No one is saying there can't be improvements other than the cylinder volume and valve area.

I'm just saying that the factory intake (air filter to throttle body) is very efficient and it's hard to change it in a way to make measureable improvements and I haven't seen anyone do so yet.

The manifold is next after the throttle body and for under $500 I have seen "measureable" gains to be had with a modified upper manifold collector (AKA plenum). IMO, the factory upper collector wasn't as efficient as it could have been unlike the air filter intake assembly up to the throttle body, which other than the "Z tube part of this" seems to be about as efficient as it's going to get.

I have also observed that others that have actually measured before and after have shown gains with a high flow cat. I have further observed, through my own measurements and others' reports, that the combination upper intake collector/high flow cats produces gains greater than the sum of each part.



2004.5 Coupe/5AT/Ivory/Everything but front spoiler/Nismo CAI/Crawford V5/Crawford Cats

 

Important to study the positive BMEP vs the negative IMEP*......how must negative energy is consummed in pumping out the spent exhaust gases and how much residual contamination [not all exhaust gets out before the exhaust valve closes [the head combustion space]. And some may reverse flow up thru intake valve when both are open.

Where the up to 5% gains come from in cats, down pipes, headers, exhaust systems.

* IMEP: also the work done in compressing the air fuel before ignition and prior to Top Dead Center.

http://me.queensu.ca/courses/MECH435/ 2.%20Engine%20Performance.ppt
http://www.factorypipe.com/Technical...BMEP/bmep.html
http://www.factorypipe.com/Technical...Deto/deto.html