Passion for performance

Jan. 1, 2020
The modifications involved in squeezing out everything a car or truck can put out often are extensive and expensive, but the attitude is “price is no object, whatever it takes.”

No doubt there are many who have a passion for performance; some for speed, some for horsepower and some for both. The modifications involved in squeezing out everything a car or truck can put out often are extensive and expensive, but the attitude is “price is no object, whatever it takes.” With such passion for performance, it might come as a surprise to learn how often an automatic transmission is left out of the horsepower upgrade equation. It doesn’t take too many engine performance enhancements to go beyond the intended torque capacity of the transmission. Something as simple as installing an aftermarket tuner into the Engine Control Module/Powertrain Control Module (ECM/PCM) can be enough to start transmission damage.

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Figure 1

I know in February’s article, “Dodge Diesel Transmission Dilemmas,” I concluded with discussing a problem that arises in an AS68RC transmission after installing an aftermarket tuning program into the ECM. The adaptive program in the ECM used to monitor and adapt shift control of the transmission would be rendered ineffective by the installed program, which over a period of time causes the transmission to self-destruct.

This is not the type of transmission failure I am referring to, as this is a unique problem where the tuner program is not cohabiting well with a computer strategy designed to operate an Asian transmission. But, this same tuner program cohabits well in computers Dodge uses to operate its transmissions.

Take for example Dodge diesels equipped with either a 47RE or a 48RE transmission. The tuner in these computers indeed unleashes the engine’s hidden potential power held back by OE programmers for emissions and fuel control restraints. Over time, this increased power can loosen the turbine shaft from the rear clutch drum housing compromising forward clutch pressure. This, in turn, causes forward clutch failure resulting in either a slip or loss of all forward gears.

Figure 2

Figure 1 shows extreme failure of this shaft, which resulted in a complete loss of drive and reverse. As you can see, the shaft snapped as if it were made of peanut brittle. This type of failure does not occur often, as it is the result of a combination of issues. Dodge diesel vehicles are prone to have issues causing the converter clutch to rhythmically cycle on and off. They also had a run of turbine shafts that were over-hardened making them fragile. Put this together with a tuned engine, and you now have a shaft being hammered on and off with TCC cycle problems accompanied by increased torque. The result is snap-crackle and pop! Typically, however, the shaft does not snap, so all that pounding is transmitted through the shaft causing it to loosen in the drum housing.

Another side effect that can occur after tuning a Dodge computer for increased power is catastrophic damage to the torque converter. An OE torque converter usually has excessive tolerance levels making it very loose internally. Second, there is a very high speed difference between the turbine and the impeller at the time the clutch is applied. Thirdly, there are dampener plate springs which load up to more than 2,000 pounds of tension when the clutch is applied during pulling conditions (Figure 2).  

Figure 3

When de-accelerating, the energy from the springs releases in the opposite direction of engine rotation. Incorporate these conditions with perhaps the use of a Jake brake, engine speed trying to go to idle, converter clutch releasing and the weight of the vehicle, a momentary tie up (torque reversal) occurs with all the stress being transferred to the converter bolts (Figure 3). The pads attached to the cover slightly flex dimpling the internal clutch surface (Figure 4). With the clutch surface slightly dimpled, apply pressure drops as it leaks past the clutch into the exhausting release circuit.

All of these combinations work together resulting in catastrophic failure of the converter clutch. Fragmented material now finds its way into the valve body and cooler. The cooler eventually clogs causing a loss of lubrication to the gear train which results in complete planetary failure. If things go your way, the flex plate will break first causing a no move preventing total transmission failure.

Just one simple modification to increase horsepower can lead to unpleasant effects on the transmission. Yet in most cases, other modifications usually are made as well, quickening its demise. Typically, most automatic transmissions are designed to handle original factory requirements only. Though the transmission is out of sight, it cannot be out of mind when horsepower modifications are being made. It might not be pleasant to drop a couple of Gs into a part of the vehicle no one can see, but in the example used in this article, at bare minimum, a customized converter is definitely required.

Figure 4

One that takes the internal tolerance down to a minimum with changes made to the fluid coupling characteristics to lessen the high-speed differential and torque reversal stress on the pads. Solid steel billet torque converters with multi-clutch disk arrangements do just that, and there are plenty out on the market. Billet made turbine and output shafts as well as modified valve bodies that increase line pressure and converter clutch apply pressure during peak load conditions are all available to accommodate upgrades.

You cannot afford to shortcut these modifications with two fingers crossed behind your back. This I promise you does not work. But when you have a passion for performance, cost is no object, whatever it takes.

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