With gauges like the AutoMeter, they are pretty obviously absolute pressure gauges as they are fudging the 0 mark to a range.



I'm still convinced the absolute pressure is what you want. But either way, depending on where you're running you can just add or subtract the appropriate factor for altitude (or more accurately the pressure altitude) of the track on the particular day depending on which gauge you get and what you want to be thinking about vis a vis pressure. I'll go back to my old flying days as a reference. On takeoff we would use 48" of boost as takeoff power - no correction for whether that was in San Diego, Albuquerque or some other place so that was obviously absolute.

Here's the correction factor based on standard pressure.

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Thanks for the great observation on the zero. You're right, of course. It's not so much that I need one or the other as that I know WHICH I have. I think you've solved the mystery.

I'm leaning toward buying a round one from Ebay and calling it good.

Dan

The standard ones are all PSIG. Note that the gauge pictured above has a zero and then readings less than zero. You can't have absolute pressure below zero. That gauge reads a differential between atmospheric and the pressure being measured. I find it interesting that it seems to be calibrated in PSI for boost and in Hg for vacuum.

In aviation they use absolute pressure gauges and I've only heard of them reading in in Hg.

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The Auber Instruments digital gage sensors are available in gage or absolute and in a number of ranges. I don't know if they zero it to ambient baro on the absolute transducer or if it'll "read" elevation with the engine off. No mention of differential sensors.

I've decided that I know too much about this and I should shut up and buy a boost gage! Too many years of dyno instrumentation.

Dan

now do you go bar or psi.... it is after all a German motor

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PSI - I don't think in Bar - or kilopascals/mm (I think that's the other unit).

I ordered up a Sunpro which will match (sorta) the water temp gage just above it. The water temp gage has a white face and the boost gage doesn't come in white - oh, well.

Dan