Adjust your GM HEI distributor for any driving condition. This kit allows you to change the amount of ignition vacuum advance for maximum part-throttle performance while preventing the harmful effects of spark knock (detonation). Dial in as much spark advance as conditions permit ? add more advance under light load conditions to improve throttle response and fuel economy, or limit spark advance in high compression engines and high load conditions to eliminate knock. Built-in stop prevents the adjustment screw from "backing out" like other designs. Easy to install. Includes 3/32" Allen wrench and easy-to-follow instructions.