on some applications, it varies between engine design. I was told by a few techy's on here that the LAP motors that carry the VVT are already equipped with oversized valves to compensate for the bolt ons. Otherwise, if built by engineers to factory setup, your intake and header when during valve overlap, will just simply bypass during that phase and exit through the exhaust causing your cold air intake and header to just stream unburnt fuel and air, thus resulting in a rich fuel smell out the exhaust pipe.
adding a bit more of info:
Its very hard to explain, but shown is a good way of knowing. All engines are in compliance with 50 state legal emissions onboard. Therefore, you get a volumetric efficiency table (ive talked about this before), but that is the standard numbers they use to adapt to any environment, altitude, and air barometric pressure. When you get your tune, say for instance Vince from Trifecta, he establishes your VE table and re-calculates a generic table that is higher based upon your region of residence. This will allow for better air/fuel ratio and expand your valves for horsepower and torque. This is what is known as a tune. He takes what performance parts you have and re-learns a data table that is permanently stored in your PCM. So when you floor the gas pedal, his data table will kick in out of the 12 that are pre-established already in your PCM's learn mode or adaptive strategy. This is your common tune. A dynotune will enhance that power even more based upon where you reside and drive your vehicle. A dynotune will measure BARO for your MAP sensor, MAF sensor input, 02 bank 1 sensor, and other conditions in the local environment.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille
Last edited by 5t3alth; 06-08-2013 at 10:40 AM.