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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Monday, February 29, 2016 5:14 AM
These clutches are unbreakable, you will kill the gearbox before you need to replace it. Good choice :)
Monday, February 29, 2016 5:57 AM
Damn, Technosquare's floor is crowded. Howard must be a busy boy. Looking forward to seeing the Fiesta on the track!
Monday, February 29, 2016 7:41 AM
The stock flywheel is called a Dual mass flywheel, they don't need a sprung hub clutch to opperate, as you said, But I hate how they feel!

Great article!
Mike Kojima
Mike Kojimalink
Monday, February 29, 2016 10:38 AM
It's not exactly a dual mass, the center hub only is sprung, it does not have any significant weight. Dual mass flywheels have a part with significant mass sprung so they act as an NVH damper.
Monday, February 29, 2016 6:47 PM
My experience with a Spec clutch wasn't exactly like that. I had a Stage 2+ on a 350whp Evo X for about 40k. The guy I sold it to had it replaced shortly after and said the clutch looked horrible, not just worn, looked like the friction materials broken up. It was good for a while, but it started feeling weird near the end of its life. It could be specifically the 2+ disk, its the hybrid one with different friction materials on each side. But overall I liked it. The one my my coworkers WRX (Stage 1) is horrible though. The pedal effort isn't even throughout the whole stroke, its relatively light through most of it and then gets really heavy right around the area where the clutch engages. I don't know if there's a worst characteristic of a clutch than that. I feel kinda bad about it too cause I was the one that recommended Spec to him cause I liked it on my Evo.
Mike Kojima
Mike Kojimalink
Monday, February 29, 2016 7:45 PM
Dual discs don't last as long before they slip in street use as single disc's because the stack height goes down faster with normal wear and you loose initial clamp faster. It's one of the drawbacks of multi disc clutches when used on high mileage street cars. As far as getting stiff at the end of the stroke, it's a normal function of a diaphragm type clutch and you could have your pedal adjusted incorrectly and pushing the pressure plate to near the overcentering point.
Tuesday, March 01, 2016 12:39 AM
I guess its possible the clutch pedal is adjusted incorrectly. The same shop did the install on both cars, and they specialize in Subies, so you'd think they'd get it right on the WRX.
Now that I reread my post, it really sounds like I'm hating on Spec. I didn't mean it to sound like that, I was just sharing my experience. The lesson I learned here is that I'll never buy an aftermarket clutch without driving it on the car I'm buying it for.
Mike Kojima
Mike Kojimalink
Tuesday, March 01, 2016 10:37 AM
It's possible that they are familiar with the car but every aftermarket clutch is different. Even different disc types with the same pressure plate can be different.
Tuesday, March 01, 2016 11:56 AM
The WRX uses a stamped steel release arm that wears pretty quickly via the pivot ball. Many people don't take much notice during a clutch install and disregard it. This leaves the geometry out of whack as the clutch wears, causing all sorts of irregularities. Mostly, the clutch pedal feels terrible and far heavier than it should. I always recommend installing a new arm and pivot ball when doing a WRX clutch job. For maybe $25, the clutch will feel about 300% better.
Tuesday, March 01, 2016 12:29 PM
if its the release arm, wouldn't it get worse over time? its been like this since the install and he has about 30-40k on the clutch now and its still the same. just to be clear, I'm not saying its not the release arm, just being thorough.

How much would the labor be to replace the release arm (time wise, ballpark)? Owner of the car isn't mechanically inclined, there's no way he'd be able to do this himself.

Monday, March 14, 2016 10:48 AM
Hey Mike,
Another great article.
2 questions. What was the blue line along the mating surface of the housing?
Also, when will you do a nice detailed write up on the k20 transmission rebuild?
Here's me hoping...
Mike Kojima
Mike Kojimalink
Monday, March 14, 2016 1:06 PM
The blue stuff is sealer, you can see Howard applying it. We are going to get a 5 speed K tranny and convert it to six speed and install an LSD on our EP3 series coming up.
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