 The Porsche-derived horizontally opposed air-cooled engine had the cylinder bore increase by Grob to make the engine 2.5-litre capacity and has had its Time Before Overhaul (TBO) increased by a technote to 1,600 hours after many examples run on condition reached up to or in excess of 4,000 hours in service.Under the current CAA LASORS document, the G109B can be classed as either a touring motor glider (TMG), or a self-launching motor glider (SLMG).The G 109B is powered by a Grob 2500 E1 horizontally opposed, four-cylinder, air-cooled petrol engine that develops approximately at 2,950 rpm.