Vacuum at idle..... kpa vs inch/hg

I feel a bit dumb asking this but I am just getting frustrated.

At idle HPT registers 60kpa. I have a gauge that is showing about -12 inch/hg at idle. Does this match and how is the conversion properly calculated. No online converter I have tried gives me these results.

I looked at some logs of my previous setup, and they also showed around 60 KPA at idle, but I seem to remember the gauge showing around 16. I am just trying to figure out if the gauge is right, or something is skewed now.

Thanks for being patient. My best 3 years of school were grade 8 !!! :doh:

100 kPa = 29.5 in/hg
60 kPa = 17.7 in/hg

Those are in absolute values. Your MAP reading in kPa is absolute pressure, but your gauge reads, well, gauge pressure. For gauge pressure, atmospheric is considered zero, so when you're in vacuum, it's a negative number.

29.5 - 17.7 = 11.8 which rounds to 12

Quote:

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Originally Posted by DSteck

100 kPa = 29.5 in/hg
60 kPa = 17.7 in/hg

Those are in absolute values. Your MAP reading in kPa is absolute pressure, but your gauge reads, well, gauge pressure. For gauge pressure, atmospheric is considered zero, so when you're in vacuum, it's a negative number.

29.5 - 17.7 = 11.8 which rounds to 12

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Thanks Dave, just the guy I thought might chime in. That is the calculation I got too. Maybe I am mistaken in remembering seeing 16 before, because this means the gauge is about right. Is -12 vacuum normal with a mild cam. For some reason I thought it would be more.

It depends on the cam, how much airflow you're using to idle, barometric pressure, and whether an elephant in Zimbabwe farts. Point being... it's in the general vicinity. Raise your rpm or use more spark and less airflow, and you'll see the vacuum increase.

Quote:

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Originally Posted by DSteck

It depends on the cam, how much airflow you're using to idle, barometric pressure, and whether an elephant in Zimbabwe farts. Point being... it's in the general vicinity. Raise your rpm or use more spark and less airflow, and you'll see the vacuum increase.

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Thanks..... I thought I heard an elephant fart... so everything is good. Runs perfectly so who cares about vacuum. LOL