WRX FHI 4 Pot Calipers

[Dorkhedeos]

Does anybody have any experience on how the WRX FHI 4 pot swap affects braking performance? I am thinking about doing the swap eventually if I can find a really good deal on them.


I know that you don't gain anything in terms of performance, but I do not want it to affect performance negatively if I end up installing them. I searched around and people say it puts a little more brake bias to the rear, but not much.



I could not find much info on how this would affect performance on the track. My main concern is that may upset the bias enough that it'll lock up the rear while trail braking. Anybody have any thoughts on this?

[jamal]

The caliper is stiffer and has slightly smaller pistons, so you will get a harder pedal and less travel, so better feel. Lighter than the 2-pot as well. If you'd like a number, they generate about 7% less brake torque (which is not the overall bias change).

The brakes do not have a set proportioning valve. Instead ebd reduces rear brake force based on wheel speeds. So it will not ever let you lock up the rears. Even the stock setup would generate too much rear force under hard braking without it.

[churchx]

Shifted bias to rear? So EBD/ABS will not let rear to lock? Sounds like underbraked front (that does most of stopping due weight shift) and because of that increased braking distances to me. And how braking will work for ones trail-braking prior EBD/ABS engage? And how it will work on car tracked in pedal-dance, that along other things switches off EBD too?
Brake retrofit from other cars with different weight distribution may somewhat work for non-tracked daily driven car with all electronic nannies enabled for those that care about slight money savings more then about performance & safety, when car is pushed near limits. But OP mentioned track use. Is that really the place/use worth compromising brake system at?

[wparsons]

Originally Posted by Dorkhedeos

Does anybody have any experience on how the WRX FHI 4 pot swap affects braking performance? I am thinking about doing the swap eventually if I can find a really good deal on them.


I know that you don't gain anything in terms of performance, but I do not want it to affect performance negatively if I end up installing them. I searched around and people say it puts a little more brake bias to the rear, but not much.



I could not find much info on how this would affect performance on the track. My main concern is that may upset the bias enough that it'll lock up the rear while trail braking. Anybody have any thoughts on this?

If it's not going to help at all, why waste your money?

[mav1178]

Originally Posted by wparsons

If it's not going to help at all, why waste your money?

because hard parking.

[jamal]

A firmer pedal, better brake feel, and less weight aren't performance gains?

[CounterSpace Garage]

Brake fluid and pads can significantly transform how the brakes perform. The better the pads and fluid, the better the consistency and intuition you get out of the brake pedal and braking performance. BBKs are great for epic consistency on track, but brake pads and fluid are still required to allow good performance out of a BBK.

[malubawla]

i have them on front and rear. slightly better brake feel, i did it for weight and ease of swapping brake pads. front are direct bolt on, rear needs a bracket.

[Dorkhedeos]

Originally Posted by churchx

Shifted bias to rear? So EBD/ABS will not let rear to lock? Sounds like underbraked front (that does most of stopping due weight shift) and because of that increased braking distances to me. And how braking will work for ones trail-braking prior EBD/ABS engage? And how it will work on car tracked in pedal-dance, that along other things switches off EBD too?
Brake retrofit from other cars with different weight distribution may somewhat work for non-tracked daily driven car with all electronic nannies enabled for those that care about slight money savings more then about performance & safety, when car is pushed near limits. But OP mentioned track use. Is that really the place/use worth compromising brake system at?

I would rather not compromise performance/safety (especially not on the track). I was hoping since the front will be slightly weaker, I would be able to run cheaper and less aggressive pads in the rear. It worked pretty well in my experience with FWD cars that weren't as nicely balanced. I don't have experience mixing pads with the 86, but I am aware that this platform has a way lower CG than any other car I have driven so it may not be comparable.

Originally Posted by mav1178

because hard parking.

Bingo!

Originally Posted by jamal

A firmer pedal, better brake feel, and less weight aren't performance gains?

Better brake feel was mainly what I was going for beside cosmetics.

Originally Posted by CounterSpace Garage

Brake fluid and pads can significantly transform how the brakes perform. The better the pads and fluid, the better the consistency and intuition you get out of the brake pedal and braking performance. BBKs are great for epic consistency on track, but brake pads and fluid are still required to allow good performance out of a BBK.

I am pretty satisfied with my current setup. I am running stock pads and Stoptech powerslots for street (ATE SL.6) and will be running XP10 and Centric blanks for the track.

I was just just looking into things I can do in the future to make it feel slightly nicer on a budget (I.e. forester clutch cylinder). However, I do not want to sacrifice performance/safety for feel.

Originally Posted by malubawla

i have them on front and rear. slightly better brake feel, i did it for weight and ease of swapping brake pads. front are direct bolt on, rear needs a bracket.

I will most likely only be doing the front unless doing both front and rear brings back the balance. I don't really like the idea of using an additional bracket on my brakes. Do you notice any increase of stopping distance?

[churchx]

Dorkhedeos: yes, some "fix" wrong bias with staggered pad choice, but that seems to me a bit wrong, to fix problem that shouldn't be brought in first place. It may work for some, but i kind of dislike such fix, as different compounds have not just one fixed friction Mu, but it changes with temperatures. And curves are not completely same and proportional for different pad compounds, so imho it may change bias depending on how hot brakes get.
On stock brakes and most properly designed aftermarket BBKs with most common square tire choice, and if extra aftermarket aero downforce bits are not very unbalanced, most run square pad choice of same compound.

Imho if anything is changed/upgraded in car, then it should be done properly, especially for something as important as brakes. There are many "native" designed aftermarket BBK options out there. Yes, more expensive, but they will improve some areas wanted (eg. more thermal capacity) without worsening others (like braking distances/stability due bias shift).

Though to me it looks that maybe you just need to experiment with pad choice to change mentioned feel, not brakes itself.

[Racecomp Engineering]

EDIT: I'm wrong on the bias change, the rear FHI 2 pots shift bias forward. Front FHI 4 pots have no effect on bias.

- Andrew

[malubawla]

I'm running the stoptech street pads and also did on my factory calipers. I would say that the wrx setup has more bite and slightly better stopping with the same pads. Braking feel is better as well

[churchx]

Racecomp Engineering: but as there are some 15" wheels that clear our stock brakes, is switch needed at all?

[jamal]

EBD works in place of a prop valve and sends equal line pressure to the front and rear until the wheel speed sensors start to notice a difference, at which point it begins to reduce rear pressure. This is maybe slightly different than just having abs itself intervene (although the ebd is performed by the abs unit). I suspect the only way to disable ebd would be to pull the abs fuse, despite what I'm reading in the mess of incorrect and contradictory information in the pedal dance threads.


Anyway, I did some math to calculate weight transfer under different amounts of braking. What's interesting is that on a stock car, the brake torque distribution exactly matches the wheels loads under 1g of straight line braking. With 4-pots it happens at around 0.9.

I would have expected ebd to be active at lower braking rates to more effectively use the rear brakes under normal street driving conditions. I might figure out what's going on with an 06 wrx later (e- ebd should start to work at only 0.7g of braking force on those, which kind of indicates actual brake bias doesn't matter much with ebd).

If you lower your car 1" that will reduce weight transfer under braking. Coincidentally, now 4-pot fronts with stock rears more closely matches the wheel loads under 1g of braking.

Also, that essex/ap kit everyone loves generates less brake torque than the 4-pots according to my numbers (299mm rotor, 1.5/1.625" pistons, I am not 100% on those piston diameters or my effective radius).