In remapping a new Jap. Impreza STI, for example, what degree of power loss can be expected in the change from 100/102RON Japanese fuel to the 98/97RON fuel found in the UK? I'm not talking about changing any of the intake/exhaust etc., just the ECU side of things. Any 'idiot's guide' explanation of what goes on with remapping would be helpful too.

 

In simple terms, the map in the engine is effectively a big table full of numbers. Kind of like a spreadsheet, where for a given engine speed (rpm) and load site (throttle pedal position), the ECU has a number telling it how much fuel to inject (mixture), and another table tells it what ignition timing to use. There are also a lot of corrective factors, e.g. inject more fuel (run richer) if the engine is cold. Later ECUs also "learn" a correction factor on ignition timing depending on whether they detect knock, and the like. In simple terms, remapping is just changing the numbers in those tables.

If you're running on 97/98 RON at the moment, you may not see any decrease in power - the engine ill now be optimised to run on the fuel you're using. However, you will have a lot smaller chance of blowing your engine up than you do if you run a map designed for higher octane.

One of the mappers will probably be along to give you a more full and accurate answer, and tell me where I've just led you astray

 

Thanks for demystifying it, Hades. I've just disovered the EcuTek site which expands on these issues, so it's beginning to make some sense!

 

Perhaps worth adding that the load site for an Impreza is almost exclusively linked to MAF sensor voltage, not throttle position.

Hence why it is so important to have a correct working one, and why so much damage can be done with a failing one.