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TO rocket said:

My understanding is that the higher the number the more vacuum therefore the middle cylinder showing a lower vacuum could indicate a leak.

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DEcosse said:

You have that backwards - higher number is LESS vacuum (i.e. higher pressure)
The cylinder pulling 540 has BETTER vacuum (lower pressure) than the other two.
Why are they different?
Because the balance screws adjust the relative 'closed' position of the throttles (they are not really closed - if they were, there would be no airflow at all and the engine would stall)
The one with 540 has the throttle 'more closed' than the other two. (or conversely the other two are 'more open')

At sea level pressure is ~ 1013 hPa (or mbar) - if you go to say 5000' (colorado for example) then atm pressure is ~ 840 hPa
At 10,000', atm pressure is ~700 hPa
Altitude above Sea Level and Air Pressure

At idle, the throttles are closed, therefor it creates a low pressure in the throttle bodies as the pistons pull against them;
when you open the throttle, it lets air in so the pressure will try to normalize to that above the throttle.
The highest vacuum, (lowest number) will be if you rev the engine then close the throttles - this creates a higher excavation rate of air from the area below the throttles, therefor the pressure drops.

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Claviger said:

MAP signals are transitory, not static. 10% throttle at any RPM isn't a set map value, 20% isn't a set map value etc. As the RPM go up, and the engine stabilizes at a specific load the MAP signal will be a lower number. You need the engine under load to see accurate values, a free wheeling motor changes MAP values too fast for TuneECU to capture.

Look at my datalog snap shot, it CLEARLY shows 940 MAP signal at WOT and 270 at close throttle run down of the motor. 0 MAP signal would be pure vacuum, absolute not relative.

MAP = Manifold Absolute Pressure, by it's very definition, the sensor is an absolute sensor.

14 years tuning Speed Density systems both NA and FI, I'm 100% positive its an absolute sensor the value of which increases in value with throttle opening.

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