I thought that as the cam got bigger the vacuum goes down. My new cam is a 24x/25x cam and according to datamaster I was pulling ~25 inches of vacuum. Is that normal?

 

Datamaster shows "negative" vacuum via the MAP sensor. You have to subtract the atm pressure from the MAP sensor output to see how much vacuum its pulling. I would expect to see something around 16-19 inHG(MAP sensor readings) with a cam that size in a 383 with over 850 idle rpm.. just a guess.

 

So if I remember correctly with 25 inches. I would take this reading and subtract the 14.7 atmospheric and end up with around 10.3 in. Is that right?

 

No, not right. You have to use the same units of measurement ("Hg) for both barometer and MAP.

Standard atmospheric pressure at sea level is 14.7psia = approx. 30"Hg. If your scanner reports MAP as 25"Hg, you would get:

30"Hg barometer - 25"Hg MAP = 5"Hg vacuum

You need to get the "exact" barometer reading from your scanner. That's one of the data fields. Barometric pressure changes with the weather and your altitude above/below sea level.

Obviously 5"Hg vacuum is a bit too low. Were you reading MAP at idle? What idle speed?

 

Oh okay
Barometric pressure was right around 30.15 and the map sensor reading was 23-25. Idle speed was pretty low, around 5-600 rpm though I have it programmed for 1000 rpm idle. Still working on that one