Really? How is it 10 pounds of boost gets 25 pounds of manifold pressure? I thought it was if you have more air going in then out it backs up causing higher pressures. But manifold pressure is not = to boost. It is a result of it though.

On a supercharged car, manifold pressure IS boost pressure. You can't read higher than the boost delivered.

Your scanner is screwy.

Then I guess I'll have to check on that.

It'* apparent that some people don't know the difference between boost and manifold pressure.

Manifold pressure covers a range of pressures from vacuum at idle to zero in a normally aspirated car at wide open throttle or in the case of supercharged cars positive pressure or boost. 10 psi positive pressure is 10 psi of boost.

I'm talking with the author of the software to correct this problem. I think I've found out whats wrong.


Proscan actual difference
idle +5 -9 15
load +14 0 15
WOT +25 +10 15

It seems Proscan is not starting at the right place.

Kind of an old post, but here it goes:

Well, it'* really quite simple, your scanner is reporting absolute pressure, not gauge pressure.

Absolute Pressure = Gauge Pressure + Atmospheric Pressure (14.7 psi)

Gauge pressure subtracts the 14.7 psi of atmospheric pressure from the real pressure because atmospheric pressure is always there. In reality, the pressure of the air in your tires is really 35 psi + 14.7 psi, or 49.7 psi. However, because the 14.7 psi of atmosphere is there pushing down on the tire, EFFECTIVLY it'* only 35 psi of air in the tires, which is why gauge pressure is almost universally used to measure pressure.

Unlike most other things, the manifold pressure inside the engine of a supercharged car can be both a vacuum (below atm. press.) or a pressure (above atm. press.).

So:
Boost refers to the gauge pressure of the manifold. 0 boost = atmospheric pressure, with the natural pressure of the atmosphere pushing air into the engine. (naturally aspirated) 10 psi boost = 10 psi EXTRA of pressure pushing air into the engine.

Vacuum is just pressure below atmospheric pressure. Even with some vacuum, there'* still pressure. If there was no pressure, no air would get to the engine. That pressure is just lower than the atmosphere, so we call it a vacuum.

Since our cars have a Manifold Absolute Pressure Sensor (MAP Sensor), it starts reading 0 in a true vacuum, it reads 14.7 psi with atmospheric pressure, and it reads 14.7psi + boost pressure when under boost. That'* why all the readings were "off" be 15. It really WAS starting at the wrong place, 0 instead of 14.7 atmospheric pressure.

Not that is was wrong, per-se, it just goes against regular convention to report vacuum in inHg and boost in psi-gauge.