Has anyone swapped aluminium pistons for stainless?

[peter de la mare]

I’m due to refurbish my CP5555 calipers next month and I’m curious if anyone has swapped the standard aluminium pistons for stainless steel ones and if any improvement in heat management has been noticed?

I have cooling ducts already and run 710 fluid, but sometimes I think more could be done.

[roastbeef]

i wonder what the difference is between the cp5555 and the cp5060 calipers. they look very similar. looks like one uses dust boots and the other uses seals. the 5060 also has ventilated pistons... not sure if that is the only difference. jritt@essex might have some good info on this.

[dogbone]

I have found my AP CP-5555 kit extremely resistant to heat for the 6 years I’ve run them. (I just moved to the Essex 9668 Radi-Cal kit in the front.)

If you have ducts and are having heat issues, you may want to try Castrol’s SRF. The wet boiling point of SRF far surpasses your 710 brake fluid. The 710 fluid wet point is 195°C. SRF’s wet boiling point is 270°C. In Fahrenheit, that’s 518°F vs 380°F. In my book, that’s a huge difference. The dry boiling points are within 20°F (with SRF being higher). But brake fluid doesn’t stay cherry and fresh for long, so for me the wet boiling point is very important.

All this to say, if I was having heat issues, the easier first step might be trying a different fluid with a higher wet boiling point, versus experimenting with something more complicated like pistons made from different materials.

[peter de la mare]

Thank you Mr Dogbone. My sessions tend to last around 20 minutes and more often than not on slicks, plus my car isn’t as light as yours, so the brakes get a real battering. The 5555 caliper is a “fast road” caliper whereas your new 9668 caliper is “general competition use” and comes with stainless steel pistons as standard. Ally conducts 15 times as much heat as stainless, I know I’m asking a bit much of the 5555 caliper, but an upgrade is going to have to wait for a while.

It’s a good call on the SRF fluid, I had been considering it anyway and as I’m minded to not miss any opportunity to help the brake system cope with heat I’m going to do both changes.

[dogbone]

Originally Posted by peter de la mare

Thank you Mr Dogbone. My sessions tend to last around 20 minutes and more often than not on slicks, plus my car isn’t as light as yours, so the brakes get a real battering. The 5555 caliper is a “fast road” caliper whereas your new 9668 caliper is “general competition use” and comes with stainless steel pistons as standard. Ally conducts 15 times as much heat as stainless, I know I’m asking a bit much of the 5555 caliper, but an upgrade is going to have to wait for a while.

It’s a good call on the SRF fluid, I had been considering it anyway and as I’m minded to not miss any opportunity to help the brake system cope with heat I’m going to do both changes.

I've literally only run the 9668 caliper one day so far. I just put them on 2 weeks ago. The other 100+ track days in the last 6 years have been done with the CP-5555 caliper. And I don't have any brake ducting.

Prior to the gutting in 2015, my car used to be full stock weight plus a supercharger (+100 lbs). (I never did any interior removal or race seats prior to the gutting/cage work.) My car weighed around 3675 lbs without a driver and the supercharger allowed higher speeds than the OEM power. I was very unkind to the braking system and it handled the workload very well. And back in 2015, I would run fairly sticky Hankook TD tires back. In the last two years, with the CP-5555 still on the car, I've been running mostly slicks---Yoko, Michelin, Pirelli and A7 and BFG R1S. Yes, the car may be lighter than yours but I'm still running a ton of power with the blower which allows those higher speeds. I think that most would agree with the idea that I'm working the brakes quite hard. Years ago, I gave a very seasoned instructor a serious pucker moment at Willow Springs going into Turn 1. He got out of the car and smiled and said, "You do understand that you are using every bit of those brakes, right?" haha! And he didn't advise me to back off or anything. Anyway, I'm just trying to paint the picture that I'm not gently gliding around the track.

The most cruel place I have ever been for brakes is Thermal Club in California. Laguna Seca is sneaky on brake systems and tortures them, but South Palm at Thermal is another level of torture. It only takes about 1:20 to go around it, but in that short time, you have several straights that take you up to 120-135mph, and they all lead into low speed hairpins---40-45mph. Bleeding that much speed, so many times per lap, and being out there for 20 minutes......holy cow.....it is the most vicious thing I've seen for brakes. My friend has CP-5555 on his E90 M3 too. His calipers were painted Orange and had been on his car for awhile. They turned bronze in less than two days at Thermal. Anyway, both of our kits took the punishment and we never felt the pedal change.

