Barometric pressure changes affect the running of the car.
The
ECU sees the manifold pressure in absolute. That means it sees the pressure in the manfold and compares it to absolute vacuum. When you measure boost with a boost guage. It is referenced the pressure to barometric. So when you have 100kpa of boost on a boost guage, you are seeing 100kpa of boost ABOVE barometric presdure. But what you really have is somewhere around 201.7kpa of air pressure in the manifold. But if the
ECU measured 100kpa of pressure in the manifold, what you actually have is probably a very slight vacuum inside the manifold. (Barometric pressure at sea level is averaged at 101.7kpa.)
Therefore if you measure the exact same MAP (manifold absolute pressure) on a day with a relatively high barometric pressure, and then achieve that exact same MAP reading on a day with a relatively low barometric pressure. Then you are actually achieving MORE boost on the day with the lower baro reading.