Ultimately, both standard and automatic transmissions do the same thing, they transfer the engine’s power to the rear wheels in a way that keeps the rpm’s in the usable power band. If we didn’t have a transmission, there would be one fixed gear ratio and our cars would be very difficult to drive. Therefore, we want multiple gears so that we can accelerate at a reasonable rate and have a reasonable top speed. This article is not going to go into the details of how each type works, but rather the differences of the two in terms of transferring power to the rear wheels.
A standard transmission has a clutch that disconnects the engine from the transmission so that you do not stall when you are stopped. Once you release the clutch and have the gear fully engaged you have a direct physical connection between the engine and the transmission. The transmission then has its associated mechanical losses.
The automatic transmission does some amazing things, but does lose power. The performance differences between the two types are that 1) the torque converter replaces the clutch and 2) the transmission in much more complicated. There is power loss in both areas.
There are two places where the torque converter has losses. To understand them we need a very quick description of how the converter works. It basically has the following parts: turbine, lock-up clutch, stator, and pump. What happens is that transmission fluid comes in through the pump and gets flung to the outside. When this happens, a vacuum is created at the center which sucks in more fluid. As the fluid hits the edge it enters the turbine and causes it to spin. The turbine is connected to the transmission, and makes it spin. So, basically fluid is moving your car. There is no physical connection like in a standard. It is this fluid connection that creates the losses.
The first loss happens because the pump and the turbine cannot spin at the same speed. Since the turbine is always a little slower you lose power. This is the reason for the lock-up clutch. It connects to the transmission directly through a shaft at a speed around 40 mph (can be as low as 10-20 mph). At this point you have almost no loss and it behaves as a manual would (from the clutch perspective). This is what is referred to as a “lockup converter.” In addition, the stator helps steer the fluid for greater efficiency. The result is that until the converter locks up, you lose power, but theoretically don’t when it locks up.
The second loss takes place in the very beginning of the launch process. When you are at a stop notice how you always need your foot on the break in an automatic. This is because the torque converter is always spinning. When you put your foot on the break, you prevent the car from moving and transferring the torque. That power has to go to something and goes to heat. Since you are using the engine’s output energy to heat up the fluid and not move the car, you lose power.
In principle, once the torque converter locks, and you are moving above 40 mph, there should not be too many losses from the torque converter. When we are on a dyno, we are always above 40 mph, so why then does the automatic still lose power? There are a few answers. The first is that at wide open throttle (WOT), which is the standard dyno setup, there is usually no lockup because the lockup clutches can not take the full engine torque. The second is that chassis dyno tests are conducted with the engine accelerating, while engine dyno tests are conducted with the engine held at a steady rpm. There is a loss associated with accelerating any spinning mass, called the inertial loss, and it is usually higher with an automatic than with a manual trans, mainly because the torque converter and the fluid inside it have a higher inertia than a manual trans clutch and flywheel. The third is the rest of the transmission. The transmission usually has four other clutches (actuated by hydraulic fluid), valves, the governor, pumps, pistons, and other stages that add complexity. All of these devices and stages steal power from the car. Since nothing is 100% efficient, anything you add will reduce your overall efficiency. In a standard you have the clutch and the transmission which only consists of gear control because the driver makes the decisions of when to shift. In the automatic, you need to add the equipment to do the decision making. A typical performance standard transmission loses about 15-18% of the engine’s power. An automatic loses 18-22%.
Thanks to Robert Smithson for corrections and additions to this article.