Clutch Basics — Pro Nitrous and Extreme Pro Stock

Looking at the numbers, there is no minimum weight requirement for Pro Nitrous cars, although they typically weigh in the 2,350 to 2,400 pound range. These cars can trip the 60-foot timers in .950 to .970-seconds and run from 2.55 to 2.65-seconds to the 330-foot mark.  Extreme Pro Stock cars weigh in at 2,425 pounds and they travel to the 60-foot clocks in .970 to .980-seconds and hit the 330-foot mark in 2.66 to 2.75-seconds.

Both Pro Nitrous and Extreme Pro Stock predominantly employ 10-inch multi-disc clutch assemblies. Pro Nitrous cars use a three-disc unit, whereas an Extreme Pro Stock car utilizes a two-disc arrangement.  The Pro Nitrous cars need the extra disc, due to the additional horsepower and torque they have over an Extreme Pro Stocker, thanks to their multi-stage nitrous oxide injection systems, also known as “Bottled Horsepower.”  Looking at the numbers, Extreme Pro Stock engines produce around 1,900 horsepower while Pro Nitrous engines put out over 2,500 horsepower.

The centrifugal pressure (or counterweight) is altered by increasing or decreasing the weight of a series of small bolts, nuts, and one-gram washers mounted on each of the clutch fingers. Remember, as the engine rpm increases to each shift point, the centrifugal force increases the clutch's clamping pressure, thereby allowing the clutch to carry the load placed on it with each successive gear change.

These clutch assemblies have two adjustments that the crew chief can use in setting up the car.  The base pressure (also known as static pressure) is a fixed pressure—a constant pressure on the clutch springs—and is adjusted by an Allen-head screw on top of each spring.  The centrifugal pressure (or counterweight) is applied by the weight of a series of small bolts, nuts, and one-gram washers mounted on the clutch fingers.  These Pro Nitrous and Extreme Pro Stock cars leave the starting line at around 5,000 rpm.  As the engine rpm increases to the shift point (typically around 8,000 rpm), the centrifugal force increases the clutch's clamping pressure, thereby allowing the clutch to carry the load placed on it with each subsequent gear change.