3701 - 2 cycle - 100hp

The 3701 is Hirth's first offering of a water-cooled engine for the aircraft market. 3701 is offered in 2 versions ranging from 80 to 100 hp allowing the same basic crankcase to be used in a wide variety of applications. A narrow engine profile with a single tuned exhaust used for all 3 cylinders allows the 3701 to be used in compact engine compartments. Pistons fire 120 degrees apart resulting in very quiet & smooth operations.

3701 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. 3701 crankshaft is 4130 chromemolly steel. Heads, cylinders, rings, block casting, connecting rods and associated components are all of the highest grade alloys available today.




Electronic fuel injection

Engine Type:

2 cycle, 3 cylinder

Fuel Pump:

Electric fuel pump.




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

HP Output:

100Hp @ 6000Rpm


108 lbs- including electric start and exhaust. With optional gearbox 127 lbs. (does not include radiator)

Peak Torque

88 Ft.Lbs @ 5500Rpm

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.


Dual CDI
(Capacitive digital Ign.)

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)

3701 is offered in two separate Hp versions. 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. Peak power is at a higher Rpm which means that the cruise Rpm will also be at a higher Rpm. By offering the 3701 with two different Hp ratings it will be possible to choose the power requirement that best fits the application.

3701 High torque/low rpm engine - Peak power - 80 Hp at 4500 RPM. (Not available until April 03)

3701 High performance engine - Peak power - 100 Hp at 6000 RPM