I could realy talk about this all day but will try and make it a little smaller.
car: Can't beat the hit of power when on boost, Free flow air filter will help but only with other supporting mods. I.E exhaust from
back, uprated fuel pump, larger intercooler and cooler runing plugs.
More dense air works best (All
rally cars are tested early in the morning when the air is best)
No backpressure needed for a
car, get the gases out shortest and quickest route possible.
N/A car: air goes in at atmospheric pressure not sucked in.
free flow filter will make the smallest amount of differance.
In order for it to make a differance that is noticable the whole system needs upgrading. larger
, ported and polished head, high lift camshaft, 4 branch manifold and a decent exhaust system (remembering back pressure required on a n/a car)
I also would fit fuel pressure reg and remapped
to except the higher fuel pressure and more air flow.
There you go thats my imput simplified hope this helps a little.