Project Fiesta ST, Improving the Drivetrain with Mountune USA and SPEC Clutch

Project Fiesta ST, Improving the Drivetrain with mountune USA and SPEC Clutch

by Mike Kojima

If you have been following our series in the transformation of our Fiesta ST from a street car to a race car, we have addressed a lot of the basics involved from getting excess weight within the rules out to building a solid cage, fuel system and safety systems.

In our prior driving of Project Fiesta ST on the track we noted that a lack of limited slip differential was a serious issue.  The car wanted to spin its inside front wheel on corner exit but the electronic nannys, namely traction control, stability control and electronic torque vectoring would kick in.

With the stock power levels and tires, this would not be so bad but with stiffer suspension, much higher cornering speeds and more power, the electronics were overwhelmed and the car would do something like a terminal shuttering understeer, not conducive to fast lap times.

To get rid of the electronics related issues, we unplugged the SRS module which the ECU uses for yaw detection to work with the vehicle dynamics controls.  Since we no longer have airbags this is not an issue!  It does throw a bunch of error codes but that’s not really an issue in a race car either.

This still leaves us with the issue of corner exit traction and being able to use all of that Ford turbo power.  To deal with our traction issues we called on mountune USA for assistance and they provided us with their limited slip differential.

We also got some help in the drivetrain department from Spec Clutches with a lightweight disc, heavy duty clutch and lightweight aluminum flywheel.


The mountune limited slip diff is made for them by Quaife.  The Quaife is what’s known as a torque biasing differential. Instead of using preloaded clutch plates or a combination of preloaded plates and cross shaft wedging action to provide locking, a Quaife uses helical gears in the diff and a pair of worm gears that attach to each axle to control torque bias.  The gears work on the principal that the worm gears can turn the helical gears but the helical gears cannot turn the worm gears. When there is equal traction the ring gear spins the differential case which the helical gears sit sideways in. They are effectively locked to the worm gears since they cannot turn them and thus the drive torque is put to the ground.  When differential action is needed, it can happen because the worm gears can turn the helical gears.  If one wheel hits a low traction condition and the speed differential between the two wheels becomes large, the worm gear puts a thrust load on the helical gear, causing it to push a clutch plate into the side the differential case.  There are six helical gears for each axle so the clamp force can be pretty large.  Typically the torque bias a Quaife diff can generate is 5:1 which means the drive wheel can get up to 5 times the torque of the slipping wheel.
The main disadvantage of a Quaife diff is that 5 times zero is still zero so if one wheel lifts off the ground while torque biasing is occurring, it will act like a conventional open differential. In this case it is very important for the the suspension to be tuned to not allow the drive wheels to lift in cornering! Due to the nature of its action the Quaife works smoothly and seamlessly without a set preload or initial breakaway torque that can hamper turn in.  This makes it an excellent candidate for a car whose drive wheels must also steer.  We had the mountune diff for months but were waiting for the differential side bearings to become available from Ford who had them on eternal backorder. Fortunately for us, mountune had them in stock, the only people in North America to have them at the moment!  We also had to order and replace the ring gear bolts as we found out that they are one use torque to yield parts.
We also selected a lightweight billet aluminum flywheel and a heavy duty Superclamp stage 4 clutch from Spec Clutches.
Our Spec flywheel is a lightweight part, CNC machined from aircraft aluminum billet to a tolerance of 0.001″.  The flywheel has a steel friction face that is replaceable, important when using abrasive full metallic racing clutch discs. Currently Spec is the only company that makes a lightweight flywheel for the Fiesta ST.

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