K&N £49, but the problem is making sure you have a good cold air feed. Our little cars respond surprisingly well if the inlet is nice and cold, it's worth making your own to go with the kit as the one supplied is something like 1 3/4" bore - you'd do better with a 3" or 4" one. Set it up so that air from the bumper is blowing directly into the end. You can spend much more if you want, but £ per bhp that's probably your best bet.
I'm experimenting at the 'mo with a panel filter in a standard airbox, but with the hot/cold air selector bit binned, and a larger inlet to the box fed from the passenger wing.
Pete.
Cinq sporting 1242cc
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