Wilwood 6-piston vs 4-piston

[Zach34]

After some searching, I'm not sure anybody is actually running the large 6-piston/4-piston Wilwood option on an 818 R or S yet.

The FFR red car pictures show the 4-piston.

I had 13", 6-piston fronts on my Roadster for some autox and two track days and I loved them. They were easy to modulate with an aftermarket manual master cylinder, and they had no problem at Auto Club Speedway.

My priorities here are:

1. reliability
2. durability
3. performance
4. weight

I'm perfectly fine accepting the additional weight of the 6-piston fronts as long as there is a tangible benefit in durability and performance. So my question is: Is there a tangible benefit in durability and/or braking performance with the big 6-piston kit over the 4-piston kit?

The pursuit of weight savings often involves compromises that amateurs like me can't afford in the long run. I think by the time the R is put on the scales it will only be a couple hundred pounds lighter than the Roadsters, so I'm leaning toward the 6-piston since I liked them so much on my Roadster.

[Santiago]

I can't speak much to durability matters across caliper designs. Now larger rotors are another matter, but if the rotors are basically the same size/width, then no discernible difference in durability (all things being equal, i.e. metallurgy, etc.).

When folks ask about "braking performance" I always wonder what specifically peaks their interest. "Brakes" are a system of different parts tasked with different duties. In a casual setting I would say "there is no such thing as "the brakes" on your car," at least when I'm talking to folks really new to cars. Calipers only do one thing: they clamp down on the rotors via the pads. They don't stop/slow your car (tires do that), they just clamp.

My understanding was that the introduction of more and more pistons was driven by a need to even out the distribution of clamping force across the pad backing plate. That alleviates some of the leading-to-trailing edge pad taper that sometimes crops up (esp. easy to see with the old single-piston calipers). Good 4-piston calipers don't usually produce a lot of taper, but folks have different experiences (esp. w/uber-grip slicks & high-downforce applications) so 6-pistons are not uncommon. This also allows a caliper designer to change piston diameters to dial in that clamping force, particularly when rotors diameters start getting huge and pad surface area increases (really I think it's just longer pads necessitate spreading out the clamp force). Also, caliper design makes a huge difference when you're looking for an enduro set-up, because those systems are set up to use ridiculously thick pads. But notice that this increase in size is not in the lengthwise direction of the pad; the calipers just get wider to accommodate mondo-thick pads. In our 12-13" range, I would be surprised to see a large difference in force distribution between 4 and 6-piston designs (but I'm no brake designer, so who knows).

As for other brake performance parameters, I don't think the caliper design has much to do with it provided that you're comparing roughly equivalent stiffness in the bodies of the calipers. Here's how I would tag the major players in different performance areas:

Stopping Power: tire compound
Brake Modulation: pad choice
Heat Range/Capacity: pad choice (and proper cooling)
Fade Resistance: pad choice (and proper cooling)
Pedal Feel: fluid choice / master cylinder (and pedal ratio)

I'm not sure what sort of "reliability" concerns you have, since the only one I can think of (outside of "are these junk parts or what?") is for enduro set ups. But that has more to do with the level of abuse sustained in that environment. Then again, some fixed caliper designs are noted for nasty pad knock-back, and this could be considered a matter of reliability. I haven't heard anyone complain about it with the Wilwood 4-piston set up.

Sorry to ramble, just always a bit confused when "brake performance" is brought up; never know where folks are coming from without more info.

Best,
-j

[Rasmus]

Well put Santiago. That's an amazing amount of information.

I would tweek your list a bit:

Stopping Power: tire compound/vehicle weight
Brake Modulation: pad choice
Heat Range/Capacity: pad choice (and proper cooling)
Fade Resistance: pad choice (and proper cooling)
Pedal Feel: fluid choice / master cylinder (and pedal ratio)
Racing Abuse: Mass of Rotor

By Racing Abuse I mean the ability to repeatably brake from high speed without the system fading. Back to back to back for 25+ minutes. The more mass you've got in those rotors the more energy they can absorb and dissipate to the air and not overwhelm your pad compound.

