[Zappo]

Actually it has to be something more than just feeding more fuel. Like, commensurate with the extra flow of fuel it will also feed more air for the optimum setting. To achieve these of course the valve opening timings will probably need to be changed.

However the question still remains why do the manufacturers themselves do not provide the same mappings? Well as GTO pointed out, one good reason is that manufacturers try to sell the same engine at two different tune levels. Second thing could always be about striking the right balance between the FE and power. A stock Swift by no means is a slacker. However it still gives a good FE. Now you may very well say that you do not mind if the FE goes down a few notches as long as you get a tarmac burning performance. Sure go ahead, but then manufacturer can not make all the cars, particularly the mass produced ones, based on the requirements of a handful of enthusiasts.

However another important thing is the life of the aggregate components. They test their vehicles for a combined total of millions of kms under different conditions to arrive at the optimum setup. When you put in an external component it may boot up the performance by as much as 20% but then is the manufacturer of that component really bothered about the life of your car in the longer run? How will the transmission handle all that boost? Is the gearbox able to handle the extra boost on a sustainable basis? Only time will tell. Obviously thats an unchartered territory and that is why manufacturers will null and void your warranty if they get to know that you have such a component added to the car.