Guys,
I would strongly urge you to purchase vacuum gauges that use a "drag needle". Vacuum can really only be observed while under load at your max cruise RPM. This means crawling down into the engine room to look at the gauge when you're crusing. Not fun. You could also do what many power boats do a mount a gauge at the panel but that is a PITA.
The alternative is to use a fluid filled "drag needle" vacuum gauge. I have been using them for about 15+ years. Racor now finally recognizes the inherent issues with non drag needle gauges and now offers
one $$$. The SBO store also has an alternative drag needle model.
Drag Needle Vacuum Gauge - SBO Store
Once installed you can check on the max vacuum ever pulled at any time even with the engine off.. You will quickly develop a sense for where a new filter should be and then can assess replacement interval via the drag needle. I simply will not install a non-drag needle gauge for any of my customers as it is simply taking their money for no no actual benefit. I have far too many customers, with installed vacuum gauges, that had no clue you need to check on them at cruise RPM or better while under load. They are essentially very expensive placebos that have been telling them nothing when they look at it at idle.. While an
at idle measurement can be used, it forces one to push the fuel filter life until the engine begins losing power. You then reduce to idle and mark the position the gauge reads
at idle. Pushing the filter to the point of restricting the engine can be dangerous and is not generally advised.
This is the drag needle gauge on my fuel polishing circuit...