What's wrong with this muffler?

[dom14]

Dunno. Maybe @nissannewby can tell you what is happening.

Ok, cool. I send him a PM. Google hasn't turned up anything on high rpm steady vacuum reading and letting it drop quickly.

[GeeYou8]

The vacuum gauge readings are exactly what I would expect, back in the 80s some cars had vacuum gauges (marked economy gauges) in the instrument cluster & that is pretty much what you got.
At idle speed the engine is sucking in air through a small gap
At higher RPM there is more air being sucked & the gap is bigger the engine will reach a speed where the manifold pressure limits it so the absolute pressure will be lower (more vacuum), with a blocked exhaust the exhaust pressure starts to limit RPM so the manifold pressure is higher (less vacuum).
When you close the throttle the engine is still sucking lots of air through a small gap so the absolute pressure drops sharply (more vacuum) until the engine slows to idle speed.

The comments in brackets are there to clarify, there is no such thing as more or less vacuum, you gauge reads "gauge pressure" not "absolute pressure" that a MAP (manifold absolute pressure) would read.
Graham

[dom14]

The vacuum gauge readings are exactly what I would expect, back in the 80s some cars had vacuum gauges (marked economy gauges) in the instrument cluster & that is pretty much what you got.
At idle speed the engine is sucking in air through a small gap
At higher RPM there is more air being sucked & the gap is bigger the engine will reach a speed where the manifold pressure limits it so the absolute pressure will be lower (more vacuum), with a blocked exhaust the exhaust pressure starts to limit RPM so the manifold pressure is higher (less vacuum).
When you close the throttle the engine is still sucking lots of air through a small gap so the absolute pressure drops sharply (more vacuum) until the engine slows to idle speed.

The comments in brackets are there to clarify, there is no such thing as more or less vacuum, you gauge reads "gauge pressure" not "absolute pressure" that a MAP (manifold absolute pressure) would read.
Graham

Thanx mate.
The reason I wondered is that I've heard idling vacuum reading is the highest we usually get.
Not sure where I heard it, but somehow stored in my head as an unclarified "fact".
It's good to know the real facts.
Cheers
Dom