mas mod

i can only assume that you mean the mas screw mod. there is a screw located on the bottom of the mass air sensor that's covered by a bit of black silicone. if you pull off your mas you can see that there is an un-metered section of the mas that allows air to flow through. it's only a small space though. the screw can increase or reduce the amount of air that can flow through this un-metered area of the mas. this allows for a small amount of fine tuning to be done to the air/fuel ratio. by backing the screw out you can increase the amount of air and lean out the mixture slightly by screwing it in you can make the mixture a bit richer. most people back out the screw until it's flush, allowing the maximum amount of air to flow and leaning out the mixture. i've found that the stock fuel mixture is a little rich. i screwed mine out and i noticed my engine was a bit more responsive but not by much at all. i believe it helps the most if you have an aftermarket intake or have done the "flip box" intake mod. some people have noticed and adverse effect by leaning out their mixture as their mixture was already on the lean side for one reason or another. it's really a try it and see what works best for you mod.