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Racecomp Engineering : Blog

GTWORX Bilstein Cup Sport Struts and Bilstein B6 comparison

We wanted to shed some light on something that people have asking about a lot lately, as well as address some misleading forum posts and social media activity. In this post, I’ll go through some of the details between our GTWORX Bilstein Cup Sport kit and the standard Bilstein B6 kit. This includes differences in the actual strut and shock bodies as well as technical differences in the actual valving of each damper.


First, we think it’s great that Bilstein USA is releasing their own B6 strut and shock for 2015+ WRX and STI! It shows how far the Subaru suspension market has come in the last couple years. When we first came to Bilstein years ago to propose our first GTWORX dampers for the previous generation Impreza, they weren’t exactly sure if this was a viable market. We were big Bilstein fans, having used them on the old GC platform, other vehicles, and some interesting JDM parts. Our goal was to convince Bilstein USA to bring that kind of shock technology to what was then the latest Impreza chassis and the result was our GTWORX Bilstein kit. We were pretty excited to be a part of making Bilsteins fixed perch dampers easily available to the USDM market…and of course we were excited to run them on our own personal cars because they are an awesome kit.


Now with Bilstein making their regular B6 kit available on the new 2015+ chassis, we are happy to offer both our own GTWORX Bilstein Cup Sport kit and the standard Bilstein B6 kit. We think both are excellent options that serve different needs, but there are significant differences.


The valving differences are particularly important, as Bilstein’s description of their B6 kits say "Bilstein B6 Performance dampers are a performance upgrade to OE dampers designed to be paired with the OE factory spring.

1. ABS bracket
a. Our GTWORX Bilstein kit uses OE ABS sensor brackets for easy installation.
b. The standard Bilstein B6 kit requires the use of zipties to secure the ABS bracket to the front strut.


Bilstein B6


2. Rear height adjustment
a. The GTWORX Bilstein kit includes an adjustable rear spring perch, allowing adjustment of rear ride height in 5mm increments. From the standard perch location, a user can bring the car up or down a total of 15mm. This is useful for getting a little more damper travel, evening out height differences when using camber plates in the front, or just getting the look and rake you really want.
b. The standard Bilstein B6 kit has a conventional non-adjustable rear spring perch.


GTWORX Adjustable rear cir-clip

Bilstein B6



3. Valving
Here’s the big one. Obviously, the actual shock valving is the most important part of equation. Bilstein’s description of their B6 kits say “"Bilstein B6 Performance dampers are a performance upgrade to OE dampers designed to be paired with the OE factory spring." Our GTWORX kit was designed primarily around our RCE Yellow and Black lowering springs. Both can work safely with OE springs or a variety of aftermarket lowering springs, but the different design goals are important.


Note that the Bilstein B6 dyno curve does not have as many data points…this dyno came directly from Bilstein and that unfortunately is how they do things most of the time. However, we can still see what we need to see. Also, gas force has been removed for all 4 dampers. Compression is on top, rebound on bottom.


All 4 curves have a similar compression values during shock piston velocities around 50 mm/sec (which is about 2 inches/sec). This is in the low speed range, which can be considered body roll and handling. Sharp and aggressive transitions may be a little higher.


A lot of low speed force can give a feeling of “platform” and responsiveness, but too much can low and mid speed damping can make the car feel very jiggly over even smooth pavement. Even the “soft” 2016 WRX inverted front dampers are overdamped in rebound, which is about 125% critically damped with RCE Yellows at 2 inches/sec. Damping ratios are often used as a measure of how underdamped or overdamped a shock is when used with a specific spring. Roughly 60-70% critically damped in the low speed range is often cited as a good target for rebound for a sporty car that still needs to soak up bumps. A damping ratio of 125% is excessive for a street car and some track cars. The STI takes that even farther which is why many complain about ride quality on that car. They are extremely firm. Too much rebound up front can also increase understeer and in the case of OE STI shocks, this is indeed too much. The WRX does digress nicely after that so it’s not as bad, but there is lots of room for improvement. On the compression side, the OE shocks build force quickly which is nice, but may have slightly too much at 1 inches/sec for optimal ride comfort. They do flatten out/digress decently in the high speed range to help soak up big bumps.


Our GTWORX Bilstein B6 kit uses the Bilstein COB digressive piston. You can see the smooth ramp up in both compression and rebound forces before digressing in the high speed range. At 2 inches/sec with our RCE Yellow springs, we’re at about 60% critically damped in rebound. We do need enough rebound in the high speed range to keep the car from bouncing excessively after big bumps when using firm springs, so we continue to build force before leveling off a little. For compression, we wanted to keep up some force in the mid-speed range and then flatten out in the high speed because we don’t want too much force to upset the car when we hit sharp impacts or mid-corner bumps.


