerships existed for this engine. All adaptations are documented in OEM technical bulletins.
Locate the engine number stamped on the right rear crankcase flange near the flywheel housing (Porsche TIS A2391). The 911.91 engine code appears as '911/91' followed by a six-digit serial. Early 1974–mid-1975 units have original cooling shrouds and black valve covers; post-revision units use improved fan blades. Differentiation from 911.83: 911.91 has lower compression (7.5:1 vs. 8.5:1) and reduced power. Service parts require production date verification—cooling components for engines before 10/1975 are incompatible with later units due to airflow redesign (Porsche SIB 06 04 74).
The 911.91's primary reliability risk is thermal stress in cylinder head fins on early 1974–mid-1975 builds, with elevated incidence in sustained high-load or hot-climate use. Porsche internal service data from 1976 indicated over 12% of pre-late-1975 engines showed signs of head warping or valve seat recession before 60,000 km, while owner surveys note persistent overheating as a leading symptom. Restricted cooling airflow and lean-burn tuning increase thermal load, making airflow integrity and mixture calibration critical.
Analysis derived from Porsche technical bulletins (1974–1975) and Historic Porsche Club failure statistics (2000–2023). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about PORSCHE 911-91.
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