I'm saying all this because I REALLY had a good run with the CP-5555 calipers. I never ever had a soft pedal-----EVER. And, again, I don't have brake ducting.

You might say, "well, if they're so good, why did you switch off them?" I got into this topic on my build thread in the Track section. Basically, my whole front kit needed to be rebuilt/refurbished. New rotors, new hats, new seals. The parts were 3 months out, and the total bill would have been $2000 to do all that. My parts could not ride out 3 months of active tracking because I was already below spec on the rotors, and the seals were popping out and the hats were 7 years old. And spending $2000 on an old kit and missing out on 3 months of the best time of year.....made me dream of doing something else. (Also, I can put the CP-5555 on another car.)

Anyway, I don't think you are asking too much of the CP5555 caliper. It is an excellent kit. In addition to looking at the brake fluid, perhaps you could use different brake pads as well. What brake pad do you use? (I've had very good luck with Cobalt Friction.)

[peter de la mare]

Wow, thanks for the lengthy reply. I actually follow your thread and I’ve read the most recent update on why you’ve ended up with a new brake kit.

Ok, I get that the CP5555 should cope, given my lower arrival speeds and lower temp climate, maybe the 710 fluid isn’t as good, or I’ve not changed it often enough. I’ve always used RS29 pads. I was running a standard rear brake setup for most of the time, but now have upgraded to the CP6602 system which should also help. SRF fluid it is.

[admranger]

Originally Posted by peter de la mare

Wow, thanks for the lengthy reply. I actually follow your thread and I’ve read the most recent update on why you’ve ended up with a new brake kit.

Ok, I get that the CP5555 should cope, given my lower arrival speeds and lower temp climate, maybe the 710 fluid isn’t as good, or I’ve not changed it often enough. I’ve always used RS29 pads. I was running a standard rear brake setup for most of the time, but now have upgraded to the CP6602 system which should also help. SRF fluid it is.

I'm with dogbone on this. SRF is amazing fluid. It's been around a long time but I'm sold on it. The only fluid that came close to it was Torque 700 due to its better pedal feel (SRF feels soft to me) but they went out of business (I think) and I still had to bleed after a track event w/the Torque fluid but not with the SRF.

I've heard the UK is a mildly moist so the high wet boiling point of SRF is your friend. :thumbs:

dogbone your brake swap logic is flawless. Now is not the time of year to lose track time in CA. If I were in the same position, I might have applied some fuzzy logic to get me to a PFC kit (but I'm a slut for PFC products, ymmv).

[dogbone]

Originally Posted by peter de la mare

Wow, thanks for the lengthy reply. I actually follow your thread and I’ve read the most recent update on why you’ve ended up with a new brake kit.

Ok, I get that the CP5555 should cope, given my lower arrival speeds and lower temp climate, maybe the 710 fluid isn’t as good, or I’ve not changed it often enough. I’ve always used RS29 pads. I was running a standard rear brake setup for most of the time, but now have upgraded to the CP6602 system which should also help. SRF fluid it is.

Sure. I hope it helps!

I still have the CP6602 on the rear (maybe not for much longer though....). It's been a good caliper. I don't like the pins that have to be hammered because that's just stupid, but it's a small price to pay for good performance. I don't know about the native British AP Racing version that you would get, but the Stillen USA version has "high temp" dust boots that crack immediately from---high temps with track use. I just ignore them. I was looking at the AP racing site. They also have a note for the rear calipers that says they're not designed to be used with the OE parking brake. Interesting, with my USA kit, I use my parking brake all the time when I put the car in the trailer. I wonder what that's about.

Anyway, I drove the car for several months on track with only the front AP CP5555 kit and stock rear. When I installed the rears, the car squatted better. Adding the rear wasn't a night-and-day difference in braking, but it was noticeable and it was welcome.

Regarding pads---I do wonder if you should try a different pad just as another variable in your experimentation. I would do fluid first, and then pads later. I'm not suggesting there's anything wrong with Pagid; I'm just saying that trying another pad would be another interesting facet of the experiment.

