Differential Tuning.
That big rusty bolt is the difference between running mid 15's
and running low 10's. If you turn the bolt counterclockwise, it will;
1. Port your exhaust and intake manifolds. (you have to pour
sandblasting material in your gas tank first, 12 pounds of
premium glass beads worked for mine)
2. Change your gear ratio to compensate for local barometric and density altitude power loss differences. (must use power
steering fluid in your diff. box for this to work)
3. Make your supercharger spin 28% faster. (add octance boost
to supercharger oil first)
4. Tune your muffler bearings.
5. Give you the ride or your life.