The "Mod Chips" being sold on ebay as a performace and or economy enhancement for your vehicle are not a performance enhancement in any way and will infact cause your car to run rich and waste fuel.
I will explain - the sellers say the resistor or mod chips should be connected in the IAT or Air Temperaute Sensor circuit. Effectively this changes the signal level output from the sensor indicating to the Engine Control Unit (ECU) that the Air is Colder. Cold air is more dense and therefore has more oxygen so the ECU thinks that the air is colder and squirts more fuel in to burn the extra oxygen. I think we will all agree that in order to get a flamable mix you need three things, Air, Fuel and a spark. The problem with this modification is that the Resistor Chip is giving a false reading to the ECU tricking it into thinking the air is colder, in fact it is not. The ECU thinks it is colder though and squirts more fuel in to burn the increased amount of oxygen. But there is NOT an increased amount of oxygen, the ECU has been tricked, so the extra fuel cannot be burnt because the Oxygen needed to burn the extra fuel is not there.
Car manufacturers spend thousands, no millions of dollars developing efficient engines, if a 1 pence resistor (sold for in excess of £10 on ebay) can really provide the benfits claimed in auctions then they would install it from new, they do not because it is not a performance enhancement and it does NOT give you better economy. If you want better performance then you have to sacrifice economy, you have to REMAP your ECU, change ignition points, increase the air flow, get a turbo or a supercharger, maybe a performace filter. An Air filter and new Exhaust are probably the cheapest/easiest performance enhancement you can make, but even this will only give you about 5 - 10 BHP, for an investment of £400 - over a £1000. Yes, I am afraid nothing comes free in this word.
Hope this helps