Located in the town of Hirtenberg south of Vienna, Austria, Hirtenberger Patronen’s (HP) primary business as an ammunition factory dates back to 1860. The word “Patrone” in their name means cartridge. Initially, HP produced handmade hunting cartridges but steadily expanded to manufacture a full range of ammunition. At its peak, during the Austro-Hungarian monarchy, the factory employed over 4,000 workers. By 1937, one million cartridges per day were being manufactured to supply the axis powers. However, following World War II and the postwar occupation period, the factory was left an empty ruin until 1955. Production recommenced during 1957 under the guidance of the former president, Fritz Mandl.
With his interest, the model engine department was started in the early 1960s to develop a twin cylinder military drone engine. Then development of a .15 and .61 engine for the consumer market followed. Only the .61 went into full production. The first model of the .61 had a bell type rear rotary valve and Schnuerle porting design, both were unheard features of in the 1960s. Soon after, the .61 evolved into a front intake version which proved to be more suitable for conventional model airplanes. With the success of the .61, the HP .40 soon followed and gained a wide following which its descendant today still carries.
In the early 1980s, the Austrian government took ownership of the munitions factory. During this time development began on a series of 4-stroke engines with a unique rotary valve design. These proved to be the quietest and most fuel efficient engines ever produced.
During the late 1980s, the engine production was neglected as it was such a small portion of HP’s business. In 1990, after years of negotiation, RJL was able to purchase the model engine department from Hirtenberger and move the engine production to Southern California.
This cutaway model, one of four donated by Jerry Nelson, shows the engine’s internals. The Silver Star and Gold Cup engines are among some of last to be made in Austria before production was moved to the USA in 1990.
Features of the HP .40 engines include: forged and bushed con rod; ringed ABC; Schnuerle ported; double ball bearings; weighs only 9.5 ounces; extremely high power to weight ratio; and the same external size as most .25’s. This particular HP engine has a black Thermex coating and a muffler weight of 3.6 ounces. The engine puts out 1.2 hp at 15,000 rpm (practical rpm is 2,200-14,000), and has a prop size of 10×6—11×5.
The above history and stats for the HP engines comes from Model Engine Company of America, who now supply HP engines.