I am in the middle of a transmission rebuild and thought to myself, "Hey, maybe some others out there in Oppoland will find some of this interesting or useful". So here goes!
Automatic transmissions are an engineering marvel really. Even to the most savvy mechanic; the complexity of an auto transmission and how it operates can be hard to get a grasp on. I am NOT going to explain how one works. There are plenty of resources on the web that can do a better job than I can at that. But I will throw out some information that anyone can understand regarding what it takes to be able to handle more horsepower through your transmission.
To make sure you can follow me, I will throw out a few basic explanations so even if you know ZERO about a transmission this will make sense to you. Your flywheel, or called a flexplate on an automatic transmission, is spun by our crankshaft. The flexplate is connected to a torque converter. Think of the torque converter as the clutch in a manual transmission. It is what couples the motor to the transmission. Technically, its a fluid coupler. It uses fluid pressure to engage or slip. The torque converter is connected to the transmission via the input shaft. Inside is where all the magical unicorns live.
Pretty basic stuff so far. Im sure you all knew that already. So what then, can be improved upon to make a transmission handle more power than it is configured in its OEM state? There are 3 basic areas that really comprise the majority of how much a transmission can handle. And its way more easy to grasp than you would think.
1. Clutches. Inside the transmission reside many clutches and "steels" inside drums. They function like a "clutch" in a manual transmission in that their job is to make contact with the next component and not slip from that position of initial impact. Just like a manual clutch, automatic clutches CAN and do slip. As you can imagine, this is bad, just like it is for a manual transmission. It can take as little as a few good slips for the clutch material to burn off and its done for good. How well a clutch "grabs" is based on two things. Surface area and fluid pressure.
Lets talk about surface area for a moment, as this is probably the more important issue at hand. In simple terms, the more surface area, the more power the transmission can handle. Think of jumping off a cliff into a lake. What is going to bring you to a stop faster? A belly flop or a dive? A belly flop, of course you silly person. Likewise, greater surface area on a clutch allows it to lock faster/harder/more instant and therefor no slipping. The same principal of upgrading to larger brake rotors. Bigger rotors let you use bigger pads for increased surface area which will slow you down faster. So lets say for a moment your transmission has 3 clutches in the drum that is associated with 3rd gear. If you were to add one more clutch, you have just increase the torque capacity by 33%. In some applications, a transmission uses clutches with material on just one side of the clutch. Others use dual sided clutches. Lets go back to the example of 3 clutches in that drum, and they are single sided. Switching over to 3 dual sided clutches, you just doubled the torque capacity of that transmission. So one of the most effective ways to increase the power handling of your transmission is to increase clutch surface area through using dual sided clutches, adding additional clutches, or a combination of both.
2. Pressure. Automatic transmissions work off of, well, as you may have guessed, transmission fluid. Just like your brakes or power steering, fluid is compressed so that it can be utilized to produce extreme force on a component. Think of simple brakes on your bicycle. The harder you squeeze the hand brake, the harder the brakes grip. Conversely, the lighter you squeeze it, the less it grips the wheel. Not enough pressure causes the clutches we just discussed to slip when they engage which leads to burning out the clutches. With so many transmissions in existence, I can't just offer a blanket suggestion on what to do here. But the bottom line is, you need to increase fluid pressure. This is usually done within the valve body. The valve body is sort of a complex electrical and mechanical component that you can think of as the heart. It routes fluid to various parts of the transmission depending on what condition the transmission is in, i.e. up-shifting at part throttle, down shifting, up shifting at full throttle, etc. The valve body has lots of little chambers in it with various little mechanical springs and valve that help route the fluid to where it needs to go. It also houses shift solenoids which is what controls the fluid pressure going to the clutches. There are often upgrade solenoids as well as valve bodies that are available that have been modified to increase line pressure going to the various circuits of the automatic transmission system.
3. And lastly, one of the more simple yet important components needed for higher horsepower is your input shaft. To put it simply, they are basically the "driveshaft" for the transmission. The converter attaches to the input shaft which turns all the magic unicorns inside. And out the other side is the output shaft which connects to your driveshaft or CV axles.
There really isn't anything special about them nor complex. The biggest issue is that they have to be able to handle the torque without snapping. I venture to say, the majority of the time, stock input shafts are capable of handing the power in most typical builds. Unless you are getting in to super high horsepower levels, its likely not something you need to worry about. Chances are your transmission will slip before it breaks an input shaft.
To lump it all up here in one pile: You want your transmission to handle more power? Look into increasing the surface area of your clutches, increasing line pressure, and if you go real extreme, a stronger input shaft. Then all you have to worry about is upgrading your torque converter, driveshaft, rear differential (RWD) and axles (IRS) so THEY don't snap.