ClubWRX Forum banner

1 - 20 of 154 Posts

·
Moderating on the run!
Joined
·
12,482 Posts
Discussion Starter #1
for six years I've run 16's and thus never been in the position to run a BBK, I have however (through much trial and error) got to the point where I could somewhat successfully track the rather pompous WRX w/ the 4/2 pots (ducting/good pads/good rotors/good fluid/ti backing plates)- always knowing that there was room for improvement w/ bigger brakes, now w/ 17's I'm looking hard at a brake upgrade.

the reason is twofold- one bigger brakes will help out at the track, two I'm always looking to shed a lb or two- I think I can w/ the right BBK.

we all know that tires ultimately stop our cars and we all know that in a one off stop that our oe brakes will rank right up there (and even surpass in some cases) w/ most BBK's- it matters not that your brake kit has more pots than Martha Stewart or that your dimpled rotor is large enough to seat six if turned on it's side :)- so no talk about wanting to stop on a dime- your oe brakes will do that (once, maybe even twice), this is about repeated stopping and shedding heat.

Choosing a BBK is not an easy task- reliability, cost, longevity, safety, and many other considerations.

Over the course of six years I've seen several Impreza BBK's come and go, so it might not pay off to go w/ a johnny come lately setup (not saying there couldn't be a worthwhile kit to come along, but w/ the significant $ a good kit will cost, wouldn't be much fun not being able to source parts in a year).

One of the first considerations of a BBK (after you've established your wheels will clear them :)) is bias. Stoptech goes into a good discussion of bias and why it's important from a performance standpoint and a safety standpoint- http://www.stoptech.com/products/BBK/balance.shtml

Bias is something I've had some experience in fiddling w/- including pad frictions, different rotor sizes and even changing piston sizes (not commonly known but the FHI four pots actually have smaller pistons than the oe two pot sliders- 40.4 mm vs 42.8). IMO the WRX like many cars is pretty heavily biased to the front, the last thing you want to do is put on a BBK that further exasperates that. A plus would be able to shift even slightly rearward.

courtesy of TCE's website- a bias calculator :)

http://www.tceperformanceproducts.com/brake_bias_calculator.html

punching in the #'s for the oe (02-05) setup we get ~ 73.6/26.4%- you don't want to stray from that too awful much (w/ the exception of shifting bias a little further rearward).

I'll post of some calculations I've run tomorrow, I've also sourced some weights of some of the more popular BBK's.
 

·
Moderating on the run!
Joined
·
12,482 Posts
Discussion Starter #4
First I'll post up some #'s so folks can play around w/ the calculator if they wish

master cylinder 02-05 1 1/16", 06/07 1"

oe WRX (02-05) slider setup
rotors F 294x24 R 266x10, front pistons 42.8, rear 38.1
H6 rotor is 290x10
FHI 4/2 rotors F 294x24, rear 290x18 front pistons 40.6, rear 38.1
oe Brembo rotors F 326x30 R 316x20 front pistons 46/40 rear 36
AP racing (5200 caliper) front rotor 330x28, pistons 41.3/38.1

when inputting the slider info put the total # of pistons in ie for the oe front you'd put 42.8/42.8, on opposed pistons- just one side ie for the front Brembos 46/40

well I crunched the #'s through the bias calculator using the FHI 4/2 pots- it definitely moves bias rearward (as expected- smaller piston size in front-same in the rear, coupled w/ larger rotors- 290 vs 266)
bias is ~ 67.4/32.6%

if you played w/ the calculator you discovered you can move bias via rotor diameter, caliper piston and pad coefficient (this is a good way to fine tune bias)

next I wanted to plug in the oe STi Brembo setup- interestingly the bias is very, very close to the oe slider setup at ~ 73.6/26.4%. the Brembo calipers sport rather big pistons in the front 46/40 and rather small pistons in the rear 36mm

