that list was a killa or turn off , ini macam mod ka , baik throw in a mivec or 93

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a/f is adjusted using piggybacks. it corrects(not really to say corrects) the air/fuel mixture to prevent getting lean(too much air) or too rich(too much petrol)
lean could lead to detonation of ur engine , and 1 piece of information , lean does gives more output ... so when u add things like "surbo" , "air compressor" bla bla bla , naturally it sucks in more air , therefore being lean and they claimed better performance.
in the case of rich , u will feel sluggish as unburned petrol being overflowed , but being rich is always better than being lean , sometimes can see flames shooting out fr ekzos but very rare in NA unless u are running on a high capacity engine ...
so to perfect the Air/Fuel Ratio , one needs a piggyback(minimum) to optimize one's engine to prevent too rich/lean ...
p/s: my explanation sucks , i really dunno how 2 put in the words , i'm better off explaining in face 2 face conversation ... lolz