Project Subaru WRX STi Part 3: Streetable Track Worthy Suspension

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Whiteline diff carrier mount
WhitelineSTI Diff carrier mount in place
Whiteline’s diff carrier mount keeps the rear differential from banging around and contributing to wheelhop.  The stock rubber bushings are retained and the kit does not increase vibration or noise.

When shifting briskly at or near wide open throttle, we also noticed a disturbing banging noise coming from the back and underneath the car. Sometimes when accelerating out of a tight corner we got shuddering wheelhop from the rear wheels. To alleviate this we installed Whiteline’s diff carrier lock and transmission mounts.  The stock diff carrier and transmission mounts are soft rubber and they allow these parts to bang around quite a bit. The Whiteline parts are made of harder 70 and 80 durometer polyurethane and stop the unwanted motion with no detectable increase in vibration or harshness.  We feel that these parts can probably go a long way to prevent drivetrain breakage as the wheelhop and banging sure felt like something was going to break.

Whiteline STI tranny mount
Whiteline STI tranny mount in place
Whiteline’s STi tranny mounts also help reduce drivetrain wind up.  Like the diff mounts they do not increase noise and vibration.

Since we were planning to so some serious track work, we also replaced our street anti lift kit with the race spec kit.  The main difference between the race kit and the street kit is a change in durometer in the bushing from 70 durometer to 90.  We noticed a slight increase in harshness but felt it was barely discernible.  We also noticed a slight improvement in steering response.

Finally following Whiteline’s recommendation we installed their rear crossmember lock.  When Whiteline was developing the WRX, they noted that after a few miles the torque load would skew the rear diff carrier to one side which messed up the alignment settings. The problem was worsened when using sticky tires.  They determined that the diff carrier was actually shifting 2-4mm under load throwing the alignment off.  This is a considerable amount and it made the WRX impossible to set up properly.  Because of this issue, the locks are mandatory for any WRX/STi that is going to be driven seriously.

Whiteline cross member lock
Whiteline crossmember lock in place
Whiteline’s crossmember lock keeps the rear cross member to which the rear suspension is bolted to from moving back and forth.  This is a serious problem with the STi.

Whiteline developed a special bolt with a large shoulder that would thread into an unused set of weld nuts in the chassis that conveniently enough lined up with a hole in the crossmember in a way almost as if Subaru intended to have this sort of lock in place as standard equipment.  The bolt solidly locates and centers the entire rear diff carrier in place allowing for precision alignments and not allowing shifting of the rear wheels location no matter how hard the side loads.  On our car this also eliminated the last bit of nibble from the rear of the car under acceleration.

Whiteline's race anti lift kit for WRX STI
We installed Whiteline’s race anti lift kit to replace our older street anti lift kit.  The race version uses 90 durmoter bushings which are much harder than the street kits 70 durometer.

2004 STi’s are plagued with soft and squishy steering rack bushings.  They are so bad, Subaru redesigned them for 2005.  The stock steering rack bushings allow the rack to move and shift back and forth.  Sometimes the steering centering even changes!  We replaced the horrible stock bushings with Whiteline’s hard urethane parts which cured all of our issues.  This was perhaps one of the worst things about our car and Whiteline’s cure is cost effective and works extremely well.  We would recommend it even for people with stock 2004 models!

Whiteline STI steering rack bushings
Whiteline STI Steering rack bushings in place
Whiteline’s steering rack bushings were Godsend in fixing our steering with a wandering center.  We would recommend this for people with stock cars.

As a last step we brought project STi to our favorite chassis shop, Westend Alignment in Gardena for a more aggressive track alignment.  We set up Project STi as follows.

Front:
Six degrees positive caster
2.5 degrees negative camber
1/8 inch total toe out.

Rear
1.5 degrees negative camber
Zero rear toe

If you were to drive this set up daily, it would probably have significant negative effect on front tire wear in which case we would probably recommend zero front toe and 1.5 to 2 degrees negative front camber.

At the track we set our tire pressures to 35 psi hot and had a go at it.  We were amazed; our car was the best handling STi we had ever driven.  Steering response was razor like with excellent turn in, the car refused to understeer and would simply go into a nice easily controllable 4 wheel drift.  Lifting the throttle or trail braking would cause the tail to step out nicely which would help get the car to pivot and rocket out of the turns.  We felt no need to fiddle with damping or swaybar adjustment as we had hit the nail on the head.  Tire wear was excellent under track conditions with no chunking and the tires wearing absolutely flat and even across the tread.

MotoIQ Project STi

Even though I have driven and fallen in love with several modified EVO’s in various track events, I was surprised to find myself liking Project STi better.  With its sharp turn in and predictable on throttle antics and wide powerband the car was a killer, turning race car like lap times and beating nearly every car in NASA’s advanced lapping class even with a young inexperienced driver.  Our little car was delivering near exotic performance on a budget.

motoiq project STi at Willow springs

In project STi’s near future we will bring you some improvements to its clutch, cooling system, brakes, chassis stiffness and then, perhaps some more power.  After this it will be time to bring on Project EVO.
Sources

KW Suspension

Technosquare

Global Performance Parts (US distributor for Whiteline)

 

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