The Bilstein B6 front struts are almost (but not quite) linear to 250 mm/sec. So for compression, there’s roughly the same amount of low speed damping compared to the other shocks, but a decent amount more in the high speed range when you hit bigger bumps. It’s not excessive and it begins to flatten out a little more after that, but I’m not sure many would want more high speed compression on their daily driver (in some specific cases you actually would, but for the most part it’s not ideal). The Bilstein B6 fronts are at about 90% critically damped at 50 mm/sec for rebound and 80% in the high range at 250 mm/sec. That’s a bit firmer than the often recommended 60-70% but this means they will not have any trouble controlling firmer springs, at least up front.


We see bigger differences when we look at the valving of the rear shocks.


The OE 16 WRX rear shocks from the inverted suspension are very soft on rebound. Running firmer springs on these shocks is very much a compromise, and you could expect a bouncy rear end due to the lack of rebound. The OE STI rear shocks (which are the same as 2015 WRX rear and 2016+ base WRX rear) do a little bit better but are still slightly underdamped in the rear in rebound. They’re fine with aftermarket springs, but really right on the edge which is why you will occasionally hear about a little bounciness from the rear. The OE STI rear shocks are at about 30% critically damped in rebound with RCE Yellow springs in the high speed range. That’s okay, but that means you get a few extra oscillations after bumps. On the compression side, they’re pretty good and IMO that’s part of what makes the car fun to drive.


Our GTWORX Bilsteins use a similar valving curve for compression compared to the STI but are a little bit more digressive. Having a good amount of low speed compression damping in the rear helps reduce understeer when powering out of corners and helps the car rotate. More rebound in the rear also helps the car turn in as well as solve the problem of the bouncy rear end. We are at about 60% critically damped in rebound in the high speed range with RCE Yellow springs, so the rear end will settle quickly after hitting bumps.


The Bilstein B6 kit has a compression curve that is very similar to OE 16 WRX. Basically the same fairly soft curve. Which is fine, but remember that the front Bilsteins are a little firmer than we want in rebound. Firm rebound in front and soft compression in the rear is generally a recipe for understeer when powering out of a corner as the front wheels want to pick up and the rear compresses more easily. On the rebound side, the curve is closer to OE STI with a tiny bit more low speed and a tiny bit less high speed. So with RCE Yellows and most aftermarket springs, we’re not making a big improvement over OE STI and we’re again right at the edge of what feels good in the high speed rebound range.


Overall valving differences
It’s important to remember that the design goal for standard Bilstein B6 is to work well with OE springs. I think they do succeed with that, and they will still work relatively well with most lowering springs including our RCE Yellows. I would expect a good overall ride and good handling.


A big focus for our GTWORX Bilstein Cup Sport kit was balance. We knew that the OE rear shocks were right on the edge of what was acceptable for rebound when used with an aftermarket spring, and we knew that the OE fronts have excessive rebound. It’s one of the best OE suspensions ever from Subaru, but it still understeers and can be pretty harsh in the front and bouncy in the rear. Addressing both of those issues not only improves ride quality, but makes the car a lot more balanced on corner entry and exit. Which means it’s a lot more fun.


4. Price
Here’s another big one. Yes, our GTWORX Bilstein kit is roughly 450 bucks more expensive than the going price on the standard Bilstein kit. Why? Well part of that is the difference in the damper bodies (OE ABS bracket, rear ride height adjustment, that sweet stealth black paint) and the other part is that our GTWORX Bilstein Cup Sport kits are low production run custom builds. There’s all our test days at the track, the autocrosses, the tens of thousands of daily driven miles and road trips we’ve put on our prototypes….but when it comes down to it, our pricing is the result of a different pricing structure based on building very small amounts of these in comparison to what Bilstein does with their B6 kit. We wish we could build thousands of these and bring the cost down, but we’re a small company serving a niche market of enthusiasts within a niche market of Subaru fans. Although the street price of the Bilstein B6 kit is around $750, the actual MSRP is $1148. We unfortunately don’t have the luxury of playing with the price quite that much.


Also, it’s important to note that a custom revalve from Bilstein is $575 for the set + shipping, + $55 an hour of tuning time.


We believe that our GTWORX Bilstein suspension is an awesome kit and we’re proud to be able to offer it to everybody. The easy installation, the durability, the ride height adjustment, the advanced digressive valving, the balanced handling, and the improved ride….we believe these are one of the best all-around suspension kits available for a fun daily driver and mild track day and autocross car at any price.


The standard Bilstein B6 kit is a good kit at a very good price, and that’s why we will offer it to our customers too. It is, however, very different from our GTWORX Bilstein kit, and anyone implying that they are the same shocks is simply being dishonest with you.


Thanks for reading and I’d be glad to answer any questions.

