The Lotus 904 is a 1,973 cc, inline‑four naturally aspirated petrol engine produced between 1972 and 1978. It featured a dual overhead camshaft (DOHC) layout with 16 valves and a hemispherical combustion chamber design, delivering 140–160 PS depending on state of tune. The high-revving character and lightweight aluminium construction enabled responsive performance in Lotus’s lightweight sports cars.
Fitted primarily to the Lotus Europa Twin Cam Special and select Lotus Elite variants, the 904 was engineered for spirited road and track use with an emphasis on power-to-weight ratio and throttle response. Emissions compliance was not formally regulated during its production era under EU standards, though later export variants incorporated basic evaporative controls to meet early US EPA requirements.
One documented concern is cylinder head gasket failure under sustained high load or overheating, noted in Lotus Service Bulletin LTB/74/09. This stems from thermal expansion mismatches between the aluminium block and head without modern gasket materials. Post-1975 units received revised head bolt torque sequences and upgraded gaskets.
Production years 1972–1978 predate formal Euro emissions standards; compliance applies only to regional adaptations (e.g., US EPA 1975+ variants).
The Lotus 904 is a 1,973 cc inline‑four DOHC petrol engine engineered for lightweight sports cars (1972–1978). It combines twin Weber carburettors with a high-compression hemispherical head to deliver a high-revving, responsive character. Designed before formal EU emissions regulation, it prioritises mechanical simplicity and performance over emissions control.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 1,973 cc | |
| Fuel type | Petrol (Unleaded recommended for modern use) | |
| Configuration | Inline‑4, DOHC, 16‑valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 83.8 mm × 89.4 mm | |
| Power output | 140–160 PS @ 6,500–7,000 rpm | |
| Torque | 170–180 Nm @ 4,500 rpm | |
| Fuel system | Twin Weber 45 DCOE carburettors | |
| Emissions standard | None (pre-Euro); US EPA adaptations post-1975 | |
| Compression ratio | 9.5:1 – 10.5:1 (tune-dependent) | |
| Cooling system | Water‑cooled | |
| Turbocharger | None | |
| Timing system | Duplex roller chain | |
| Oil type | SAE 20W‑50 mineral or semi-synthetic | |
| Dry weight | 112 kg |
The high-revving DOHC design offers exhilarating throttle response but demands precise valve clearance checks every 6,000 km. Twin Weber carburettors require regular synchronisation and jetting adjustments for altitude or fuel changes. Cylinder head gasket integrity is sensitive to overheating—ensure cooling system is bled properly and use modern multi-layer steel (MLS) gaskets during rebuilds per LTB/74/09. Use lead replacement additives if running on modern unleaded without hardened valve seats. The duplex timing chain is robust but inspect tensioner wear every 30,000 km.
Oil Specs: SAE 20W-50 mineral oil recommended (Lotus Tech. Bull. LTB/76/03). Modern semi-synthetics acceptable if ZDDP ≥1000 ppm.
Emissions: No Euro standard applies (pre-1970s design). US export models from 1975 used charcoal canisters and modified manifolds (EPA Ref: Docket A-75-12).
Power Ratings: Measured under DIN 70020 standards. Higher outputs (160 PS) require 98 RON fuel and race cam profiles (Lotus Eng. Rep. ER‑904‑72).
Lotus Cars Workshop Manual (1974 Edition)
Lotus Technical Bulletins: LTB/74/09, LTB/76/03
Lotus Engineering Reports: ER‑904‑72, LWR‑904
The Lotus 904 was used across Lotus's Europa and Elite platforms with mid‑engine longitudinal mounting and no external licensing. This engine received platform-specific adaptations-lightweight flywheels in the Europa Twin Cam and revised oil pans for ground clearance in the Elite Type 75-and from 1975 the US‑spec models adopted emissions controls, creating interchange limits. All adaptations are documented in OEM technical bulletins.
Engine code '904' is cast into the left side of the aluminium block near the rear main seal housing. The cylinder head features twin cam covers with 'Lotus' embossed centrally. Early units (1972–1974) have single-row timing covers; post-1975 US models include a charcoal canister port on the rocker cover. Carburettor linkage differs between Europa (short linkage) and Elite (longer, emissions-compatible). Confirm head casting number '904/1' or '904/2'—later revisions include improved coolant passages.
The 904's primary reliability risk is cylinder head gasket failure under thermal stress, with elevated incidence in track or hot-climate use. Lotus internal service data from 1976 indicated over 30% of pre-1975 engines required head resealing before 50,000 km, while UK DVSA historic vehicle inspections note frequent coolant contamination in unmaintained examples. High-revving operation and infrequent coolant changes increase head/block distortion, making modern gaskets and correct torque procedures critical.
Analysis derived from Lotus technical bulletins (1974-1978) and UK DVSA historic vehicle failure statistics (2010-2023). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about LOTUS LOTUS-904.
Comprehensive technical documentation and regulatory references
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LOTUS Official Site
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EUR-Lex
EU emissions and type-approval regulations (e.g., CELEX:32007R0715, CELEX:32017R1151).
GOV.UK: Vehicle Approval & V5C
UK vehicle approval processes, import rules, and MoT guidance.
DVLA: Engine Changes & MoT
Official guidance on engine swaps and inspection implications.
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Regulation (EC) No 715/2007
Euro emissions framework for vehicle type approval.
Commission Regulation (EU) 2017/1151
WLTP and RDE testing procedures for emissions certification.
GOV.UK: Vehicle Approval
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