Fuel Trim Parameters

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This tab contains settings relating to the fuel trim of the engine.


Fuel Trim



Overall fuel trim - an additional compensation applied evenly to the main fuel tables.



The 'Calc' button enables the overall fuel trim to be calculated from changes in fuel pressure or engine displacement.


Cranking fuel trim - applied when starting the engine. Note that the cranking fuel is automatically compensated for the injector size, but the overall fuel trim is not applied to the cranking fuel.  Usually the cranking fuel can be left at zero.


Throttle tip in trim - applied to the throttle top in extra fuel, which is added when the throttle is rapidly opened.  As with the cranking fuel trim, the throttle tip in fuel is automatically compensated for the injector size, but the overall fuel trim is not applied to the throttle tip in fuel.


Cylinder Fuel Trim



By changing the cylinder fuel trim it is possible to add or remove fuel for individual cylinders. Note that the injector duration as datalogged is the duration for injector #1. Changing the fuel trim for cylinder #1 will result in the datalogging showing the duration for cylinder #1, not the rest of the cylinders. Warning: Do not set the cylinder fuel trim from reading the spark plugs. The only reliable way to set the fuelling for individual cylinders is using EGTs or individual lambda probes on each exhaust runner.


Cold Start Fuel Trim



Post Start Fuel Adjustment - alters the amount of fuel added immediately after starting the engine. This extra post start fuel is normally slowly reduced to no extra fuel over the period of 10-30 seconds. Changing the post start fuel adjustment will alter the amount of fuel and also the enrichment time period.



Post Start Fuel Rate of Decay - determines the time taken for the extra post-start fuel to diminish to zero. Higher values (rates) of decay will decrease the fuel more quickly than lower values.


There are four steps to the decay table.  Step 1 is used until the post-start fuel has diminished to 50% of its initial value, step 2 is used until 25% and then step 3 is used until the post-start fuel decreases to zero.  Step 4 is a special case when the intake air temperature is very high (about 70 degrees Celsius / 170 degrees Fahrenheit).


Cranking Fuel


The injector pulse while cranking is calculated from the engine coolant temperature, then compensated for rpm and air pressure.  This injector duration is then further compensated for injector size, overall fuel trim and finally cranking fuel trim.