Sport aviation engines for Ultralights, Homebuilts, Kitplanes, Gyrocopters, Helicopters, Powered Parachutes, Trykes, Hovercraft & Air Boats.

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3703 - 3 cylinder - 2 cycle water cooled 100hp

The 3703 is Hirth's water-cooled alternative to the popular 3003 air cooled engine. 3703 is offered carbureted or with optional fuel injection. Standard engine is pre-mix and oil injection is available for either the carb or fuel injected versions. A narrow engine profile with a single tuned exhaust used for all 3 cylinders allows the 3703 to be used in compact engine compartments. Pistons fire 120 degrees apart resulting in a very smooth power curve. 3703 fuel consumption rivals similar Hp 4 cycle engines.

3703 incorporates Al-Nikasil coated cylinders for superior performance and reliability. Nikasil provides for a super low coefficient of friction, reducing engine heat and wear. The pistons and cylinders expand at the same rate thus providing for a seizure resistant engine. 3703 crankshaft is 4130 chrome molly steel. Heads, cylinders, rings, block casting, connecting rods and associated components are all of the highest grade alloys available today.

Alternate Views:

3703S - 100Hp electric start &
3 carburetors.
3703S - 100Hp elec,st, carbs & G-50 gearbox
3703ES- 100Hp electric start
and fuel injection.
3703ES 100Hp with electric start, fuel inj. & G-50 gearbox
Oil Injection Option

Hirth's latest design.

Engine Type:
2 cycle, 3 cylinder
HP Output:
100Hp @ 6000Rpm
Peak Torque
88 Ft.Lbs @ 5500Rpm
Dual CDI
(Capacitive digital Ign.)
Triple 34mm slide or optional electronic fuel injection
Fuel Pump:
Auxiliary external pneumatic - 12VDC with fuel injection
Fuel mixed with oil @ 50:1 or optional oil injection
Counter clockwise, viewed from output end
12 volt electric,
250 watt/20 amp alternator standard
Water cooled
108 lbs- including electric start and exhaust. With optional gearbox 127 lbs. (does not include radiator)
Reduction Units:
G-50 gearbox available in 2.16:l, 2.29:l, 2.59:l, 3.16:l and 3.65:l ratios or optional G-40 gearbox can be used if clutch is required.
Drive Coupling:
Elastomeric dampener with G-50 gearbox or optional centrifugal clutch with G-40 gearbox. (2.03:1, 2,25:1, 2.64:1, 2.96:1. 3.33:1)

3703 is the high performance version of the 3702 engine. This is done through engineering by changing the piston port timing. Placement of the port timing determines what Rpm the peak torque is achieved. When peak torque is arrived at low Rpms the peak Hp of the engine is less but the torque curve in mid range is very flat. Such an engine gives up some thrust at full power but has high torque in the mid range allowing for very low engine RPM at cruise power settings. The same engine with the port timing set high will produce much more Hp and thrust at full power but gives up some torque in the lower midrange and the torque curve is not as flat. Peak power is at a higher Rpm which means that the cruise Rpm will also be at a higher Rpm.

3703 High performance engine - Peak power - 100 Hp at 6000 RPM.

Engine Print

Customer Photo Gallery

3703 installed on MX II
Picture #1 Left Side
Right Side

3703 installed on Earthstar Thundergull II
Picture #1 Left Side
Right Side

3703 installed on Dolphin Floatplane
Picture #1 Left Side
Right Side

3703 installed on Skymaster PPC
Picture #1 Left Side
Right Side Rear View

3703 installed on Gyrocopter
Picture #1 Left Side
Right Side

3703 installed on Infinity PPC
Picture #1 Left Side
Right Side

3703 installed on Fusion PPC
Picture #1 Right Side

3703 installed on Skyboy
Picture #1 Left Side
Right Side

3702 on Genesis Aircraft
Picture #1 Picture #2
Picture #3 Picture #4