LTT stands for “Long Term Trim”. It’s used extensively in most modern OEM ECUs as a way to ensure proper engine functioning when using the same map over tens of thousands of units, with the inherent manufacturing differences between them and to cater for the wide range of conditions in which they are required to function.


Proper functioning of any engine rests on having the correct Air-Fuel Ratio (AFR). As the name suggests, it's the ratio between the air's mass and the fuel's mass that's being burned in the cylinder. There are some key terms here:

Stoichiometry = the ratio at which all the fuel is burned (14.7 for pump gasoline)

Lean mixture = more air than fuel (>14.7)

Rich mixture = more fuel than air (<14.7)

Lambda reading = Current AFR / Stoichiometric AFR


By defining the desired AFR for specific conditions (normally a 2D table against RPM and Load) and by using a wideband sensor to measure the actual AFR, we can make real-time corrections as to how much fuel is being injected in order to reach the desired AFR. In short, if the current AFR is smaller than the target AFR, then it means the mixture is too rich and we need to make the injection pulses shorter. Similarly, if the current AFR is bigger than the target AFR, this suggests we are running too lean and more fuel needs to be added.


The “Long Term Trim” uses those real-time corrections and compounds them over time. As more real-time trim values are gathered and stored, the initial fuelling values as the engine enters a set of conditions (RPM and Load) become more precise and in need of less corrections from the wideband sensor.


A step by step article is available with more information on our website here. (Note some variables/tables are called "WB LTT Trim" in older firmware, 4.0 and above use "Lambda LTT Trim" - the functioning is the same however.