He posses magical powers that exceed the best and brightest masters in the field. He can do things, in grandma's garage, that even the richest car manufacturers failed to do with millions invested in the best gear man can buy. He can even do it better in the garage where racing teams failed after 100+ years of motorsport.
Chiptuners's are naturally smart and anything that happens is an act of CIF and Johnny is worshiped as only real God. Slow damage to engine and nature is not considered as a fault of Johnny McFuck.
If issues appear, Johnny posses the magical personal trait and explains to you it is not his fault.