Why you should ensure injectors match before a vehicle is tuned.
Firstly, a little background on how the correct amount of fuel to add to an engine is determined.
Air flow into the engine is measured (through a variety of methods) and the Engine Control Unit (ECU) opens the injectors to add the appropriate amount of fuel.
During “closed loop” operation a lambda sensor is used to measure the oxygen content of the resulting exhaust gases, allowing the ECU to adjust the fuel dose, if necessary, and achieve a desired air/fuel ratio.
During “open loop” operation the ECU will use predetermined values (maps) for fuelling and will not adjust the fuel dose from lambda readings. During calibration, a calibrator may use an external wideband lambda sensor to determine the air fuel ratio, and adjust the ECU maps accordingly.
These modes of operation and tuning are effective but do have some weaknesses.
After combustion, exhaust gases combine in the manifold and exhaust system, creating a mixture – a combination of the output from all cylinders. It is this mixture that flows past the lambda sensor(s) and these readings that will be acted on by either the ECU or calibrator.
If the amount of fuel being injected into each cylinder were to differ, this would be impossible to determine from the lambda sensor readings.** There could be more fuel being injected into one cylinder, and less into another, but as the exhaust gases are averaged the overall result may look to be correct. Example pic below:
A variation in the amount of fuel being injected will mean differing conditions in each cylinder.
This can cause reductions in economy (more fuel may be injected into a cylinder than is required), power output (detonation may be encountered on an overly lean cylinder) and in the worst cases may lead to engine wear and component failure.
What can be done to avoid this? Ensuring that our injectors are functioning correctly and are matched in flow.
Engine designers, developers and builders invest their time and money calculating, measuring and recording data with the goal of achieving even output from all cylinders. To stay in line with this, the injectors need to be matched as closely as possible.
If you present your car for tuning and your injectors don’t match, at best you’re in for a sub-optimal result, at worst, the tuner may be unable or unwilling to tune your vehicle.
TLDR; Make sure your injectors match; don’t guess, or assume, check.
**Individual lambda sensors for each cylinder are not seen outside of the engine manufacture & development world, or high-end motorsports.