okay its a common question so heres some info.

induction kits are intended to allowthe engine to draw more air in.
more air means more fuel can be added so more power

there are many types.
i'll try and cover the basics here but theres plenty of different types available.
budget also plays a part here.
the most commonly bought are the straight on the top of the TB K&N,green,pipercross etc type

in that picture you will see a pipercross breather filter as well.these are now an emissions fail(though some have passed MOT's with them)
the problem with this type of induction kit is they sit right on top of the engine so are surrounded by hot air.
not good for performance.
you can fit a cold air feed which helps but is not perfect

now other options involve fitting the filter remotely to move the filter ot a cooler part of the engine
this is liams home made one

if your making your own then you need an 899cc bob for the TB(available from fiat,scrappies etc) a reducing elbow silicone hose either samco or SFS and of course your chosen filter.
obviously there are other options and varients but this just covers the basics

heres arc's with the BMC CDA



this is a GSR kit available from oldschool https://www.fiatforum.com/member.php?u=10010
The GSR Orderline is 020 7005 0909(no tech info)


the one thing to say about the GSR is its a proven product with proven performance gains.