Bristol Pegasus
The AEHS is grateful to Bruce Vander Mark, who supplied the manual from which most of these articles are drawn.
Bristol Pegasus II.L and II.M
Specifications
Compiled by Kimble D. McCutcheon
Published 29 Jan 2025; Revised 2 Feb 2025
Bristol Pegasus |
The Bristol Pegasus aero engine series, designed by Roy Fedden, was developed from the earlier Bristol Mercury and Jupiter engines; the model first ran in 1932. The Bristol Pegasus II.L and II.M engines, introduced in 1936, were geared air-cooled 9-cylinder supercharged fixed radials with 5.75" bores, 7.50" strokes, 1,752.79 in³ displacement and 5.5:1 compression ratio. Propeller rotation was anti-clockwise when viewed from the engine rear. The diameter was 55.3" and the length was 42.3 (inertial starter to reduction gear front). The engines were attached to the mounts using 18 3/8" bolts on a 27 1/4" pitch circle using either solid or flexible mounts. Over 30,000 Pegasus engines were produced into the 1940s. The AEHS is grateful to Bruce Vander Mark, who supplied the manual from which most of these articles are drawn. |
| Part 1: Specifications | Part 2: Description |
| Part 3: Supercharger, Lubrication | Part 4: Accessories |
| Part 5: Magnetos | Part 6: Carburettor |
| Part 7: Automatic Throttle Control | Part 8: Starter |
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| Bristol Pegasus | |
| Engine Model | ||||||
| Characteristic | II.L.2 | II.L.2a | II.L.3 | II.M.2 | II.M.2a | II.M.3 |
|---|---|---|---|---|---|---|
| Airscrew Ratio | 0.666 | 0.655 | 0.500 | 0.666 | 0.655 | 0.500 |
| Normal Engine rpm (5 min max) | 2,000 | 2,000 | ||||
| Normal Airscrew rpm | 1,332 | 1,310 | 1,000 | 1,332 | 1,310 | 1,000 |
| Maximum Engine rpm | 2,300 | 2,300 | ||||
| Maximum Airscrew rpm | 1,532 | 1,507 | 1,150 | 1,532 | 1,507 | 1,150 |
| Engine Takeoff rpm (calculated) | 1,900 | 1,835 | ||||
| Rated Altitude (ft) | 2,000 | 5,000 | ||||
| bhp, Normal rpm, Rated Altitude | 580/600 | 560/580 | ||||
| bhp, Normal rpm, Sea Level, Full Throttle | 610/635 | |||||
| bhp, Normal rpm, Sea Level, Max Boost | 610/635 | 620 | ||||
| bhp, Normal rpm, Sea Level, Rated Boost | 565/585 | 530/550 | ||||
| bhp, Max rpm | 630/650 at 2,500 ft | 615/635 at 6,500 ft | ||||
| Supercharger Rated Boost (psi) | +0.25 | 0 | ||||
| Supercharger Max Boost, Takeoff and Climb (psi) | +1.25 | +1.5 | ||||
| Supercharger Gear Ratio | 5.7:1 | 7.0:1 | ||||
| Supercharger Impeller Diameter | 10" | 10" | ||||
| Supercharger Impeller rpm, normal engine rpm | 11,400 | 14,000 | ||||
| Supercharger Impeller Drive bhp, Rated Power | 12 | 26 | ||||
| Supercharger Output Volume, Rated Power ft³/min | 1,015 | 1,015 | ||||
| Intake Valves per Cylinder | 2 | |||||
| Exhaust Valves per Cylinder | 2 | |||||
| Valve Actuation | Push Rod | |||||
| Intake Port Diameter | 1.900" | |||||
| Exhaust Port Diameter | 1.771" | |||||
| Valve Seat Angle | 45° | |||||
| Inlets Open, Close | 12° BTC, 50° ABC | |||||
| Exhausts Open, Close | 65° BBC, 31° ATC | |||||
| Inlet, Exhaust Duration | 242°, 276° | |||||
| Inlet, Exhaust Valve Lift, Hot | 0.478", 0.471" | |||||
| Inlet, Exhaust Valve Lift, Cold | 8.484", 0.475" | |||||
| Inlet, Exhaust Rocker Clearance (Cold) | 8.484", 0.475" | |||||
| Carburetor | Claudel Hobson AVT.80.B Duplex | |||||
| Specific Fuel Consumption, Takeoff Power | 0.665 lb/bhp/hr | |||||
| Specific Fuel Consumption, 90% Power | 0.565 lb/bhp/hr | |||||
| Specific Fuel Consumption, Rated Power | 0.561 lb/bhp/hr | |||||
| Specific Fuel Consumption, Best Economy Power | 0.475 lb/bhp/hr | |||||
| Fuel Consumption at Rated Power | 43 Imp gal | 42 Imp gal | ||||
| Fuel Specification | D.T.D.134 | |||||
| Fuel Specific Gravity at 15°C | 0.745 | |||||
| Mixture Control Range | 50% to 60% | |||||
