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Diff's Hot, Take it Easy - Mishimoto's Focus RS Rear Diff Cooler R&D

Mishimoto

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#1
Part 1: The Rear Differential

At Mishimoto we are big fans of our little blue car. Over the past few months you could say we have formed a bond that only burnouts and track days can build. In testing various Focus RS parts (and maybe doing a few donuts, for science) we noticed that it is very easy to make the rear differential in the RS angry, and you won’t like it when its angry. The rear diff makes the rules. Overheat it, and you’re likely to end up parked for the day. When the rear diff heats up, the ECU in the RS pulls power in an attempt to cool it down. Therein lies the problem: We have a car that begs to be driven to the absolute limits and rear diff that is programmed to shut down when it reaches a certain temperature. Don’t get us wrong, the problem doesn’t seem to be with the diff itself, but rather with its inability to dissipate heat fast enough to keep up with this blue bat out of hell.


Focus RS rear differential


Focus RS rear differential


Focus RS rear differential

As a peace offering, we decided to kick off R&D of a rear diff cooler and we hope it pleases this beast.

The Rear Differential

Given our cooling theory, we did what any other gearheads would do with access to a development facility such as ours: We took the entire subframe out to get a better look at the angry little ball of metal. The first step is often the hardest, and in this case it is no different. Dropping the rear subframe was no easy undertaking given that the driveshaft had to be disconnected in a very specific way to keep from damaging it. It’s safe to say that there is nothing straightforward about the design of this car, but that is what makes it interesting.


Mishimoto engineer scanning the Focus RS rear differential

Before we removed any part of the RS, we used our 3D scanner to get a better idea of the space we had to work with in the rear of this car. By collecting measurements this way, it saves a substantial amount of design time and often results in more accurate prototypes.


Focus RS with the rear subframe removed

With the subframe finally out, we could take a look at the design of the differential and how we could incorporate the cooler. Check out some shots of the subframe after we removed it from the RS!


Focus RS rear subframe


Focus RS rear subframe


Focus RS rear differential

Coming Up!

Now that we have a better look at the rear of the Focus RS, we can begin to create some prototype coolers. Stay tuned for more on our prototype designs, and in the meantime let us know what you think about this project in the comments below.

Thanks for reading!

-Sara
 


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Mishimoto

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Thread Starter #4
Hi everybody,

We’ve made some more progress on this kit and we’re chomping at the bit to share more information with you. We’ve spent countless hours street testing over the past several weeks trying to learn more about the relationship between the PTU, ECU, and RDU, as Diamaan mentioned in his last post. We’ve learned a lot from this street testing, but we’re finding that we can’t push the car hard enough on the street to match what you all are seeing at the track, without getting arrested anyway. So, we’re gearing up for another track day to push the RS and its PTU and RDU to the limit; but before we do, we want to get your thoughts, opinions, and feedback on what we’ve discovered so far.

The PTU and the RDU are a tight pair, we knew that already. What we needed to learn was at exactly what temperature the RDU is placed in thermal protection mode and how the PTU and ECU determine the RDU oil temperature, without a temp sensor directly in the RDU. Dan, the project engineer, fitted the PTU and the RDU with sensors and began monitoring their temperatures. After a lot of hard driving (definitely not a lot of donuts and drifting), we were able to determine that the PTU and RDU temperatures increase at the same rate. From this relationship, the AWD system can accurately infer the RDU gear-oil temperature from the PTU temperature, and place it in thermal protection when needed.



Armed with that knowledge, Dan was able to use a potentiometer in place of the PTU temp sensor and mimic its signals to the AWD system. Referencing the PTU temp sensor chart in Ford’s factory service manual, we found that the RDU’s clutch packs are disengaged when the PTU signals a temperature between 290°F and 310°F. This relates to an RDU temperature of about 220°F.



With the temperature at which the PTU and AWD system signal the RDU to disengage the clutch packs determined, we knew our goal for creating a cooling system. The RDU must remain under 220°F and the PTU must remain under 290°F in order to keep the RDU functioning and power transferring to the rear wheels.



Rear differential overheating issues are nothing new for the Focus RS. We knew that this issue would need to be dealt with eventually as soon as we brought the car into our R&D facility. If you’ve read our blog for the RS oil cooler, you know that we’ve already taken the RS to the track once before. But the oil cooler wasn’t the only part being tested that day; we were also testing ducting for a rear differential cooler. We wanted to test multiple options and make sure that we were able to build the best possible ducting that would get the future diff cooler the most possible air.



With the rear ducting tested and chosen we had to build the RDU cooler itself. As you all know, the RDU is an extremely complex unit. The RDU case houses a gear drive connected to the drive shaft that is constantly rotating and receiving power from the engine, whether the rear wheels are receiving power or not. Also within that case is a hydraulic pump that feeds two solenoids. These solenoids are what control the engagement of the clutch packs and thus the distribution of power to each rear wheel. Depending on the amount of pressure delivered to each clutch pack by the solenoids, the clutches are allowed to slip independently of each other. This slip controls how much power is directed to each wheel, it’s also where the heat that we’re trying to dissipate is generated.

Those clutch packs are filled with their own fluid and are considered non-serviceable parts. What that means for us is that there’s no way to tap into that fluid to cool it. Therefore, we must address the oil within the main RDU case that envelops the clutch packs and the gear drive. To cool this oil, Dan built a system that utilizes a thermostatic controller to run a Tilton differential pump based on a temperature sensor mounted in the RDU. The pump circulates gear oil through an external Mishimoto dual-pass, bar and plate, liquid-to-air oil cooler. That cooler receives air from the ducting developed earlier and channels it out through the rear valence. Aside from adding an external cooler, this system increases the effective fluid capacity of RDU.



So, we have a plan to cool the RDU and keep the clutch packs within it from overheating, but that won’t stop the PTU and AWD system from disengaging those clutch packs and robbing your rear wheels of the power they need. Without cooling the PTU, the cars computer systems will still think the RDU is overheating and shutdown the fun. So Dan began developing a cooler for the PTU itself. In stock form, the PTU is cooled by the engine coolant via a built in heat exchanger. To cool the PTU, Dan tapped into the coolant before it enters the PTU and added a transmission-style heat exchanger. On top of that cooler, he mounted an electric fan controlled by the thermostatic controller that activates the RDU fluid pump. When the RDU pump is powered, the electric PTU fan is engaged. All of this is mounted under the transmission and fed air from the ducting built into the stock splash shield. No cutting required.



Now comes the fun part. As you can see, we’ve put a ton of work into this project; it has been a challenging and complex project and we’re pretty proud of how far it’s come, but we need to know if you feel the same. With all its complexities, we’re looking at a price range of $2,800 to $3,000 for this entire kit; including a PTU cooler, RDU cooler, direct-fit bracketry, ducting, lines, fittings, oil pump, thermostatic controller and temperature sensor. Of course, as with all of our products our distributors may be able to get you a lower price and there may be a discounted pre-sale; but we wanted to hit you with the hard numbers first. While this kit is still a prototype, and we’re still testing and developing, we wanted to hear your thoughts. Let us know what you think and any feedback you have!

Thanks!

-Steve
 


GhostRS

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#5
Out of curiosity, how fast does this happen at a said track day?
Has it happened at every track day you've done?
 


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