autovac

An autovac is a small auxiliary fuel tank mounted high on the engine block or firewall, fed by vacuum from the engine's intake manifold and gravity-fed to the carburetor below it. It solved a real problem in early automotive design: mechanical fuel pumps were expensive, prone to failure, and took up valuable engine space. By using engine vacuum, free and already present, to pull fuel from the main tank into the auxiliary chamber, autovacs eliminated the need for a separate pump.

The device works by allowing intake manifold vacuum to act on a flexible diaphragm inside the autovac body. This diaphragm pulls downward, expanding the chamber volume and dropping internal pressure below atmospheric. Fuel from the main tank is drawn up through a one-way valve into this low-pressure space. Once the diaphragm relaxes as the engine idles or decelerates, atmospheric pressure on the main tank pushes fuel into the auxiliary reservoir. A float valve inside the autovac prevents overfilling; once full, fuel cannot be drawn in further.

Autovacs appeared on many production cars from the 1920s through the 1950s, particularly on models where manufacturers wanted to avoid the cost and mechanical complexity of electric or mechanical fuel pumps. They performed acceptably on level terrain and at moderate engine speeds, but failed reliably on steep grades, hard acceleration, or high-altitude operation. The vacuum signal weakens at wide-open throttle precisely when fuel demand peaks, starving the carburetor. On hills, gravity works against the system when the main tank sits below the auxiliary tank.

Why it disappeared

Modern carburetors and fuel injection systems demand consistent, pressure-regulated fuel delivery that vacuum-operated devices simply cannot provide. The autovac also left the fuel system vulnerable to air leaks in vacuum lines, which caused vapor lock and hard starting. Once electric fuel pumps became cheap and reliable in the 1960s, the autovac became obsolete. It survives today only on museum pieces and restoration projects, where understanding its operation is essential for getting old engines to run again.

Restorers often find autovacs either seized internally (the diaphragm cracks or the float sticks) or completely missing, replaced by a mechanical pump during a previous repair. Replacement parts are scarce; rebuilding a failed autovac requires careful disassembly and often new diaphragm material. Some shops have retrofitted period-correct restorations with small electric pumps disguised inside replica autovac housings to achieve reliability without altering appearance.