What makes vacuum?

[PropXprt]

I had a friend ask me today, "why does your boost gauge read vacuum?" I had a difficult time giving him a clear answer. Would someone give me a clear definition or a link to a good resource so that I may explain this to him better.

 

[tpi_roc]

As your piston pulls down with the intake valve open and the exhaust valve closed, it pulls air from the intake, when the throttle blade is closed it has a limited supply it can pull in so it creates a vacuum. Your boost gauge is tapped into your intake (if its aftermarket) so thats why you read vacuum there, if it was before the TB, you probably wouldn't.