| Fuel Working Head at Carburetor, Minimum, Maximum | 0.875 feet, 12 feet | |||||
| Carburetor Venturi Diameter | 68 mm | |||||
| Carburetor Main Jet Size, cc/min | 1,825 | 1,960 | ||||
| Carburetor Power Jet Size, cc/min | 675 | 700 | ||||
| Carburetor Slow Running Jet Size, cc/min | 340 | 340 | ||||
| Carburetor Enrichment Jet Size, cc/min | 1,500 | 1,100 | ||||
| Fuel Pump Type | Bristol Vane Type | |||||
| Fuel Pump Rotation Speed | Engine Speed | |||||
| Fuel Pump Maximum Suction Head, Delivery Head | 12 feet, 12 feet | |||||
| Fuel Pump Relief Valve Cracks | 2.5 psi | |||||
| Magneto Type | Two B.T.H. SC.9 5B | |||||
| Magneto Rotation Speed | 9/8 Engine Speed | |||||
| Magneto Rotation Direction | Anti-Clockwise | |||||
| Ignition Timing | 35°BTC | |||||
| Spark Plugs per Cylinder | 2 | |||||
| Firing Order | 1-3-5-7-9-2-4-6-8 | |||||
| Oil Pressure | 60 psi | |||||
| Oil Inlet Temperature | 50° - 80°C | |||||
| Oil Pump Pressure/Suction Capacity Ratio | 1:1.246 | |||||
| Oil Pump Drive Ratio to Crankshaft | 1:1 | |||||
| Oil Specification | D.T.D.109 | |||||
| Oil Consumption | 1 - 2 Imp gal/hr | |||||
| Generator Drive Speed | 2.5 x Crankshaft | |||||
| Telemeter Drive Speed | 0.25 x Crankshaft | |||||
| Gas Distributor Drive Speed | 0.5 x Crankshaft | |||||
| Gun Gear Drive Speed | 0.5 x Propeller | |||||
| Hand Starter Options | Eclipse Hand Starting Gear, Hand Inertia Starter | |||||
| Electric Starter Options | Eclipse Hand & Electric Turning Gear, Inertia Starter | |||||
| Gas Starter Options | Bristol R.A.E. Mark II, Viet, Saintin | |||||
| Weights | ||||||
| Complete Engine, Dry | 970 lb | 980 lb | ||||
| Exhaust Ring | 50 lb | |||||
| Exhaust Stacks | 7.5 lb | |||||
| Exhaust Heater Pipe to Induction Elbow | 6 lb | Not used | ||||
| Flexible Mounting Unit | 4.0 lb | |||||
| Fixed Mounting Unit | 3.25 lb | |||||
| Hand Turning Gear with Handle | 21 lb | |||||
| Turning Gear, Hand and Electric, with Handle | 30 lb | |||||
| Hand Inertia Starter with Handle | 26 lb | |||||
| Hand and Electric Inertia Starter with Handle | 37 lb | |||||
| Bristol Gas Starter | 52 lb | |||||
| Gas Starter 3-Way Cock with Switch | 2.0 lb | |||||
| Gas Starter 4-Way Cock with Switch | 2.9 lb | |||||
| Hand Doper (primer) | 1.0 lb | |||||
| Hand Doper (primer), Auxiliary Tank & Piping | 4.5 lb | |||||
| Fuel Pump | 1.6 lb | |||||
| Fuel Pump Drive | 1.3 lb | |||||
| Fuel Pump Flexible Drive Connection | 0.5 lb | |||||
| Fuel Pump Flexible Drive Shaft/ft | 1.25 lb | |||||
| Auxiliary Drive | 3.75 lb | |||||
| Generator Flexible Drive Connection | 0.6 lb | |||||
| Generator Flexible Drive/ft | 1.9 lb | |||||
| Electric Generator Drive, Bulkhead Type, | 4.3 lb | |||||
| Gun Gear Twin Unit | 4.0 lb | |||||
| Inter-Cylinder Baffles | 12.5 lb | |||||
| 3-Blade Metal Airscrew Hub | 55 lb | |||||
| Wooden Airscrew Hub | 29 lb | |||||
| Air Intake | 8 lb | |||||
| Gas Distributor Piping and Check Valve | 10 lb | |||||
| Spark Plugs | 3 lb | |||||
| Lubricants for Temerate or Tropical Climates | ||||||
| Wakefield | Castrol Aero "C" | |||||
| Shell Mex | Aero Shell | |||||
| Sternol | D.T.D.109 | |||||
| Silvertown | Speedolene P.4 | |||||
| Vacuum | Mobiloil Aero W | |||||
| Lubricants for Arctic Conditions | ||||||
| Wakefield | Castrol "AA" | |||||
| Vacuum | Mobiloil "A" | |||||
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| Pegasus II.L.2 and II.L.3 Power at Altitude, Power, Fuel Consumption, Throttle Power, Throttle Consumption | ||||
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| Pegasus II.M.2 and II.M.3 Power at Altitude, Power, Fuel Consumption, Throttle Power, Throttle Consumption | ||||
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| Carburetor Calibration | ||||
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