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Diff’s Hot, Take it Easy – 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
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
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
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 subframe
Focus RS rear subframe
Focus RS rear differential
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

6 thoughts on “Diff’s Hot, Take it Easy – Part 1: The Rear Differential”

    1. There are several sensors going into the differential, but we are not confident that these are temperature sensors. We are still looking into how the temperature is read. It is very possible that the temperature reading is calculated, similar to engine oil temp. Either way, we are in the process of obtaining more information before we make a conclusion.

      -Sara

    1. Great questions! At this point we are looking at a fluid cooler. We do not have a completed prototype laid out yet, but we are most certainly going to account for and eliminate any starvation issues if we proceed with a fluid cooler. We will share our prototype as soon as it is complete here on the blog, so check back for more details.

      -Sara

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