I couldn't find out piston info on StopTech (but will :)), but they note that their WRX kit was put together w/ bias in mind- presumably shrinking piston size in the front, my guess is their STi front kit sports larger pistons. StopTech front rotor is 328x28

the AP racing (330x28) kit using FHI 2 pots as the rear yields ~ 72.1/27.8%

oe Brembo front/FHI 2 pot 74.8/25.2%

I'm going to try and get the aftermarket Brembo info as well- their 328x28 kit

weights that I have are as follows

oe slider front rotor 14.2, oe rear 9.5 lbs front calipers 13.1/rear 6.9
FHI 4/2 front rotor -same, rear 11.1, front calipers 12.1/rear 4.1
Brembo front rotor 21 lbs, rear 17 lbs, front caliper 8.7 lbs, rear 5.9
Stoptech front rotor 13 lbs (two piece), front caliper 7.9
Brembo (GT) front rotor 12.9 (two piece), front calipers 7.2
AP racing front rotors (two different rotors-a AP slotted and a made for AP DBA slotted drilled) don't have exact weights yet but the AP slotted is evidently much lighter, the DBA evidently longer lasting, both two piece
front caliper weight 6.9

that's enough info for a bit :)
 

·
Registered
Joined
·
1,564 Posts
I'll input what i can...

I have what is an often overlooked street kit, which is often considered too track biased (due to lack of dust shields). I'm running the Wilwood 6 piston front calipers and rear 4 pistons. Personally i've been pretty happy running it on the street. Having had no issues with it. The only downside is the lack of pad selection. Wilwood has a good amount but there are basically no other pad manufacturers making pads for the front calipers in any case. I had a set of Ferodo DS2500 pads modified to fit...

On track performance has been a massive improvement. It is definitely more planted during braking, and is more biased to the rear than the standard setup. I have not had the pedal go to the floor yet, with only a bit of fading on semi-slicks. On street tires (Falken FK-452) the tires were giving up before the pads... It should also be slightly lighter then the stock ones, but i can't remember the exact weights..
 

·
Moderating on the run!
Joined
·
12,482 Posts
Discussion Starter #6 (Edited)
^ thanks- I think it's the lack of dust boots that your referring to, most (all?) full out race kits include no provision for boots.

I think there is now a Wilwood kit offered that is a little more street orientated (w/ boots)

edit- TCE does offer such a kit

Also worth mentioning a potential drawback of going to a BBK is knockback, the WRX and 04 STi are especially prone to knockback. This is due to the rather weakish design of the front wheel bearings (the 05+ STi have a much beefier setup). Knockback is not fun, basically after cornering when you go to apply brakes- it goes to the floor- a quick tap of the brakes before will "cure" it, but not fun

more on knockback here:

http://www.stoptech.com/tech_info/wp_knockback.shtml
 

·
Moderating on the run!
Joined
·
12,482 Posts
Discussion Starter #7
Found on some info on the Brembo GT kit- it uses a 328x28 rotor (weights listed above) piston size is 40/36, this yields ~ 70.1/29.9 bias (w/ the FHI two pots)

this kit while very pricey has gotten really good reviews (as have the very pricey AP kit)

Still looking for the StopTech piston diameters
 

·
Moderating on the run!
Joined
·
12,482 Posts
Discussion Starter #9
certainly :) they use two pot sliders up front, one pot rear- piston diameters are the same as the 02-05 WRX's 42.8 front/38.1

front rotors are 316x30, rear 290x18

bias is ~ 73.3/26.7%

can't find accurate weights, but from my reading pretty heavy, but would make a nice economical track setup to be sure :)
 

·
Moderating on the run!
Joined
·
12,482 Posts
Discussion Starter #11
I've got AP listed above- the four pot (330x28) one- they make a bigger kit, not many would need it :)
 

·
SuperNova
Joined
·
11,905 Posts
What about Alcon?

http://www.alcon.co.uk/advantage_extreme.htm

Front:

