Shift Change – Automatic Transmission Cooler R&D, Part 1 – Stock Review and Design Plans

There’s just something special about sitting basically on the pavement in your sports coupe, gear lever in one hand, rev-matching 100% of the time simply because you have the option. For us automotive purists, that is the dream, which is why it seems almost blasphemous to resurrect one of the most legendary names in the sports car segment without the opportunity for us to row through the gears manually.

In the A90 Supra, Toyota and BMW decided that we humans wouldn’t be able to manually shift as efficiently as the Supra needed so that responsibility is up to the ZF 8HP automatic transmission. If we’re honest, it almost makes you forget that you’re missing the 3rd pedal at your feet. Lightning-fast changes in the planetary gear sets know what you’re thinking quicker than you do, and add a degree of smoothness that is typically sacrificed in a dual-clutch. However, as advanced as ZF’s automatic gearbox is, it can still fall victim to an age-old issue – heat.

Even with its superb performance, it’s still hard not to yearn for a traditional 6-speed manual.

Vehicles with automatic transmissions usually are sold with the notion that the fluid which lubricates the gears is designed for the lifetime of the car, and outlive humankind. Sure, for regular, civilized commuter driving, the health of your automatic transmission fluid will last. However, we’re talking about the Supra here. A car that was dreamt up by enthusiasts for folks that want to go fast, which is where the fluid’s lifespan could suffer.

Generally speaking, your automatic transmission operating at its peak at around 180°F. The fluid has warmed and is at its correct viscosity for proper lubrication while minimizing friction by creating a slick barrier between the plethora of moving parts. Once you put the hammer down, and those gear sets and clutches start working harder, your ATF temp is going to spike. Extreme heat can cause the fluid to thin, meaning more friction, and even cause the fluid to burn, which is less than ideal for keeping the gears meshing smoothly, hence adding an external cooler to the system.

Toyota/BMW already knew the importance of keeping the fluid within it’s operating temperature when they placed this cooler on the Supra. It’s a sizable core, measuring in at 19 x 6 x 0.75″ with a volume of 85.5 cubic inches. This is more than enough to keep the ATF cool during a regular commute, but sportier applications would require more cooling capacity. They also made the conscious decision to mount this cooler at the front of the cooling stack, giving it first dibs to the fresh air and further increasing its cooling capacity. Even given its stature from the factory, those looking to extract every ounce of performance from their Supra might require even more cooling power for the ZF.

BMW opted for a full-aluminum construction making for a more sturdy transmission cooler straight out of the box.
This cooler has a large amount of surface area, which is beneficial for heat dissipation, but it lacks in thickness.

When it comes to our vision of cooling the Supra’s transmission, it almost goes without saying that bigger is better, and given the prime real estate that the OEMs gave the transmission cooler, we have plenty of room to expand. Even with that amount of space, we don’t want to overdo it. Expanding this cooler too far could result in an excessive fluid pressure drop, and run the risk of overcooling the fluid, and cause a detriment in the performance of the transmission.

For that reason, Jason extracted our stock unit for a closer examination of our new design parameters. Through a series of scans and measurements, he was able to determine the perfect size of our new core and create a fitment prototype. This framework serves as a tool for any real-world design adjustments that would be needed, and can physically resemble our planned enhancements. We’re planning on putting most of our improvements into the cooler’s core, especially by almost doubling the core thickness, as well as expanding the other overall dimensions, for a more efficient means of keeping the transmission fluid cool under extreme conditions.

We intend to retain a few aspects from the stock design, for the sake of simplified installation. For instance, it’s pretty much a guaranteed fact that if BMW designed something, quick disconnect fittings will be used. So, it’s no surprise that the lines to and from the transmission cooler are just that. Since these are hardlines, and also don’t need an overhaul, we’re utilizing the same connection style on our inlet and outlet ports. Also, the stock cooler features all-aluminum construction, and we saw no reason to change that.

Even though the manual option is sorely missed, the ZF 8HP almost fills the empty hole in we enthusiasts’ hearts. Still, with such a potent piece of mechanical technology, the lubricating fluids must stay cool to keep the Supra operating at peak potential. Make sure that you stay tuned for the first look at our new cooler and how it stacks up against the stock unit.  

Thanks for Reading!

-Nick

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