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 9…
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 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.
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.
The most common questions about engine codes, what they mean, how to find them and how this database works
The 911.91 is a transitional, emissions-focused engine with reduced performance but improved regulatory compliance. Pre-late-1975 units suffer from thermal stress issues, while later revisions improved cooling. With correct oil (20W‑50 mineral), frequent changes, and proper K-Jetronic calibration, it can be reliable for vintage road use. Classic Porsche specialists consider it a historically significant but maintenance-sensitive engine.
Top issues include cylinder head thermal stress, K-Jetronic control pressure drift, oil sludge in the dry-sump system, and cooling fan belt failure. These are documented in Porsche Service Information Bulletin SIB 06 04 74. Regular thermal and fuel system inspection is essential.
The 911.91 was used in the 1974–1975 Porsche 911 and 911 T models for global markets, particularly in regions with early emissions regulations (U.S., Germany, Sweden). It was never installed in S, E, or RS variants. No other manufacturers used this engine; it was exclusive to Porsche’s compliance-focused lineup of the mid-1970s.
Yes, but cautiously. Restoring higher-compression pistons (+15–20 PS) and revising cam profiles can recover lost output. However, this may compromise emissions compliance. Modern ignition upgrades help, but over-revving risks thermal damage—especially on early units with original cooling. Stock internals support ~165 PS safely with supporting mods.
Approximately 14.2 L/100km (city) and 10.0 L/100km (highway), or about 20 mpg UK combined. Real-world usage in classic 911 T models typically yields 19–22 mpg UK. Requires only RON 95 petrol, making it more fuel-flexible than earlier high-compression variants.
Yes. The 911.91 is an interference design. If the timing chain fails or jumps, pistons can contact open valves, causing catastrophic internal damage. However, chain issues are rare; thermal stress is the greater concern. Proper maintenance minimizes this risk significantly.
Porsche specified SAE 20W‑50 mineral oil meeting PS‑1974. Modern classic-specific oils (e.g., Millers Oils CFS 20W‑50) are acceptable. Avoid synthetics unless engine is rebuilt for them. Change every 5,000 km to manage thermal stress and maintain oil pressure.
Comprehensive technical documentation and regulatory references
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