Are they really effective????
In a word, no. Buying fuel saving devices and fuel additives is just a waste of good money.
If you want to save on the cost of gasoline, you can try converting your car to auto-lpg. If you don't like LPG, then changing your driving habits and keeping your car in good "tune" is the only way to go. If yo still want to save on fuel, going for a smaller car would be the answer.
NO.......altering the design of the engine will only make your FC worst....