The SRT Piston or Slick Rough Track Piston is a piston design to allow more bypass through the piston when hitting a bump. Image that your car is in posture, down on the nose speed through the corner when all of a sudden you hit a large hole. Bam just like that the shock is going to move a lot of oil from one side to the other and the shims are not going to allow it to happen. This will completely upset the car, breaking traction on the tires and allowing the guy on the outside of you to pass. Grip in the slick is the name of the game and when you can keep your tire attached to the track this will increase you r grip all the time. Now to determine hole size and when you think you would need a piston like this, well let's just say ALL the time, because it just seems to work well in all conditions.
15 Series come in two lengths, 7" and 9" .