C A T A L I S T C O N V E R T O R S
Between the manifold (the system of pipes accepting the exhaust gasses from the enige) and the exhaust you will find the catalitic convertor. This expensive piece of metal allows the hot escaping gasses to react with oxygen to bether, cleaner products. The product, platina isn't used by this reaction, it's just acting as an activator, an element that encourages the reaction.
To have the best gain, a big surface of platina must be created. This is done by creating hundreds of channels in the convertor. The gasses will pass through those channels and will be converted to better products. This automatically means that the convertor is restrictying the free flow of the gasses. Faster escaping gasses *can* mean a better performance figure.
There are two ways to remove the restricting role of the catalyst
1) Fitting a (rather expensive) sport catalitic convertor
2) Removing the convertor by mounting a cat-bypass.
The pipe, that will replace the convertor
Just before you read any further, I need to warn you that this small power gain (if there is one) isn't allowed by the law and ... maybe more important ... this modification will harm the environment. The pipe/cat-bypass can be ordered by Mike Satur for a very low price.
A powergain of 1-2bhp can be expected (but isn't garantueed at all), although the noise level while accelerating will be enormous. If you decide it to use on the public road, I advise you to drive carefully in cities. It could be very annoying to have such a ear-bleading car passing by.
before: with the convertor still in place
after: with the cat-bypass mounted instead of the convertor
NOTE: The more modern versions of the MGF (MkII, Trophy, Freestyle, ...) have an EBD4-compliant MEMS, which will check the correct functioning of the convertor. Replacing the catalyst, will trigger a warning at MEMS, which will cause the MIL light to turn on, and the engine will be set to 'limp home mode', resulting in poor performance figures.