Hi Keith

Strictly speaking there is no vacuum in the inlet manifold on any diesel or petrol engine come to that. To have a vacuum would mean that the manifold would be "empty of matter" & clearly this is not possible. In reality what we have is a degree of vacuum or a relative depression.

Moving on from this, if we look at a basic system, where we have no throttle or intake restriction device, the question is, where will the greatest negative pressure difference exist?

In the intake port close the inlet valve?
In the larger void of the inlet manifold?
Either side of the air filter?
Under the bonnet?

Obviously the intensity of the depression reduces as we move further away from the source of intake (the descending piston on an intake stroke)

In a similar way that a high pressure pump on a common rail system can not generate any pressure, unless there is some restriction or resistance to the flow of fuel. If you uncrack an injector pipe, the pressure is lost. Turning this on it's head & applying the concept to intake pressure & flow, without some form of resistance to air flow, the level of vacuum or depression is reduced significantly.

This is all granny & egg stuff to you Keith I am sure, but some might be a bit confused about what is being debated here. I agree with you when you questioned Marks "No depression" comment. Even an air filter will cause some degree of depression.

Cheers for now.

Dave