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. |
