What the ratio tells you
An engine is an air pump. How much air it moves depends not only on what's pushing into the intake, but on what the exhaust side is pushing back with. Turbo engines in particular can have exhaust manifold pressure that climbs faster than boost, and that back pressure affects how well each cylinder fills.
The engine pressure ratio divides intake manifold pressure by a second pressure, so the ECU has one number that captures how the engine is breathing. NSP can then use that ratio as the load axis in your tuning tables.
Ratio source options
| Ratio Source | What it compares | What it shows | Needs |
|---|---|---|---|
| MAP / EMAP | Intake manifold pressure against exhaust manifold pressure | How efficiently the engine moves air from intake to exhaust | An exhaust manifold pressure sensor, configured separately |
| MAP / Barometric | Intake manifold pressure against barometric pressure | How the engine is breathing compared with outside air pressure | A barometric pressure sensor, configured separately |
The ratio won't calculate correctly until the second sensor is set up. Configure exhaust manifold pressure or barometric pressure first and check both sensors read sensibly before you switch any tables to the ratio.
Display settings
- Display Min: the lowest ratio any gauge in the software will show.
- Display Max: the highest ratio any gauge will show.
These only affect how the channel is drawn on gauges, not the calculation. Set a range that covers what you see in your logs so the gauge is readable.
When we use it
MAP / EMAP is most useful on turbo engines where back pressure changes a lot through the rev range, for example with a small, restrictive turbine or a wastegate that's struggling. MAP alone can look the same at two points where the engine is actually breathing very differently, and the ratio separates them.
MAP / Barometric is a simpler way to account for altitude and weather, without an exhaust sensor.
Changing a table's load axis changes what every cell means. Treat it as a retune of that table, not a quick swap, and verify on the dyno.
Related: manifold pressure and the exhaust settings. Wondering if an EMAP sensor is worth it on your build? Ask us.
