OEM cars are usually built with compromises leaning towards "street manners", not meant to be "beat on" and with "comfortable" shifts. But there are different "duty" levels of transmissions. (i.e.: 904, 998, 999) I've always heard that heat is the most common cause of transmission failures, which I believe applies mainly to street & dual purpose applications. I was fortunate enough to get a '72 340 Challenger as my first car. Not sure if it was totally stock or just slightly modified at the time, but I went through three stock HD 727's in a year because I abused it on the street and just didn't know any better. It wasn't parts breakage, it was mainly band & clutch slippage. The local trans shop finally put in a shift kit and it made an obvious difference. Why? Because quicker, firmer shifts reduced overlap and slippage which generate heat. But there are other considerations also. Firmer shifts from a shift kit or aftermarket valve body are harder on bands & clutches which aren't designed to be hit much harder. Those should be upgraded also. Trans fluid is also a factor. Many have used Type F trans fluid because it's supposed to help the bands & clutches grab & hold better than other fluids. Performance synthetic fluids can also helpful because synthetics tend to handle more heat before starting to break down. Then there are the usual things regardless of application like a good trans cooler, higher capacity pans and regular servicing. There are little miscellaneous mods that trans builders can do to increase the durability as well, just like tweaks to an engine - clearances, fluid pressure, opening or restricting oil flow, etc. A proper torque converter matched to how the vehicle is driven is also important because it generates the most heat. You want an efficient converter because the more flash/stall it has, the more heat is generated. "Regular", non-lock-up converters don't form a solid path of power to the rear wheels, so there is always slippage there. But the converter has to compliment the cam and rest of the combo too, so they all work together, in the same range. It's difficult to say how much HP something can handle. I think it mostly boils down to how long a transmission built to a certain degree with certain parts can live behind x-amount of torque or HP. No simple answer. I'm no trans expert, just trying to provide a better perspective. Advice from this forum and from reputable trans builders will help in finding the right build for your needs. I was a "dumb teen" too, but eventually learned a lot, often the hard way. But there is always something more to learn. I wish the internet and these forums were around then. You're on the right track.
