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# EFI Tuning Fundamentals: Absolute vs Gauge Pressure

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## Absolute vs Gauge Pressure

### 01.52

 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.

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