[Jaime]

You can always create enough braking force by selecting the right master cylinder, so force is never the reason to choose a particular diameter disc.

I always thought that the primary reason for big brakes was high thermal mass. Pads have a set max temperature, so the game is to slow down for a corner without losing braking. The amount you need slow down has nothing to do with the brakes (unless you have to take it easy due to poor brakes), it's simply "as fast as you can go into the turn" minus "as fast as you can navigate the turn". How long it takes is nearly irrelevant because you can't dissipate a meaningful amount of heat in the two second you are braking. So, you need enough heat capacity to take your steady state temperature and add the heat of the corner to it, without cooking the brakes. More metal does this. It's also the reason some brake pads have copper back plates - to get the heat out of the pad and distributed into the lump of iron.

Better cooling will lower the average temperature, which will give you a bit more headroom next time you brake.

Pedal Feel: fluid choice / master cylinder (and pedal ratio)

Component design also plays a big role in feel. Hard or braided lines feel different than rubber lines. Calipers built more solidly feel different too. Really expensive calipers are sometimes made from one piece of metal with the piston bores on both sides bored all the way through from one face, or from the center gap if the manufacturer is really tricky (Brembo does this on some calipers).

These calipers are bored through from the side facing us and the entry hole of the tool is capped with the funky screw caps in the picture...

ISRBES.jpg

[Santiago]

I like the additional category Rasmus added - I was trying to think of a way to say something similar, but dropped it. My experience has actually shown that pad compound helps here as well. Ex: In the Mustang I've found that when comparing pads of equivalent heat range from Carbotech and Hawk (Carbo XP-12s vs Hawk DTC-70s), they don't take the same level of sustained abuse over their lifetime. When new, sure they're a match in this regard (you can beat on them all day long), but once they're well-worn, it's a different story. Carbotech pads experience far greater drop off when they wear past 25% original thickness, while Hawk pads are just beasts. I can run Hawks ridiculously low and not fade them provided I'm not running any harder than normal, but the Carbos won't take it even if I'm driving the exact same way. Now, I still run Carbotechs, but that's because their modulation characteristics are heaven and their initial bite is ferocious compared to the Hawks. Still, if you were looking to get "the longest life" out of your pads, Hawks win...of course, they do so at the expense of the rotors (and I run 2-pc rotors that are more costly to replace than pads).

I'm not disagreeing with anything Jaime said. In fact I was trying to avoid it for simplification when I said "...provided that you're comparing roughly equivalent stiffness in the bodies of the calipers." The real heavy hitters in the caliper design field have come up with much stiffer calipers than what you'd find in your average production car. However, we're not really comparing these in the thread - it's Wilwood's 6-piston vs. 4-piston...so I was trying to get away with a simpler statement. You caught me. =)

One point of clarification though; when I discussed the 6-piston caliper advantage in clamping force, I wasn't referring to absolute clamping force. That's as Jaime says easy enough to address with master cylinder selection. The advantage is in how that force is distributed across a pad, thus minimizing the fore-aft pad taper vs. a less well designed caliper. Again, I don't think it matters much in the range of sizes we're talking about. The 4-piston calipers are going to do a very similar job at distributing X-amount of force as the 6-piston design. That's because the pads are what, 3.5" to 4" long? Compare that to one of those really long 6" pads found in a huge 15" rotor set up. Now having more pistons across that length spreads out X-amount of force more advantageously (note: I didn't necessarily say "more evenly" since this isn't desired...and the reason you see staggered piston sizes in 6 and 8-piston set ups).

