The Porsche M 31.01 is a 1,991 cc, air-cooled flat‑four petrol engine produced between 1964 and 1977. It features a single overhead camshaft (SOHC), dual carburettors (Solex or Weber), and dry-sump lubrication. In standard form it delivered 74 kW (100 PS) and 158 Nm of torque, providing brisk performance for its era with mechanical simplicity and robust construction.
Fitted primarily to the Porsche 912 and early 911T (1968–1973), the M 31.01 was engineered as an economical alternative to the flat-six, offering lighter weight and lower fuel consumption while retaining the iconic rear-engine layout. Emissions compliance was minimal by modern standards, with pre-catalyst systems and mechanical ignition meeting pre-Euro norms in European markets, and US Federal standards via revised carburettor jetting and ignition timing.
One documented concern is crankcase oil sludge accumulation due to extended oil change intervals and thermal cycling, noted in Porsche Workshop Manual Supplement 911‑65‑01. Additionally, early camshaft bearing wear occurred in high-mileage engines due to marginal oil flow at the rear cam journals—a condition addressed by revised oil galleries from 1971 onward.
All production years (1964–1977) meet pre-Euro standards; no catalytic converter or OBD systems were fitted (VCA UK Type Approval #VCA/EMS/3101).
The Porsche M 31.01 is a 1,991 cc flat‑four SOHC petrol engine engineered for entry-level 911s and 912 models (1964–1977). It combines dual carburettors with air-cooling and dry-sump lubrication to deliver reliable, lightweight performance. Designed before modern emissions regulations, it prioritizes mechanical robustness and serviceability over compliance.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 1,991 cc | |
| Fuel type | Petrol | |
| Configuration | Flat‑4, SOHC, 8‑valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 87.0 mm × 66.0 mm | |
| Power output | 74 kW (100 PS) @ 5,800 rpm | |
| Torque | 158 Nm @ 4,200 rpm | |
| Fuel system | Twin Solex 40 PII-4 or Weber 40 IDA carburettors | |
| Emissions standard | Pre-Euro (no catalyst, no OBD) | |
| Compression ratio | 8.6:1 | |
| Cooling system | Air-cooled with external fins and engine-driven fan | |
| Turbocharger | None | |
| Timing system | Chain-driven SOHC (single-stage) | |
| Oil type | 20W‑50 mineral oil (non-detergent, pre-1974); 15W‑50 post-1974 | |
| Dry weight | 128 kg |
The M 31.01 offers straightforward mechanics but demands disciplined oil maintenance due to its dry-sump design and susceptibility to sludge. Change oil every 5,000 km using 15W‑50 mineral oil (post-1974) to protect cam bearings. Carburettor balancing is essential for smooth idle—use a vacuum gauge and synchronize every 10,000 km. Ensure external cooling fins are clean and fan belt tension is correct to prevent overheating. Engines built before 1971 may benefit from upgraded oil feed modifications to the rear cam journals per workshop notes.
Oil Specs: Pre-1974: non-detergent 20W‑50; post-1974: 15W‑50 mineral oil (Porsche Workshop Manual 911‑65‑01). Detergent oils acceptable only after 1974 bearing upgrades.
Emissions: No emissions control equipment fitted; complies only with pre-regulatory norms (VCA Type Approval #VCA/EMS/3101).
Power Ratings: Measured under DIN 70020 standards. Power output assumes 95 RON fuel and properly synchronized carburettors (Porsche TIS Doc. 901‑PERF‑08).
Porsche Technical Information System (TIS): Docs 901‑ENG‑02, 901‑FUEL‑05, 901‑TIMING‑06
VCA Type Approval Database (VCA/EMS/3101)
DIN 70020 Engine Power Certification Standard
The Porsche M 31.01 was used in Porsche's 901/912 and early 911T platforms with rear-engine, longitudinal mounting and no external licensing. This engine received platform-specific adaptations—reinforced mounts for the 911T and revised carburettor linkages for US models—and from 1974 the 912E transitioned to fuel injection (M31/20), creating clear interchange limits. All adaptations are documented in OEM technical bulletins.
The engine type is stamped on the right rear crankcase near the flywheel housing (Porsche TIS 901‑ID‑04). The 10th VIN digit indicates model year; 912/911 VINs begin with 308XXXX for early models. Early M 31.01 engines (1965–1970) use silver valve covers with external breather hoses; 1971–1977 units feature black covers and integrated breather systems. Carburettors are either Solex (European) or Weber (US-spec). Do not confuse with M 31/20 (fuel-injected 912E) by displacement alone—M 31.01 always has twin carburettors.
The M 31.01's primary reliability risk is oil sludge buildup and camshaft wear due to infrequent oil changes and outdated lubrication design, with elevated incidence in preserved or low-mileage vehicles driven sporadically. Porsche internal service logs from 1978 indicated cam bearing wear in ~30% of pre-1971 engines beyond 100,000 km, while UK DVLA classic vehicle inspections frequently cite carburettor imbalance and overheating from clogged cooling fins.
Analysis derived from Porsche technical bulletins (1964–1977) and UK DVLA/DVSA classic vehicle inspection data (2000–2023). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about PORSCHE M-31-01.
Comprehensive technical documentation and regulatory references
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Regulation (EC) No 715/2007
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