What a map is
An engine control unit works from tables called maps. Each map gives a value, such as boost pressure or injection time, for every combination of two inputs, such as engine speed and fuel quantity. The factory values in these maps are already calibrated for the engine.
What a percentage tune is
A percentage tune takes a map and raises every value in it by the same amount, for example 15 or 20 percent. It is quick and needs no knowledge of that engine, and it does add power. But it changes factory values that were already correct, everywhere in the map, including where the engine never needed a change.
A calibrated remap works the other way. The factory values stay as they are. The tuner extends the fuel quantity range the maps cover, and calibrates the new area for the extra fuel.
Percentage tune
Every factory value changed
Calibrated remap
Factory values kept, new range added
Two grids representing the same map. In the percentage tune every cell is raised by 20 percent. In the calibrated remap the factory cells are kept unchanged and two new columns are added at the end of the fuel quantity axis.
Injection duration
Changing injection duration is a normal part of a remap. How it is changed is what matters.
The duration map turns the fuel quantity the ECU wants into how long the injector stays open. The factory values are already calibrated for those injectors. The correct approach is to keep them, extend the fuel quantity (IQ) axis so the map covers the larger quantities a remap uses, and extrapolate the durations for that new range.
A percentage-scaled duration map changes the factory values too, so the injector delivers more fuel than the ECU thinks it is delivering, at every point in the map:
- The smoke limiter and other fuel limits work from the quantity the ECU believes it injected, so they no longer protect the engine properly.
- Extra fuel lands at idle and part throttle, where it was never needed: more smoke, worse drivability, higher consumption.
- The fuel consumption shown on the dashboard is calculated from the quantity the ECU believes it injected, so it reads lower than what the car actually uses.
Boost
A turbo can only deliver so much boost at each engine speed. The correct approach is the same as for duration: extend and extrapolate the fuel quantity axis of the boost map, set boost for the new, larger quantities, and keep the factory values below them.
A flat percentage increase instead raises the boost request everywhere:
- Low in the rev range, the turbo cannot reach the new target. The ECU keeps chasing it, which can cause overshoot and boost-deviation faults, and put the car into limp mode.
- High in the rev range, the turbo is pushed outside its efficient range: more heat and turbo speed for little extra power.
- Altitude correction maps adjust boost for thinner air at altitude, and they also need calibrating for the new boost levels. A percentage change to these maps does harm too, because the turbo is working at its limit there.
The torque to fuel conversion
Modern ECUs work in torque. The gearbox, traction control and the ECU's own limiters are all told how much torque the engine is making. The ECU turns a torque request into a fuel quantity using a conversion map, often called Nm to IQ.
If that conversion is percentage-scaled, the engine injects more fuel for each newton metre it believes it is making, so it produces more torque than it reports.
- An automatic gearbox sets its clutch pressures from the reported torque. When the reported figure is too low, the clutches are under-clamped. This is a common cause of slipping and bad shifting after a remap.
- Traction control and the torque limiters also work from the wrong number.
When both maps are scaled, the errors add up
The Nm to IQ map asks for more fuel than the torque figure admits, and the scaled duration map then injects more than that request. The ECU's picture of the engine is wrong twice, and everything that relies on it, the gearbox included, is working from a figure even further from reality.
Removing the protections
Some files go further and switch off limp modes completely, or disable exhaust gas temperature (EGT) monitoring, to hide the faults the tune itself causes instead of fixing them. The engine then keeps running in conditions where it was designed to protect itself.
What a calibration does instead
A calibration keeps what was already right and changes only what the extra fuel needs. It takes longer, needs knowledge of that ECU, and is written on the read from that exact car, which is why it costs more.
- Keeps the factory values, which are already calibrated.
- Extends the fuel quantity axis and calibrates the new range for the extra fuel.
- Sets boost to what the turbo can deliver at each engine speed, including at altitude.
- Keeps the torque model accurate, so the gearbox and the rest of the car know what the engine is really doing.
- Leaves the engine's protections working.
How to tell what you have
- Look at the whole power curve, not the peak. A spike followed by a drop, or a curve that changes from run to run, is a warning sign.
- Take a datalog. Compare requested and actual injected quantity, injection duration, requested and actual boost, and requested and actual rail pressure, and check for stored faults.
- Symptoms to watch for: smoke under light load, boost faults, limp mode, and an automatic gearbox that slips or shifts badly after the remap.
We will check your tune for free
Not sure what is on your car? Send us the tuned file and the original read, through a support ticket or by email, and we will check it free of charge. You get concrete evidence of what was changed in the file: which maps were modified, and whether they were calibrated or percentage-scaled.