Project Subaru WRX STi Part 4: Brake System Upgrades


Last in line for our brake upgrades we replaced our stock, stale never been changed before Japanese fish oil brake fluid with Motul RBF 600.  Motul’s RBF formula is one of the best fluids, performing nearly as well as or better than uber expensive exotic stuff like the legendary F-1 brake fluid standards Castrol SRF and AP650.  When brake fluid gets really hot it boils.  Boiling fluid has bubbles in it that are filled with compressible gas. The compressible gas causes the brake pedal to drop to the floor.  This is another type of brake fade that must be avoided.

Project Subaru WRX STI
The rear STB does not interfere with trunk storage.

Motul’s dry boiling point is an amazing 600 degrees F.  Motul works so well that our teams race cars need only bleed the brakes once a season, instead of the typical once a race.  Most brake fluids absorb moisture from the atmosphere.  When the brake fluid absorbs moisture its boiling point rapidly drops rendering it less effective.  Even though the brake fluid is contained within the brake system, it still can quickly absorb a surprising amount of moisture from the atmosphere that is inadvertently drawn into the brake system. Some brake fluids can deteriorate greatly from this phenomenon with as little as one heat cycle of the brake system. Motul resists absorbing moisture from the atmosphere better than most brake fluids so its inherently high boiling point lasts longer than most fluids.  If you do manage to boil Motul, surfactants in the formula cause the bubbles formed to rapidly dissipate and you get a recovery of your brake pedal feel.

Project Subaru WRX STI
The front STB is a pretty straight forward piece.  It made a feelable difference in steering response and smoothed out the ride.

The change to the feel of our modified brakes is confidence inspiring.  The brake pedal is now firm and high.  Modulation of the brakes is now much easier due to the improved pedal feel.  Under track conditions we are not observing any fade at all.  The only negative is a slight amount of brake squeal for the first few minutes of operation on the street when the pads are dead cold, an acceptable trade off for greatly improved fade resistance.  From our initial track test it appears as if our brake pad life is greatly improved as well.

Project Subaru WRX STI
The rear STB mount bolts to the rear shock mount and snakes around and underneath though this hole in the parcel shelf to bolt to the cross bar.  Despite its complicated bends, it is quite effective making a more feelable difference than the front bar.

Our last mod for this installment is the addition of Whiteline’s front and rear Strut Tower Braces.  Although the STi has a strong WRC inspired chassis that exhibits good rigidity to begin with, it like most cars responds to an increase in stiffness.  The Whiteline braces are made of aircraft aluminum alloy with brackets that are designed to place the brace in an area that does not interfere with the Whiteline Group 4 struts damper adjustment. Despite the stiff chassis, we noted a difference in the car’s handling.  Steering response is slightly improved and the ride is also improved, a sure sign that the braces are working well.  We also noticed that the car is now more sensitive when it comes to making adjustments to the damping and swaybar stiffness.  This is another sign that the bars are doing their job.

We felt that the rear bar surprisingly seemed to make a bigger difference than the front bar did.  Although the braces did not make a huge world stopping difference, they did make a positive contribution to Project STi’s handling and are the icing on the cake rounding out what has become a very good handling car that also has decent road manors to boot.

In our next installment we are going to address our weak clutch with some trick parts from ACT, improve cooling with some stuff from ARC and get even more power with the help of some bolt on engine parts.  Project STi is getting ready to wreak havoc on the track and teach Project EVO a lesson in speed.  Stay tuned for more.


To read the rest about Project Subaru WRX STI click here!



KW Suspension





Hawk Performance Brake Pads


Bits and Pieces Motorsports



Leave a Reply

Your email address will not be published. Required fields are marked *