how is it possible that the engine produces different MAP readings at the same rpm?

 

Imagine sitting in your car in the driveway in neutral and holding the RPMs at 2500.

Now imagine going up a very very steep hill at 2500rpms.

On the steep hill you will have the gas pedal pressed down much more. The throttle blades will be open much more.

Generally speaking - the more the throttle blades are opened, the higher the kPa and the smaller the vacuum.

Tim

 

Here is something from Holley's EFI tuning guide.

 

thx