In part one we showed you all the internal parts of a Th-700R4 converter (same converter design used with the Th-2004R), in part two we showed you just the converter lock up components and how they interfaced with each other to provide a direct mechanical connection between the engine and the transmissions input shaft. In Part three we explained the two hydraulic circuits and one electrical solenoid that control the converter clutch movement. In this part we are going to try to explain the inherent problems caused by an overdrive gear, how it effects the converter fluid temperature and how we have found the best way to control the converter clutch.

With the overdrive gear added to an automatic transmission comes many wonderful benefits. Lower engine rpm with its associated improved fuel economy being high on the list of these benefits. As with most things mechanical, when you gain something, there is a loss of something. With overdrive gearing, the loss is torque.

A transmission is a torque multiplier. The amount of resulting torque is determined by a specific gears ratio. For example, first gear in a Th-700R4 is 3.06:1, meaning it takes 3.06 turns of the input shaft to get one full turn of the output shaft. 100 pounds feet of torque going in comes out twisting with a force of 306 lb. feet of torque. This is terrific for pulling hard or accelerating from a stop quickly but is very limited by the engine rpm capability. Who wants to drive at 20 miles per hour all day? Other gear ratios give different torque outputs for different driving situations. The old reliable Powerglides, Th-350 and Th-400 transmission had a one to one ratio high gear as does the Th-700R4 and Th-2004R in third gear. A one to one gear gear results in the engine and the output shaft of the transmission turning at the same speed with no torque multiplication. Overdrive gear ratios loose torque because they are less than one to one. The Th-700R4 has a .70:1 overdrive gear. This means you will loose 30% of the torque being applied by your engine as it's transferred through the transmission.

Lets set up driving situation to help explain what happens. You have a vehicle with an overdrive automatic transmission and a 3.73:1 rear end gear ratio. You're driving along at 70 miles per hour in third gear (one to one, 1:1) in a situation that requires your engine to produce 100 pounds of torque to maintain a steady speed. This 100 lb. of torque doesn't get multiplied by the transmission since you are in a one to one gear, so it's twisting the driveshaft with 100 lb. of force, this is multiplied by 3.73:1 which results in 373 lb. of torque being applied to the pavement. If you add throttle, the engine will make more torque, consequently the vehicle will speed up accordingly. Conversely, if you reduce the amount of throttle, the engine will produce less torque, the vehicle will slow down accordingly. I have intentionally ignored the torque converter to keep from confusing this explanation. For the sake of explanation assume there is a slight amount of rpm loss across the converter coupling and a certain amount of work induced heat placed into the fluid as it passes through the converter.

When the disc is in its engaged position, the input shaft/drum assembly of the transmission will always be turning at the exact same speed as the engines crankshaft thereby eliminating any heat production inside the torque converter. When the converter clutch disc is in its disengaged position there will almost always be a difference in rpm speed between the engine crankshaft and the input shaft/drum. This difference in speed will be discussed in great detail in part four of this series, "When accelerating or maintaining vehicle speed, the input shaft will always be turning less rpm than the engines crankshaft. When decelerating, the input shaft/drum will generally be turning faster than the engines crankshaft. This difference in rpm produces a heat by product into the fluid inside the converter. The amount of heat input into the transmissions fluid and how rapidly this heat input occurs depends on many factors. For this discussion, lets assume the following is true: the greater the rpm differential between the driving and driven sections the greater the heat input into the transmission fluid. Our testing as demonstrated this can be quite a small amount when driving around town using normal light throttle to extremely rapid heat build up under hard work situations like towing, climbing steep grades, doing a burn out or power brake or other similar events.