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IMHO, they are the biggest waste of money. If I were you, I'd use that money and get some good plugs and wires, along with other general maintance items, like air filters and good oil.
If you put one of those things in, along with the fuel line magnets, a can of slick50, a chrome muffler tip, and quad platinum tip spark plugs, -you will be going down the street making fuel!!!