What is the normal vacuum reading at idle for a Mazda RX-7 FD?

For a Mazda RX-7 FD, the normal engine vacuum reading at idle depends largely on the engine's porting:

• Stock Ports: A healthy stock-ported engine typically pulls between 16 and 20 inHg of vacuum at idle ^{1 7}.
• Street Ports: Opening up the engine with a street port naturally lowers the vacuum, making 15 to 16 inHg a normal baseline reading ¹.
• Bridge Ports: Aggressively ported engines, such as a half-bridge setup, will pull much lower vacuum. It is normal to see readings around 10 to 13 inHg with a raised idle speed of 1400-1500 RPM ³.

If you have a stock or street-ported engine and your vacuum suddenly drops below normal (for example, down to 10-12 inHg), or if the car develops a high or surging idle, it is a strong indicator of an unmetered vacuum leak somewhere after the throttle body ^{7 5}.