I have used the very useful MEMSfcr website to have a look at my SPi ECU settings (1998 JDM Cooper spec car). My car has had some basic tuning in the past, with a modified head, high ratio rockers, exhaust system, air filter etc but no bottom end modifications.
Some questions for those with experience of these things, and based on the assumption that the entire SPi system/ECU/engine is well maintained and running well without any faults....
1. Can the long term fuel trim reported by the ECU give an indication of how much air the engine is getting in the combustion chambers? I am aware that the ECU will make corrections to injection duration to hit its target air/fuel ratio up to a point. Can I assume that if the LTFT is positive, then the car needs more fuel because there is more air in the combustion chambers?
2. If the above is true, if I did further modifications to get more air into my combustion chambers, would I be correct in thinking the ECU LTFT value would eventually increase as the ECU learns how to deal with this additional air, adjusting its injector map trims to give the appropriate amount of fuel (for its target air/fuel ratio).
3. If that is true, for a given car/engine/ECU, is the LTFT a good indicator of how 'tuned' the engine is compared to baseline factory SPi spec?
The further modifications I have in mind are larger throttle body and potentially bore to 1380. I know that the ECU only has a certain amount of inbuilt adjustability to the fuelling maps.
4. Does anyone know what the highest value for the fuel trim settings are in the ECU? For example, can it adjust up to 25 or 30? If I knew what the ECU fuel trim limit was and I had reached it, then I would know that any further tuning to increase air mass in the combustion chambers would offer no further power benefit, with the standard ECU and fuelling at least.
Thanks all!
