What /\ he said.
All engines have a peak volumetric efficiency, I.e. one rpm where everything marries up perfectly for the strongest torque. OEM engine designers use specific cam profiles, turbo sizing, exhaust and intake diameters, airbox size, plenum volume, throttle body size etc to make every other rpm around that VE point the best compromise possible. They also tailor the gearing to suit the torque curve i.e. Honda use very short gearing to increase wheel torque to overcome the engine's low flywheel torque....and so on, and so forth.
The best compromise usually means keeping the gas speed up, both into and out of the engine, which means deliberate restrictions.....which appear counter intuitive to enthusiasts and less experienced tuners. As soon as you change intake diameter, exhaust diameter, cam profiles, turbo size etc you move that peak torque zone further up the revs, and one thing I have definitely noticed with these bath sponge on a pipe filters is the reduced gas speed at <1750rpm causes a slower, rougher idle, incorrect MAF readings, and a noticeable flattening of the throttle response at low revs.
So to summarise all of that waffle into one easy to digest sentence - aftermarket intakes are good for one thing only: increasing power past 5000rpm.
Claims of 40hp from Revo for their intake (with a 2+ tune) and 25hp+ from Racingline are grossly exaggerated in my opinion, and experience, so take the marketing with a dose of table salt.
