Diff's Hot, Take it Easy - Rear Differential Cooler R&D, Part 4: A Tale of Two Track Days Part 1

The term "track day" around here is always surrounded with a certain level of enthusiasm. We're all aware of the extra labor that goes into a trip to Raceway Park in Englishtown, NJ, but everyone involved is more than willing to pitch in for the chance to see our RS thrashed around the track. Think of it like one of those field trips you would take in middle school. Sure, there would be a test on everything seen that day, but that was a small price to pay for a day out of the classroom. After spending almost a week preparing it was time to take the class on their field trip.

Hot Oil and Broken Eggs

Our other product coordinator Steve Bryson took the journey up to the track as the perfect opportunity to snap some roller shots.

Like any of the previous trips to the test track, we started at sunrise, meeting at HQ in New Castle, DE before shipping up the NJ turnpike. Even though our team spent over a week preparing for the day in Englishtown, there was still plenty to do once we arrived before the RS could hit the track. It was all hands on deck to set up our pit area, do a final shake down of the car, and most importantly, ensure that our course was free and clear of any debris that could cut the day short.

Raceway Park might not be the biggest, or closest road course in the area, but what it does offer is the perfect environment for heating up all the systems, especially the differential units on the RS. Just looking at the aerial layout of the track's outer loop, it's almost as if it was specifically designed to torture the RDU. With only one straight that passes by in an instant, the differentials, suspension and brakes get put through the gauntlet of switchback turns, elevation changes, and long sweeping bends.

A suited-up Eric Plebani gets ready to set off for a round of testing to the RS's AWD system.

With the pit and testing gear ready to go, we put the RS at the hands of our Director of Innovation and seasoned race-team driver Eric Plebani with the goal to put the RDU into thermal protection. If you recall from our previous posts, the temperature of the rear differential is inferred by way of the gear oil temperature sensor in the PTU. Through our preliminary testing, we found that the magic number for thermal protection is between 290°F and 310°F, calculating to an inferred RDU temperature of 220°F. During the stock comparison test, Eric reached that temperature in a matter of 9 laps, or about 10 minutes of driving.

With our armory of data logging equipment, we were able to single out the oil temperatures for the stock equipment. Without extra cooling the PTU plateaued right at about 260 degrees, with the RDU not far behind.

Eric's driving skills are complimented with the ability to relay the RS's mechanical and technical activity back to Dan in the pit. The collaborative effort allows for a real time technical diagnostic gateway of communication for our development team to get the whole picture whether the car is crushing the tarmac or in the pit.

Wanting to extend that lap count, our team got to work installing the differential cooler on the RS before setting off once again. With our full PTU and RDU cooling system on the car, we were able to extend the Focus' time on the track by about 3 minutes. Even by running external coolers on the gear oil for both units, we weren't able to ward off the strict mathematical calculation hell-bent on putting a damper on track days. We all know how the saying goes, "You have to break a few eggs to make an omelet." We broke the eggs, but at the end of the day, it looked like we ended up just scrambling them instead of going for the full omelet.

After installing the cooling system, we were able to drop the RDU temperature to under 200 degrees, which is far from the danger-zone that would require thermal protection. We would need to come up with a more intricate solution to ward off the RDU shutdown.

Change of View

With such a complicated and cutting edge AWD system equipped on the Mk.III RS, Ford also included state of the art programming to go along with it. While our engineers are masters at their craft, re-coding the fortified ECU in the RS to remove or alter the RDU safeguards is much easier said than done. That being said, both Eric and Dan returned to our R&D facility hell-bent on finding an alternate solution.

For as many times the RS has been to the track, it's been stripped down twice as many times in the name of science and speed.

The main roadblock in the path of a more consistent all-wheel-drive system in the Focus RS is the close-knit relationship between the differential units and the ECU. Ideally, the solution would be to break up that relationship by way of re-tuning the Focus' computer. This solution is tricky, and the premise behind creating this kit is to keep the power flowing through the rear wheels without shredding the clutch discs.

There is no temperature sensor mounted on the RDU of the RS, but rather the ECU infers the temperature from the slipping clutch discs. With the many different inputs that actually determine the RDU's temperature, the three main variables in the equation are the PTU's gear oil temperature sensor, the throttle position, or how hard you're mashing the pedal, and the wheel speed. In reality, the actual temperature is generally lower than the inferred calculation. It's clear to see that Ford wanted to play it safe and keep the inferred temperature higher to avoid the risk of replacing RDU units down the line.

Since it's hard to drive conservatively on the track, and the wheel speed is determined by how much lead is in your foot, Dan and Eric figured that the temperature would have to change. The best way to go about changing the ECU's point of reference is by transplanting the PTU's gear oil temperature sensor, since it's the main variable in the RDU's heat calculation that isn't dependant on the driver.

Pulling daily driving data is just as important to the process, since not every RS lives at the track.

What the pair discovered was that they could relocate the oil temperature sensor in line with the PTU's coolant supply after it passed through the stock PTU heat exchanger, giving the ECU a temperature reading of around 215°F rather than climbing to the 260°F range, shutting off the RDU, all while following the same flow as the original reading. Think of it like a dyno graph. We want the new reading to follow the same curve as the stock reading, just lower on the graph. This is the method Ford would use to recalibrate the sensor, but we just used a different technique.

With the PTU temperature sensor transplanted with this housing, Dan hopes to finally resolve the chronic RDU shut-off.

Simply put, we're tricking the ECU, changing its point of view, so to speak. However, we did our homework to ensure that it was more than just a ruse. During our preliminary bench testing of this new configuration, we still ran our own temperature probe into the gear oil of the PTU and on the RDU to ensure that even with transplanted temperature sensor, we were keeping the finicky RDU safe. It also goes without saying that it's highly recommended not to relocate the PTU temperature sensor without also properly cooling the differential units.

With a new configuration sorted, and a solution in sight, Dan and the rest of the engineering team work on prepping the RS for another trip to Raceway Park for its final round of testing. In the meantime, one of our drivers gave one of our PTU cooling prototypes some real world torture testing, not in the depths of the Sahara, but rather flying through the woods of New England. More to come soon!

Thanks for Reading!

-Nick