I have a box from ebay on my car, It was £99 From here http://cgi.ebay.co.uk/Ford-Focus-TDC...item35a698dece

It has made a fair difference to my car, Its not a fast car now but there is a noticable difference, There is a screw to adjust the box but I haven't changed it. It took me a while to decide whether to buy one or not but I don't regret it.