The engines were developed as competitors to the
Rotax 582 with an identical mounting bolt pattern to the 582, allowing either to be a drop-in replacement. The engines are similar to the
Rotax powerplant in being liquid-cooled, two-cylinder in-line engines, with dual
capacitor discharge ignition. The 3502 was developed from the 3503 as a de-rated version and produces , in comparison to the 3503's . Both engines have the same bore, stroke and compression ratio as the three-cylinder in-line liquid-cooled
Hirth 3701 and the bore, stroke, compression ratio and displacement of the twin-cylinder
Hirth 2704 and
Hirth 2706. Both the 3502 and 3503 are equipped with dual Bing 34 mm slide carburetors or optionally electronic fuel injection. The cylinder walls are electrochemically coated with
Nikasil and the crankshaft is forged from
4130 steel. Standard starting is
recoil start with electric start as an option. The reduction drive system available is the G-50 gearbox, with reduction ratios of 2.16:1, 2.29:1, 2.59:1, 3.16:1, or 3.65:1. The engines run on a 50:1 pre-mix of
unleaded 93 octane auto fuel and oil, or optionally oil injection. ==Variants==