AIR-COOLED HEATING
Classic 911s used air to cool the engine, yet heat its passengers. Here’s how
While the 911 has been revered for pushing the boundaries of sports car technology and performance, the heating of air-cooled cars is often thought of differently. Water-cooled 911s rely on a simple, proven system, whereby a small auxiliary radiator behind the dash is fed with hot coolant from the engine. This system is easy to blend hot and cold air, then feed it to where required. With air-cooled cars – specifically pre-964 – the system is totally different, and has a reputation for confusing controls, and working ineffectively.
That system works using a pair of external manifolds on the exhaust, through which air is forced through, forwards to the cabin. The exhaust heat is used as a heat source and the engine cooling fan used to move the warmed air. The amount of heat was originally metered by a pair of levers between the front seats, which each operated a mechanical valve before each heat exchanger, thereby opening or closing the hot air supply to the cabin. Closed, air was directed through pipework in the sills to the passenger compartment. Opened, holes in the valves simply vented heat away under the car.
1968 saw dash controls added, allowing passengers to direct heated air to windscreen or footwells, and additional fresh air. A scuttle fan meant fresh air could now be pulled into the interior, and a mixer valve allowed further control to balance fresh and heated air. In 1974, an auxiliary fan was added in the engine compartment, to solve the issue of reduced airflow when idling in traffic. For the 1975 Turbo, and regular models from 1977, the inboard red levers were replaced by a rotary control dial between the seats.
Being part of the exhaust system and made of mild steel components, issues affecting operation (and expect them) centre around corroded exhaust valves, pipework, or blower motors. Replacement of valves is not overly expensive, and with a well-maintained system there should be ample, controllable heat.
For the 1989 964 model, a new design brought everything together in a single, easy-to-understand, dash-mounted control. The heat exchangers changed to being made of stainless steel, and a mixing chamber was used to blend heated air from the engine with ambient air before it went into the cabin.