• May 22nd, 2018
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Trials and tribulations of the 68RFE. Part Two: QT100 and the Big Stack
BD's approach to this challenging transmission, continued...

There are two fundamental ways to increase a transmissions clutch holding capacity; increase the hydraulic apply pressure or increase the clutch capacity. Part one of this series showed the improvements done to the valve body which allow us to run high hydraulic line pressure. Here we will discuss some of trials and tribulations on the physical parts when adding more clutches and the effects on those parts due to increased line pressure.

QT100 Separator Plate and Reaction Plate

Figure 1: 68RFE cross section

We will focus on the clutch pack that is the most prone to premature wear and failure, the overdrive clutch pack. The Overdrive (OD) clutch pack, which is located in the input drum is the perfect candidate for an increased number of clutches. The most common way to make these fit would be to shave down the OEM metal reaction plates that directly apply the clutches. You can get away with this on some transmissions, however the 68RFE is another story.

Figure 2: Allison reaction plate on the left vs a factory 68RFE reaction plate on the right

The factory OD/UD and OD/Reverse reaction plates are made of inexpensive powdered metal. These plates were designed to handle stock pressure at full thickness. While they do perform okay with increased pressure at full thickness, when they get machined down to fit more clutches they bend significantly - so much in fact that it will only contact half of the friction surface it is trying to apply.

Figure 3: Worn OD clutch due to "coning" of the reaction plate

This problem gets exaggerated significantly when these plates are machined down to add more clutches. If the factory separator plate is machined, it can bend enough to start applying the adjacent clutch pack causing premature wear. Figure 5 shows how much the reaction plates bend at full line pressure. You can see the machined plate bends significantly more.

Figure 4: Measuring deflection within the input drum

QT100 Reaction plate Vs OEM

To solve this problem, we investigated different possible materials and found a high strength steel known as QT100. We were able to reduce plate deflection down to a level lower than stock even with increased line pressure. We have MORE clutches and LESS deflection. Every BD 68RFE transmission and all build-it-kits come with these two billet-machined QT100 separator plates.

Overdrive/ Reverse Reaction Plate
Outside Deflection Additional Inside Edge Deflection (Coning)
Inches % Inches %
Stock 0.023 0% 0.012 0%
Machined Stock 0.032 39% 0.023 92%
BD (QT100) 0.016 -30% 0.008 -33%
Overdrive/ Underdrive Reaction Plate
Outside Deflection Additional Inside Edge Deflection (Coning)
Inches % Inches %
Stock 0.0255 0% 0.007 0%
Machined Stock 0.0275 8% 0.009 29%
BD (QT100) 0.0245 -4% 0.0015 -79%

* All test done at 250psi line pressure
Figure 5: Reaction plate deflection

Figure 6: BD billet reaction plate OD/UD Vs a stock OD/UD reaction plate

Big Stack OD Shaft

There is a limit to the number of clutches one can add to the input drum, this is constrained by the space between the separator plates and the length of the fully splined section of the shaft. In the 68RFE, the billet reaction plates allow us to add more clutches, but to go any further we would run out of spline engagement on the overdrive shaft. The clutches would no longer engage with the shaft correctly and could slip off causing failure. As such we came up with a novel solution that keeps the stock input drum, removes failure modes, and can handle as much torque as a large diameter input drum without the need to increase the hydraulic apply area.

Figure 7: Stock OD shaft spline taper beside a big stack of clutches

The BD "Big Stack" consists of a few parts, the heart of it is the overdrive shaft. To increase the spline length on the shaft we start with brand new OD shaft and machine off the clutch spline section. We then billet machine a longer spline and weld it onto the shaft.

Figure 8: Big Stack OD shaft spline Vs the stock OD shaft tapered spline

The challenge with an increased clutch count on the shaft is how to physically fit them within the drum. Even with the stronger, more rigid QT100 separator plates we did not want to make them any thinner. In order to fit the increased OD clutch count, we needed to also offset the OD/Rev reaction plate. By machining a new reaction plate and offsetting it, we were able to use a different reverse clutch pack and use that space in the overdrives.

Figure 9: Overdrive/Reverse reaction plate offset needed to fit more clutches

In the end you get 16 overdrive clutches with the BD big stack, compare that to our previous builds that utilized 14 overdrive clutches or stock that used a measly 12. Keep in mind these are full thickness steel and clutches, not thinner ones.

Both the thermal mass and holding capacity are increased by 33% over stock. This improves heat dissipation between shifts as well as total transmissible torque. The increased line pressure, alongside the increased clutch count gives you a 110% (or 2.1x) increase in torque holding capacity in the OD clutch pack, all while still using the stock input drum.