There's absolutely nothing that can transform the attitude of your manual shifted street or race car like having a proper faceplated racing transmission. A Dogbox, or crashbox allow you to change gear at ANY RPM, and as quickly as you can move the shifter. Between the gear-whine from the straight cut gears, the bang as they hit each shift, and the speed with which they work, there's nothing like one! If you know a better single modification, I'd like to hear about it!