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If you think that you can feel an increase of ~10 HP compared to 400+ (an increase of 2.5%), you're kidding yourself.
As far as adding up advertised gains, here's my example. I gained ~45 HP from a tune (+20 HP), a full exhaust with HFC (+25 HP), a pulley (+10 HP), and a filter (+10 HP). That doesn't add up.
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2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R |