- Andrew

Project Texas STU

2015 was an incredible year with the new WRX/STI, and we figured it couldn’t end any better than with a project car build!

It all started back in September when the customer initially contacted us. He wanted a competitive STU car that could be daily driven and street friendly. After numerous back and forth emails, phone calls, and contact with SCCA, we gathered a list of parts that would address the following: brakes, power, suspension, wheels/tires, and most importantly, safety.

The car arrived from Texas the first week of December. First order of business was to inspect the car and make sure it was in the same shape it left Texas in. We then measured the stock ride height and began to tackle the suspension. Out came the front control arms, rear lateral links, dampers, swaybars, and end links. In went new Whiteline front control arm bushings, Cusco rear lower control arms, Cusco front strut bar, RaceComp Engineering Tarmac 2 coilovers with RCE street camber plates, RCE front and rear swaybars, and a set of Whiteline adjustable end links. To help eliminate movement within the chassis, Whiteline rear subframe inserts and rear differential inserts were installed, completing the suspension upgrades.

Next on the agenda were the brakes. The factory STI has good braking capabilities but we knew the customer would be pushing the limits of this car. Up front, we installed a set of Performance Friction 2 piece rotors. Not only do they provide additional cooling, but they weigh 7lbs less than the factory rotors. Finishing off the brakes is a set of Pagid RS4-2 Blue pads, along with Goodridge stainless steel lines and Motul RBF 600 fluid.

Having gone through many wheel and tire setups on our Clubsport WRX build, we already had an idea for a wheel and tire combination for Project Texas. WedsSport TC105N in 18x9.5 +35mm were the perfect candidate with a simple motorsport look and weighing in at less than 18lbs! We decided on the new sticky Bridgestone RE-71R in a 255/35/18. No modifications to the front fenders were necessary but the rears were slightly rolled to avoid any rubbing on large bumps. Front camber was set at -3.0 and rear camber was set at -2.2. Zero toe all the way around. The new setup shaved off 7 lbs. per corner for a total of 28 lbs. in all. Not too shabby!

We knew from the beginning that handling was the primary focus of this car. However, that doesn’t mean we couldn’t show the engine any love. A Cobb Tuning downpipe was installed and mated to an RCE Clubsport cat back exhaust, providing the nice deep rich boxer rumble we have all grown to love. We contacted one of our tuners, Bren Tuning in MA, who sent us an STU spec tune to load up on the Cobb Tuning Accessport.

Knowing that this car may very well see some track time, a Perrin oil cooler was installed to combat the Texas heat.

With the basic modifications all finished, next in line were the seats and harness. A set of Recaro SPG XL seats were delivered, along with Recaro side mounts and sliders mounted on Planted Technology seat brackets. The stock front seats were removed and the new Recaros were mounted in place.

At this point, the car was essentially done. A quick trip to the alignment shop and we were ready to call the shipping company to pick up the car. That same night we received a call from the customer inquiring about extra safety measures. One thing lead to another and it was decided that a back half roll cage would be installed. We called Mitch at Piper Motorsports and set up a date to drop the car off. The roll bar meets SCCA and NASA regulations consisting of 1.75x.120 DOM tubing. In no time the car was back in our hands looking better than ever. Mitch’s attention to detail is second to none and we couldn’t be any happier with the outcome. More importantly, we knew the customer would be more than satisfied upon receiving car.

With the car 100% done, it was time for a photoshoot…with 2 other matching cars!

After a little over a month with the car, it was finally ready to head back to its owner in Texas. We were happy to see that it would be accompanied by a trailer full of Porsches!

More Pictures

Links to products used:

RCE Tarmac 2 Coilovers
RCE Street Camber Plates
RCE Swaybars - 2015+ STI
Cusco Lower Control Arms
Cusco Front Strut Bar - Type OS
Performance Friction 2-Piece Rotors
Pagid Racing Blue brake pads - Front
Pagid Racing Blue brake pads - Rear
Cobb Tuning Downpipe
Cobb Tuning Accessport - STI
Whiteline Front Control Arm Bushing
Whiteline Rear Subframe Bushing Insert
Whiteline Rear Diff Insert
Whiteline Steering Rack Bushings
Kartboy Short Shifter w/ Bushings

Secret Santa RCE line !

OK WIVES, GIRLFRIENDS, FIANCEE's, special BOOS etc.....if your dude/partner has a wish list and it has RACECOMP parts on it.....dont hesitate to reach out to us so we can plan/scheme some secret Santa stuff just in time for the HOLIDAY/CHRISTMAS ETC ETC day thats approaching ! call from a friends phone, go in to a bathroom, text me, what ever so you can do to surprise him or her with some springs, swaybars, coilovers, camber plates, and be the motorsports Santa this year ! 443 370 9000 text. Secret Santa RCE line ! BOOOM !

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