What Engine Pressure Ratio is
An engine is an air pump. Intake manifold pressure alone tells you how hard the intake side is being pushed, but not how easily the air is getting out the other end. Engine Pressure Ratio divides intake manifold pressure by a second pressure, giving the Elite a better picture of how efficiently the engine is moving air. The result can then be used as the load axis for your tuning tables.
Ratio source options
| Ratio Source | What it compares | What you need | What it tells you |
|---|---|---|---|
| MAP / EMAP | Intake manifold pressure against exhaust manifold pressure | An exhaust manifold pressure sensor, set up separately | How well the engine moves air from intake to exhaust |
| MAP / Barometric | Intake manifold pressure against atmospheric pressure | A barometric pressure sensor, set up separately | How the engine is breathing relative to the air around it |
Why MAP / EMAP is interesting on turbo engines
On a turbo engine, the exhaust side is often at a higher pressure than the intake, because the turbine is a restriction. Two setups making the same boost can have very different exhaust back pressure and very different cylinder filling as a result. Comparing the two pressures captures that. If you're considering it, read our exhaust manifold pressure sensor article first.
MAP / Barometric
This ratio takes altitude and weather into account, since the same manifold pressure represents a different amount of work at sea level than it does in the mountains. It needs the barometric pressure sensor function configured.
Display Min and Display Max
These only change how the ratio appears on gauges in ESP, setting the lowest and highest values a gauge will show. They don't change the calculation. Set them to cover the range you actually see in logs so the gauge is readable.
Setting it up
- Install and configure the second sensor first (EMAP or baro) and confirm it reads correctly with the engine off.
- On the Pressure Ratio tab, choose the ratio source.
- Set Display Min and Max to a sensible range.
- Log the ratio across your normal driving and dyno runs before you decide to base tables on it.
Switching a table to use Engine Pressure Ratio as its load axis means the existing values no longer line up with what they were tuned for. Treat it as a retune, start conservative and check every area in your logs.
FAQ
Do I need an EMAP sensor to use this?
Only for the MAP / EMAP option. MAP / Barometric uses a baro sensor instead. For sensor choices, see the Haltech range.
Is this worth it on a street car?
Most street builds tune happily on MAP. The ratio options earn their keep on high-output turbo builds where back pressure changes a lot. Book a free consultation if you're unsure.
