The K&N doesnt give a noticable increase because it provides little or no torque increase. the torque comes from the tubing that the ait filter is connected to. bolting a filter right onto the top of the Throttle body takes away any flow from the induction and so could quite possibly reduce torque over the standard filter, although the standard filter doesnt really work as a tube would so its probably about the same.
The K&N probably would produce slightly more Horse power BUT unless you drive around at 5000rpm all day then you wont notice it. and if you do drive around at 5000rpm all day then i suggest you dont as your head gasket wont see the end of 2005
the advantage of the GSR is that it does inhale colder more dense air from beside the engine rather than the hot air that gathers at the top of the engine bay (hot air rises remember). Also because of the tubing, it creates more torque which will improve the pulling power in the lower/mid rev range and so you will notice a difference in day to day driving, not just doing 90 up the M1 for example.
The longer the tubing on an induction kit, the more torque you will unlock. obviously there will be a limit to how effective this is. If you have a nice long tube that starts off wide at the filter and then reduces in diameter towards the TB it will give you major (well, for a cento anyway) increases in torque and probably a nice increase in hp too. this allows the throttle body to suck in large ammounts of air at the filter which is then accelerated into the engine by the reduction of the diameter in the pipe towards the throttle body. larger diameter followed by smaller diameter = faster air flow.
The induction piping also needs to have smooth curners, no sharp right angle bends. smooth ones are fine, just make sure the tubing turn is curved rather than a dead right angle.. this will disrupt the airflow and although will still probably give you an increase in torque and hp, it wont be as good as it could be.
so basicly, unless your going to go fully custom and have a think about getting it made (bolting some samco hoses together doesnt work as the joiners disrupt airflow) then i suggest the GSR. it doesnt matter if it was engineered for the 30mm TB, it will still give a much better performance over the standard filter, and definatly over the K&N.
It might not sound as nice over the K&N but who cares, all sound and no go is typical chav nature anyway
. youve got a 1.1, make it faster, not sound nicer. thats what the exhaust is for
as for the 40mm TB. it should work fine with the GSR. the tubing DOES drop bellow 40mm in one place (the accelerated part just before the TB) as this needs to fit onto the Fiat bob that sits on the TB. isnt only 1 or 2mm out so you really wont notice any difference. and if your that bothered, you can file out the bob that extra few mm.
GSR all the way because its good. and yes, 40mm TB great.
