+25-40 BHP Engine Mods...what you should know : eBay Guides

I'm sure lots of you have seen and maybe even brought one of these, but do you really know what it does?

Basically, you fit the resistor to your airflow sensor and all it does is trick the ECU into thinking there's a load of cold air coming into the engine, so the ECU compensates by injecting more fuel into the cylinders. Overfuelling causes borewash which will damage the bores and rings!
It's not a brilliant idea. The resistors can be brought from Maplins etc for just a few pence, and no, having them "graded to the correct level" won't make an difference, it will cause damage.
As for the "variable power" modual...same thing, only it's a variable resistor aka a potentiometer. There's an article about these here:
http://en.wikipedia.org/wiki/Potentiometer.
It's a bit like driving about with the choke peg out back when auto choke was not the norm...
Give these items a wide birth unless you want a big bill at the end of it all. I'm sure this will make me one of the least popular people on Ebay, but I'd rather you hear some truth about the subject and be given a second opinion. If you sell these, please think about what your actually selling. Would you drive round in one in your car if you fully knew what it did? Feel free to call me "unhelpful" if it helps you sleep at night ;)
In truth there is no cheap and fast route to gaining more power from an engine. A modified intake and exhaust system may see 5-9bhp increase in most normally aspirated cars, but in truth for more power, you need more capacity or forced aspiration by turbo or super charging. Neither of these routes are cheap, and can have teething problems.
My advice, is if you want a faster car, better tyres and brakes are the first thing you should sort out, followed by your suspension...either that or wait till you've secured enough driving experience to be trusted by your insurance company...and keep your money in the bank till then. True, half of your friends will be driving about it 1100cc Hatchbacks with exhausts that have been made from catering sized Baked Bean tins, chrome wheels etc etc...and will still be paying for all those Halfords bolt on goodies on finance by the time your driving around in an Imprezza WRX or Mitsi Evo 8...it's worth the wait.
It's your choice, but if you care for your car, your engine, and your bank balance, look for other ways then these "mod chips".