Steeda's Patented Timing Adjuster lets you advance or retard your ignition timing up to 10°, so you can tune your car for optimum performance. Advancing your ignition timing can net up to 10 horsepower and better gas mileage. Supercharged Mustangs can also benefit by setting the timing back a few degrees, which allows you to run more boost and avoid detonation. We went the extra mile designing this part, to make sure it will fit with Steeda underdrive pulleys or Vortech and Paxton superchargers. Will not work with "piggyback" style underdrive pulleys