What Goes Into Your Aftermarket

Published in the December 2008 Issue December 2008 Feature

It takes more air and more fuel to make more horsepower in a diesel. But something people overlook when improving their trucks' performance is upgrading that factory `slushbox' that's bolted up behind the motor. Sturdy, reliable, transmission upgrades are a very important part of planting all that new-found power to the ground. But what actually goes into improving the factory transmission? With the help of the guys at ATS Diesel, here's a little behind-the-scenes look at what it takes to build a tranny you can put through its passes.


In most factory transmissions the weakest link is the factory torque converter. While they are built plenty sturdy enough to handle the factory output of your truck, most fall short after the baseline horsepower and torque output have been increased. The factory designs are most commonly improved in generally the same way. Adding a billet cover, increasing disc surface, and using stronger, more reliable materials are among the most common.

ATS Diesel's Five Star Torque Converter with what they call an innovative fluid-assist technology further enhances power transfer through the transmission and to the ground. Using a five-disc design, the ATS converter can provide plenty of clamping force during full lock-up. The improved efficiency equates to lower fluid temperatures, better fuel economy, and a maximum torque multiplication.


The transmission valve body looks to be the most technical piece in a transmission, and for good reason. With multiple springs, fluid valleys, and small moving parts, an improperly set-up valve body could quickly lead to a transmission's early demise. An aftermarket valve body can improve shift strategies, shift firmness, and extend the life of your gear clutch packs. The upgraded valve body with new valve designs, stiffer springs, and worked-over oil circuits will improve clutch pressure. Again, all this equates to cooler transmission temperatures and improved transmission performance. The valve body plays a very important role in the performance of your transmission.


Inside your transmission, each specific gear you run through on a daily basis is composed of separate clutches. Much like the OEM torque converter in your transmission, the stock gear clutches are usually more than enough for the stock horsepower and torque output of your truck. But as more and more power is sent through your transmission, clutch slippage can occur; slippage creates heat, and heat in the end is what can kill your transmission prematurely.

The aftermarket combats these slippage issues with improved clutch designs. Factory materials are improved along with the number of clutch surfaces each gear will use. The stronger biting material and added clutch surface will create a stronger bond that can eliminate slippage as power is sent through the transmission.


The next area that needs to be looked at when upgrading your transmission to withstand the added power from your tuning software and injector upgrades is the internal shafts. The input, intermediate, and output shafts work your gear drums and clutch packs to send power all the way from the front of your transmission and torque converter out through the back to your transfer case or rear driveline. Most often the input shaft is the weakest link and is the most commonly upgraded shaft.

An aftermarket torque converter like the Five Star offered by ATS Diesel can apply a much stronger torque multiplication into the transmission, and it is the input shaft that helps to transfer this power. So when upgrading to an aftermarket converter, upgrading your stock input shaft should be looked into as well, especially if you have plans of towing heavy or drag racing your pick up. The added shock load the drive train and transmission must uphold under those harsher conditions can really put a strain on the factory shafts.

The output shaft in a factory transmission must also endure some pretty intense strain and power when pulling heavy or when the tires are really fighting for traction. The output shaft is the final link in the transmission and connects to your transfer case or driveline to send power to the drivelines. Most splined factory shafts hold up exceptionally well when you consider the load the small diameter steel parts are constantly subjected too. But everything has a breaking point and most often the section of the shaft where the splines end is the weak area. To combat this, ATS Diesel now builds their output shafts with a machined locking tab that will key and match up perfectly to the transfer case shaft, decreasing the load on the actual splines of the output shaft. ATS machines these shafts to work in conjunction with each other one set at a time, to ensure the perfect fit.

This story just barely scrapes the surface of what actually goes into building a tough reliable transmission that can withstand the torque and power these diesel motors can make. Thanks to companies like ATS Diesel building performance aftermarket transmission systems, we can continue pushing the limits with these trucks.

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