I have just finished doing some experimenting with preload on the Duster. Our first passes was with .085 (we used an old brake adjuster feeler gauge) of clearance between the spring and pin, with me in the car. With the air gap the car would pick up the front end. When we set the pin on the spring with me in it, it would not pick up up the front end and the rear body separation during staging was very pronounced. I was trying to bring it out at about 3,300 rpms. The 60ft is a tad better with the gap. It left me with the impression that there is a difference in pressure and hit. The pressure on the tire is greater with the preload, but there seems to be a bigger "hit" with the gap.

We then swapped out the old six cylinder springs for the Caltrac monos and went to the top hole and a gap. This gave us the lowest and most consistent 60ft we have had with the car and this was leaving at 1,700 rpm. I wanted to see if letting the converter swing a bit would hurt or help. Next time out we will try coming out about 3,500-4,000 (whatever the brakes will hold) and see what that does to the 60 ft. In the past the higher launch rpm has created a better 60ft. I think it also has a lot to do with getting the motor in its happy place. With the 590 cam I think the 3,500 mark is a more preferable spot than 1,700. I try to get my videos posted so you can see, but for the time being, here is one of last passes with the monos.