Do not bother buying one of these chips.  They Knacker you engine and they cost about 3p each.  It is a resistor that you can find on the circuit board in your PC TV Or Stereo.   Do not buy as described Below.

I bought one out of interest and I got a small resistor as expected

Having been a member of various car forums and clubs and been asked whether these 'tuning chips' work a bazillion times by other members, and after seeing how many are being listed on Ebay, i thought it was time to set the record straight on what they are, and the harm they can do to your engine. Firstly, they neither chip or tune your engine - only a proper ECU chip, mapped to suit your car with the correct fuelling/air ratio can tune it (superchip, seb etc) and these are usually 100+ pounds to buy and are a replacement EPROM for the board in your cars ecu, NOT a 3p resistor available from any branch of maplins or RS.  So what does this resistor actually do? In a nutshell it cons your ecu by sending it false information about the temperature of the air going through the airflow meter or IAT sensor on your cars engine. It basically makes the ecu think the air is much colder than it actually is. The colder the air, the denser it becomes and its this information that is used by the ecu to adjust the amount of fuel needed for the conditions its sensing, and proper running of the engine, except with one of these resistors fitted, the fuel system of your vehicle is delivering fuel at a higher rate than the engine needs. Yes you might feel a small improvement, but too much or too little fuel is pretty serious as it can lead to detonation, lumpy idle, higher emissions, lower mpg and, in extreme cases, even melted pistons!! Anyone whose ever had their car in for repairs at a garage will know how costly in terms of inconvenience and money this can be. Please remember what your parents probably told you many times - if something seems too good to be true it most probably is!! There are very few tuning mods that deliver 20+ BHP for a couple of quid.....if it was that simple to gain horsepower from an otherwise standard engine car manufacturers development costs would be a lot lower and everyone would own a racing team ;)

If you're serious about tuning your engine, join a forum that covers your make and model and speak to the people who have already done it. They will tell you what works and what doesn't, whats good value for money and whats a con, and you'll meet friendly, like minded people as an added bonus :)