Transmission Power Flow

Transmission Power Flow

The following sections describe the power flow paths in a typical four-speed manual transmission.


The input shaft rotates at engine speed whenever the clutch is engaged. The clutch gear is mounted on the input shaft and rotates with it. The clutch gear meshes with the counter gear, which rotates around the contershaft.

The counter gear transfers power to the speed gears on the mainshaft. However, since speed gears one, two, three, and four are not locked to the mainshaft when the transmission is in neutral, they cannot transfer power to the mainshaft. The mainshaft does not turn, and there is no power output to the driveline.

All gear changes pass through the neutral gear position. When changing gears, one speed gear is disengaged, resulting in neutral, before the chose gear is engaged. This is important to remember when diagnosing hard-to-shift problems.

First Gear

Power or torque flows through the input shaft and clutch gear to the counter gear. The counter gear rotates. The first gear on the cluster drives the first speed gear on the mainshaft. When the driver selects first gear, the first/second synchronizer moves to the rear to engage the first speed gear and lock it to the mainshaft, the first speed gear drives the main (output) shaft, which transfers power to the driveline. A typical first speed gear ratio is 3:1 (three full turns of the input shaft to one full turn of the output shaft). So, if the engine torque entering the transmission is 220 foot-pounds it is multiplied three times to 660 foot-pounds by the time it is transferred to the driveline.

Second Gear

When the shift from first to second gear is made, the shift fork disengages the first/second synchronizer from the first speed gear and moves it until it locks the second speed gear to the mainshaft. Power flow is still through the input shaft and clutch gear to the counter gear. However, now the second counter gear on the cluster transfers power to the second speed gear locked on the mainshaft. Power flows from the second speed gear through the synchronizer to the mainshaft (output shaft) and driveline.

In second gear, the need for vehicle speed and acceleration is greater than the need for maximum torque multiplication. To meet these needs, the second speed gear on the mainshaft is designed slightly smaller than the first speed gear. This results in a typical gear ratio of 2.2:1, which reflects a drop in torque and an increase in speed.

Third Gear

When the shift from second to third gear is made, the shift fork returns the first/second synchronizer to its neutral position. A second shift fork slides the third/fourth synchronizer until it locks the third speed gear to the mainshaft. Power flow now goes through the third gear of the counter gear to the third speed gear, through the synchronizer to the mainshaft, and driveline.

Third gear permits a further decrease in torque and increase in speed. As you can see, the third speed gear is smaller than the second speed gear. This results in a typical gear ration of 1.7:1.

Fourth Gear

In fourth gear, the third/fourth synchronizer is moved to lock the clutch gear on the input shaft to the mainshaft. This means power flow is directly from the input shaft to the mainshaft (output shaft) at a gear ratio of 1:1. This ratio results in maximum speed output and no torque multiplication. Fourth gear has no torque multiplication because it is used at cruising speed to promote maximum fuel economy. The vehicle is normally downshifted to lower gears to take advantage of torque multiplication and acceleration when passing slower vehicles or climbing grades.


In reverse gear, it is necessary to reverse the direction of the mainshaft (output shaft). This is done by introducing a reverse idler gear into the power flow path. The idler gear is located between the countershaft reverse gear and the reverse speed gear on the mainshaft. The idler assembly is made of a short drive shaft independently mounted in the transmission case parallel to the countershaft. The idler gear may be mounted near the mid-point of the shaft.

In other transmissions, there are two separate idler gears, one near each end of the shaft. The reverse speed gear may be an independent gear located at the rear of the mainshaft. The reverse speed gear is actually the external toothe sleeve of the first-second synchronizer.

When reverse gear is selected, both synchronizers are disengaged. In the transmission, the shifting linkage moves the reverse idler gear into mesh with the first/second synchronizer sleeve. Power flows through the input shaft and clutch gear to the countershaft. From the countershaft, it passes to the reverse idler gear, where it changes rotational direction. It then passes to the mainshaft and driveline. In the transmission, the reverse slides the reverse speed gear forward until it meshes the gear idler gear. Power flows from the input shaft and clutch gear to the countershaft. It then passes through the front idler gear (direction change), rear idler gear, rear speed gear (direction change), and out through the mainshaft to the driveline.


As discussed earlier, when a large gear drives a smaller gear, an overdrive condition occurs. The large driving gear may rotate three-quarters of a revolution while the smaller driven gear rotates one full turn. Overdrive permits an engine speed reduction at higher cruising speeds. Because the engine (rpm) is running slower, fuel economy is greater. However, engine torque also drops, so power is sacrificed for better mileage.

Overdrive gears are usually located in the transmission housing. The gear ratio of this fifth gear is 0.87:1. The reverse gear train is designed with spur-type gearing. Unlike the four-speed transmission covered earlier, reverse shifting in this transmission is controlled by a synchronizer. As you can see, this synchronizer is also used to control engagement of fifth gear overdrive.

Power flows for first, second, third, fourth, and fifth gears are similar to those in the four-speed transmission described earlier. In each case, a shift fork moves the appropriate synchronizer to lock the required speed gear to the mainshaft. Power flows through the input shaft to the counter gear, and back through the mainshaft to the driveline.

When reverse gear is selected, the fifth/reverse synchronizer is moved by the fifth/reverse shift rail and shift fork. The synchronizer locks the reverse gear to the mainshaft. The clockwise rotation of the input shaft and clutch gear drive the counter gear in a counterclockwise direction. The reverse idler gearing is driven clockwise by the counter gear. The clockwise turning reverse idler gear drives the reverse speed gear, synchronizer, mainshaft, and driveline counterclockwise. The vehicle can then back up.

More Manual Transmissions And Transaxles

Copyright @ Carkipedia All rights Reserved. Sitemap