I'm on the fence with "braided lines." I've heard as many "advertising-bs" claims on that as I've heard folks swear by them. The anti-camp claims that braided lines are good for abrasion resistance, not feel. The advertisers want you to think they're magic. One thing that I have more faith in is maximizing your hard-line vs. flex-line. Keep the flex lines as short as you reasonably can and you'll improve pedal feel regardless of braiding or not. Maybe the braids do something positive for feel...just haven't seen any good evidence for it.

Best,
-j

[FFRSpec72]

I run the challenge series and a couple of other leagues and I run carbotec xp10 pads all the way around and have no issues with usage. These pad last me an entire season of practice, qualifying and racing, so I run about 20 to 25 races and some are very demanding tracks. I plan to run caebotec on my 818R also

[Zach34]

I think when I say "braking performance" I'm referring mostly to what Rasmus defined as "race abuse". I could certainly lock up the wheels with the fox-body brakes on my roadster, but the big Wilwoods were just easier to control and felt exactly the same lap after lap on the track. I don't want to get the 4-piston set-up, then a year down the road find myself on a bigger track where the 6-piston might offer better fade-resistance.

Would you guys expect more race miles out of a larger pad vs a smaller pad, controlling for brand/compound and race conditions? The additional cost of the larger 6-piston setup could be a wash if you get longer life out of the pads/rotors.

There's also the not-to-be-overlooked detail of wheel options, and I wonder if this might be why FFR chose the smaller Wilwood package for the red car.

[longislandwrx]

What's the target weight of your vehicle? At a sub 2000lb fighting weight the Wilwood 4pot system with a proper pad choice should be more than you ever need. I think this has been validated by everyone who has tracked their car already. Wayne, FFR, brando, I imagine would all agree.

There just seems to be little justification for the increased weight and cost of the 6 pot system, looks maybe? piston envy? the fact that you can get all 4 in nickel finish?

If you plan on running plush seats full ICE, subs, a portly passenger and you're encroaching on 3200lbs maybe you could make a case.

I think your pad life difference will be negligible, perhaps even the opposite of your expectations, there's just not enough data out there to know for sure yet.

[Rasmus]

I will drive my 818 on racetracks at speed. I will AutoX my 818, alot. I've driven my WRX on racetracks for 14 sessions. That stated, I've never had the 818 running, much less driven it on the track. Here's my brake plan for my 818R:

Front: Wilwoods 140-9193, 4-Piston Front Kit, 12-in Rotor Diameter, 0.81-in Rotor Width, up front. I'd love a blank face rotor but will probably take their slotted one. Under no circumstances will I use a drilled rotor.
Rear: Subaru's 2006 Aluminum 2 Piston caliper and matching blank face rotor.

I'll run the 12" Wilwood kit for four reasons:
1. I can run 15" wheels. Which gives me lots of options for tire sizes and compounds. If I want to run 24.5" tires on 17 inch wheels for street use, I can. If I want to run 23" tires on 15" wheels for AutoX for the better gearing, I can. If I want to run 24.5" tires on 15 inch steel wheels for track-days, I can. Running 15" wheels with higher profile tires on the track for me is desirable. I've had to pay for my sins while getting a little off track with low profile tires before. I've bent up wheels running too low a profile (235/40-P17) when I run over the curbing.

2. The system weighs less

3. It's cheaper.

4. It looks on paper to have enough mass to take the abuse I'd give it on track days for the weight of the 818 with sticky tires.

Now if it doesn't have enough mass and I'm over heating the system, I have three backup plans:

A. Brake cooling ducts fed from NACA ducts in the radiator tunnel that passively push air to the inside of the rotor hat.

B. If I need more cooling. Adding in brake duct fans to actively push air to the rotor hats.

C. If, for some bizarre reason, I get to be a monster driver that can overheat even that: water cooling, in addition to the forced air.

That is, spray a fine mist of water in the braking duct's air stream.

[Bill Waters]

I am sure that you have made the right decision, Rasmus.

I got the 6-piston setup because it is liable to provide the best cooling and reliability over time and settles the question of whether I have enough brake, but not because of any empiric proof that it's the better choice in an 818R.

