Both of these 7004R converters could be rated at a 3,000-rpm stall speed. The difference is that the small 9.5-inch unit (245mm shell) would go into a very light car with an engine that produces peak torque high up in the rpm range. The larger 10.15-inch unit (258mm shell) would go into a 3,800-pound car with a healthy big-block, making lots of torque way down low. By choosing an application-specific converter, you are assured that it will deliver the performance and driveability you expect. Both of these converters were used in our '75 Laguna project, illustrating the difficulty of empirically rating a converter by its stall speed alone.