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EFI Tuning Fundamentals: Absolute vs Gauge Pressure

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Absolute vs Gauge Pressure


00:00 - In the last few modules we've discussed air pressure.
00:02 However, when we're talking about air pressure, we need to be a little more specific, since it can be displayed in two ways, absolute pressure or gauge pressure.
00:11 The way pressure is displayed has a huge impact on what it means, so you need to understand what you're looking at.
00:18 Absolute pressure measures pressure relative to pressure we would find in an absolute vacuum.
00:24 Since this scale references absolute zero, all measurements are positive numbers.
00:30 On this scale, Atmospheric Pressure, under standard conditions, would read 101.3 kpa or 14.7 psi.
00:38 Gauge pressure, on the other hand, measures the pressure relative to the current atmospheric pressure.
00:43 Or in simple terms, how much higher or lower, the measured pressure is, compared to the atmospheric pressure.
00:49 On this scale, we'd read zero kpa or zero psi under normal atmospheric conditions.
00:56 A simple way to remember the difference between the two, is to think of how a tyre pressure gauge reads.
01:01 With the tyre pressure gauge, what we want to know is how much higher the pressure in the tyre is compared to the atmospheric pressure.
01:09 So, if the tyre pressure gauge was just sitting on a workbench it would obviously read zero.
01:14 Attach it to your tyre, however, and it will read the pressure in your tyre.
01:18 Let's say 35 psi, for example.
01:20 This is gauge pressure, because it is relative to atmospheric pressure.
01:25 The air pressure in the tyre is 35 psi higher than atmospheric pressure.
01:30 Now why this is important is that many ECU's will use pressure as a load axis for many of the tables.
01:37 Some ECU's will represent pressure in absolute, while others will use gauge.
01:42 If you don't know what you're looking at, then you can potentially end up making changes in completely the wrong place.