The Porsche 911 brake light uses P21/4W and P21/5W and 1157 bulb sizes for the rear lighting system. Available data covers the range from 1965 through 2009 for the specified vehicle components.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1157 bulb is the most frequently specified brake bulb across the Porsche 911 range, covering model years 1965-2005 with exceptions in 2006, 2008, and 2009, as well as 2007.
The 1157 follows the ANSI standard used in the US and Canada. It is a dual-filament bulb rated at 12V with a 21W high-intensity filament for braking and a 5W filament for running light functions, mounted on a BAY15d bayonet base.
The P21/4W appears in the Porsche 911 brake light position for 2006, 2008, and 2009. It follows the ECE standard used in Europe and worldwide, and operates at 12V with a 21W and 4W dual-filament configuration on a BAZ15d bayonet base.
Although the 1157 and P21/4W are both dual-filament 12V bayonet bulbs with similar wattage on their primary filaments, they use different base types. The 1157 uses a BAY15d base and the P21/4W uses a BAZ15d base, making the two types physically non-interchangeable despite their functional similarities.
The secondary filament wattage differs between the two main brake bulb types: the 1157 carries a 5W secondary filament while the P21/4W carries a 4W secondary filament, resulting in a slight difference in running light output between the two standards.
The center high mount stop light on the Porsche 911 used the P21W bulb from 1981-1994. The P21W is an ECE-standard single-filament bulb rated at 12V and 21W, fitted with a BA15s bayonet base.
For the center high mount stop light, the specification shifted to the bulb size 74 for 1995-1998. The 74 is an ANSI-standard miniature wedge bulb of the T5 form factor, rated at 12V and 1.4W, representing a substantial reduction in wattage compared to the P21W it replaced in that position.
The P21W and the P21/4W share the same 21W primary filament rating and both follow the ECE standard, but the P21W is a single-filament bulb on a BA15s base while the P21/4W is a dual-filament bulb on a BAZ15d base, making them incompatible with each other's sockets.