IMO if you change something on the car but is regualr wear and tear then it is still stock..IE.. Spark plugs/ wires, fliters oil and air, break pads exc exc...
There are way to many loop holes to draw a line for "stock" to "Modified Motors"
Is Putting in a K&N void the stockness? No I belive not. The motor is still stock, how dumb would it sound going to the track and having some guy ask you...hey is your car stock?? Nope Ive got a K&N Filter you better watch out.
now like what 1105 said, what if you port the "Stock" heads or Intake? Well thats modifying it so I would consider that non stock. Once you change a factory part w/ an aftermarket part and that part give you more performance that can actually be mesured then I belive you dont have a stock motor anymore b/c it is not in stock preformace as it was when it came off the show room floor. IE...Mass air..TB, Intake, Heads.
But then there is the other side of the coin, what if you bump the timming, you dident add or take anything off the car, but you gained HP from it so is the motor still stock
UGGG this is way to confusing, my head hurts........