[Santiago]

piston envy?

I love it...that was damn funny... =)

I've run 4-piston Stoptech calipers on a 3,700 lb Mustang for years now, and they have been excellent. I'd be really surprised if the 4-piston set up from Wilwood isn't up to any task you throw at even a heavy 818.

Zach, just to reiterate, take a look at the list of major contributors to braking performance. When you say you are worried about getting "brake fade" on a bigger track, you're pad selection and heat management system is going to have a much bigger impact on this than the caliper design. Look over the back-up plan Rasmus suggested. He's thinking about the same thing, but his plan of attack (a good one) has nothing to do with the calipers. Caliper design has little to do with fade resistance.

As an aside, fluid boil is another matter and is influenced by caliper design/material selection, since aluminum pistons will transmit more heat to the brake fluid than stainless pistons will...a sad fact that Stoptech never seemed to care about (so I run titanium shims, but honestly haven't tried any back-to-back tests with or without them).

Anyway, when you reflect on your experience with the Fox-body calipers vs. Wilwood products, you're comparing cast OEM calipers to a significantly better product all around. I wouldn't think to draw parallels of what to expect comparing the two Wilwood products. You don't have "crap vs. good design" anymore, you've got "good design solution for X" vs. "good design solution for Y."

I'm with the others here, there's no evidence to suggest we're in a Y-situation. The experience folks have had so far suggests we're in the X-situation. Worth noting, John George has run blisteringly fast laps for FFR and commented that he didn't even feel he was using the brakes full potential.

Best,
-j

[C.Plavan]

4 piston is overkill. Either route you choose will be more than enough.

[Sgt.Gator]

I agree, six pistons is overkill. Four will be plenty. AP Racing makes real racing brakes for Formula, GT, NASCAR, and most of those are 4 piston setups. The only six piston ones are for endurance racing in very high speed races in heavier high HP cars. Essex is the US distributor for AP acing, they have an interesting comparison page between their own AP Racing BBK kits for street/track cars (think the usual Wilwood/Brembo BBK kits) vs their brakes for real competition cars.
http://www.essexparts.com/shop/compl...e-systems.html

And this is their take on the OEM Brembo STI brakes vs their competition setup. I may swap in their STI comp brakes in my LGT and use the STI/Brembo in my 818R. Not sure yet.
http://www.essexparts.com/shop/compl...ti-bundle.html

[SixStar]

pUncG9F.jpg

[Drew P]

4 pot is the way to go especially in a car this light. I know people with 6 pot calipers on their STi track car and said that they were so grabby that they needed to make a lot of adjustments to feel comfortable with them and were thinking about pulling out the power assistall together. And that is on a much heavier car. I dont rememer if they actually pulled the power assist or not though.

[Doowop]

Now, to add to the confusion, I am not at all familiar with mid engine RWD cars. Unlike a regular Suby, the weight distribution is very different on the 818. Is it right to always make comparison with a Front engine AWD or FWD car, or is there something else to it when looking at the 818. I guess, it would be like a porsche, which i basically know nothing about when it comes to race brake setup...

[SixStar]

I'm not concerned. The nice thing about cars is most of them come with a big lever that allows you to adjust braking force with your foot.

[Zach34]

Well, the oft-unmentioned but ever-present factor of the dollar has annoyingly invaded my thought process, and after weighing the options, I managed to score an STI donor. Removing the $3200 Wilwood brake option from the kit price is a huuuuge relief. I know the Brembos will be at least good enough to get me on the track, and with luck will be all I need for quite some time. The cost of adapting the STI front spindles and rear axles is trivial in comparison (looks like $400 for custom rear axles, and some time with my welder for the front spindles - hopefully no surprises). I also get tougher spindles, bolt-in hubs, and the 6-speed.

After all, I'm inexperienced on the track and will be going autox -> track days -> license -> NASA/SCCA. I don't need to crush the brake option right off the bat.