[Gmecm] MAP and Vac

[Robin Handley]

Having started to think about adjusting SA for low CTS values, I spotted
that the Sunbird/$58 CTS correction table for SA is indexed by 'vacuum',
rather than 'MAP'. I've plotted this quantity on the same axes as MAP, and
the two look like a mirror image of each other, with the mirror at ~60kPa.
So, vacuum goes up as MAP goes down, which makes sense. But I don't
understand why vacuum exists as well as MAP, and what the relationship is.
Can anyone help?

I also spotted that, whilst idling, vacuum was constant and had no noise on
it. This changed when the fan kicked in, and then went back to being
constant when the fan switched off. I don't understand why!

Robin