<table class="txtgrey" bgcolor="#ffffff" border="0" cellpadding="0" cellspacing="4" width="600"><tbody><tr><td>Subaru WRX (Dual Machined Wheel PCD)</td> <td>Ø343X32</td> <td>F98H01-02S</td> <td>4 pot mono </td> </tr> <tr> <td nowrap="nowrap">Subaru WRX (Dual Machined Wheel PCD) </td> <td>Ø365x32</td> <td>F98H02-02S</td> <td>4 pot mono</td></tr></tbody></table>

Rear:

<table class="txtgrey" bgcolor="#ffffff" border="0" cellpadding="0" cellspacing="4" width="600"><tbody><tr><td>Subaru STi</td> <td>Ø343x25</td> <td>R36X07-12S</td> <td>4 pot 2 piece</td></tr></tbody></table>
 

·
Registered
Joined
·
1,667 Posts
You're correct to expect a BBK to extend your braking distances as well as be more consistent from repetitive hard braking... But there's absolutely no real reason for a BBK to stop at further distances than an OEM brake set unless the pads being used need to be brought to temp first (such as track pads). If both brake systems can lock the wheels, you have potential to stop as short as the tire's grip limit will allow (at a given tire temp). Since the tire is the ultimate limiter, BBK or OEM should both stop within inches of eachother from identical stops/conditions.

The difference-
One of the primary reasons OEM brakes typically stop in shorter distances is the fact that the OEM system is balanced for the platform. The f/r calipers/piston sizes/rotor sizes are dialed in so each wheel maintains that last little limit of traction before locking up.

If you're looking to truly optimize a BBK, consider getting an adjustable master cylinder (or brake force distribution block of some sort). Seems that BBK's are relatively generic between models so there's no way they could fine tune the last few % for weight shift or weight distribution per each wheel for every platform available. The only difference is that the OEM brake set is specific to that vehicle and can be balanced to achieve optimal power distribution at threshold.

Anyways, that's how I see it.
 

·
Registered
Joined
·
1,564 Posts
^^^ Be nice if their was a way to recalibrate the factory ABS to account for BBK.
The ABS just works using the sensors... Doesn't matter if you have a BBK or not, it'll still only kick in if you're locking up. Nothing really to be recalibrated for...
 

·
Premium Member
Joined
·
26,609 Posts
My question for Mike was a two part question. If he doesn't like the ABS, doesn't use it, and is mostly using the car for autoX and track days, he should look into cutting out the ABS unit completely. It's easy to bend up some new lines and install a manual bias adjuster and gauges. Then he can choose whichever brakes he wants and adjust bias however he wants it on the fly.

He should be able to find whatever he needs here. http://www.portcityracing.com/general/assets/swf/all-in-one/index.html
 

·
SuperNova
Joined
·
11,905 Posts
The ABS just works using the sensors... Doesn't matter if you have a BBK or not, it'll still only kick in if you're locking up. Nothing really to be recalibrated for...
I was under the assumption that the ABS sensors were designed to operate under a table of preset parameters or functions much like the engine functions do. Put on a tire out of height range or step outside the predetermined stopping force and it goes whacky.

Which is why on my old 4x4 I had to recalibrate the computer for my larger tire/wheel combo so my ABS would operate properly.

Do Subarus act differently?
 

·
Registered
Joined
·
189 Posts
I like the idea of brembo GT fronts with oe brembo rear. I get a 68.5% front bias with this setup.
 

·
Registered
Joined
·
189 Posts
I was under the assumption that the ABS sensors were designed to operate under a table of preset parameters or functions much like the engine functions do. Put on a tire out of height range or step outside the predetermined stopping force and it goes whacky.

Which is why on my old 4x4 I had to recalibrate the computer for my larger tire/wheel combo so my ABS would operate properly.

Do Subarus act differently?
It depends on the ABS system. I've read that on modern Porsches they can accept a 5% (or was it 10%) difference in tire diameter from stock and be fine. 5% of 25 inches is 1.25 inches which is a lot of wiggle room for a sports car. I would expect Subarus are similar in this respect. A larger wheel and tire package on a 4X4, I imagine, would be much larger than an inch over stock though.
 
1 - 20 of 154 Posts
Top