[dogbone]

Originally Posted by admranger

dogbone your brake swap logic is flawless. Now is not the time of year to lose track time in CA. If I were in the same position, I might have applied some fuzzy logic to get me to a PFC kit (but I'm a slut for PFC products, ymmv).

You know----fuzzy logic is the basis on which all track upgrades are based. It's the guy equivalent of the wife coming home after a lonnnnng day of clothes shopping and exclaiming, "Honey, I saved SO MUCH MONEY today!!"

For the track guys, it's like "Well honey, this $4000 mod will save me 2 seconds on the track! 2 seconds! Do you realize how much time that is? I've been scratching for tenths of a second! When considering cost versus time saved, this is by far the best value!---TOTALLY worth it! The kids don't need any dental work soon, do they?....."

[inTgr8r]

^

Been there done that

[dogbone]

Originally Posted by inTgr8r

^

Been there done that

[Redd]

If you're concerned about heat transfer from pad to fluid via the pistons, why not consider titanium brake shims? They do work.

[peter de la mare]

Originally Posted by Redd

If you're concerned about heat transfer from pad to fluid via the pistons, why not consider titanium brake shims? They do work.

Thanks, I'll check them out. I believe that stainless is less conductive than titanium, but a doubly whammy of stainless piston and ti shim might be even better?

[Redd]

Ti has 1/2 to 1/3rd the thermal conductivity of steel, and 1/10th that of aluminum. Plus a shim would be both cheap and easy to install.

[peter de la mare]

Originally Posted by Redd

Ti has 1/2 to 1/3rd the thermal conductivity of steel, and 1/10th that of aluminum. Plus a shim would be both cheap and easy to install.

Are you sure about that?

I’ve read a few pages that suggest stainless steel is the least conductive.

Approximate Thermal Conductivity of common caliper piston materials (BTU/hr-ft-F):

*Aluminum: 136
*Stainless Steel: 8
*Titanium: 13

[Redd]

https://www.engineeringtoolbox.com/t...als-d_858.html

[///Mobbin]

I run a Stoptech ST40 setup in the front still, OE rear, and used to run Motul 600 and 660 with PFC08 pads. After boiling my fluid a few times, and encountering some pedal drop to the floor excitement, I knew I needed to change some things with my setup. I wasn't ready to route cooling ducts yet so I decided to maximize what I had. I switched to Castrol SRF, Pagid pads, and 1mm Ti shims. The Pagid pads have some ceramic content which helps keep some heat out of the fluid. Then the Ti shims don't fit with new pads but I add them to my setup the next time I go out. New thick pads tend to be good enough when the pads are new.

Maybe marginal improvements from each item but together they worked and solved my problem. Of course this is still with street tires, Cup 2's, RE-71r's, etc. I am fairly certain that for more aggressive tires I will need to upgrade my brake setup again.

Another anecdote, once you upgrade your suspension or at least your spring rates, the rear brakes contribute more to the overall braking force and take some of the work away from the front. This can help and is a good reason to also run rear track pads at this point.

Another thought, I had been running Pagid RS-14 blacks up front but find they lock up my street tires more easily than I'd like, saving them for a more aggressive tire setup and going with RS-29 fronts and RS-19 rears for now. I may also try Cobalt Friction at some point since everyone seems to love them.

Hope this helps.

[///Mobbin]

Also, when I do upgrade the 9668's are definitely on my list, thanks for all the details shared on these recently by Dogbone and others!

I'm curious if they make a similar rear kit or if they are planning one? I'm weird and like things to match. (Yes, I know I never bought the rear ST40 kit lol, but I planned to!)

[dogbone]

Originally Posted by ///Mobbin

Also, when I do upgrade the 9668's are definitely on my list, thanks for all the details shared on these recently by Dogbone and others!

I'm curious if they make a similar rear kit or if they are planning one? I'm weird and like things to match. (Yes, I know I never bought the rear ST40 kit lol, but I planned to!)

Essex has created a Radi-Cal rear kit----the 9449 caliper. (I've been considering upgrading to it.) I've seen the rear kit at Racewerkz. The caliper is actually pretty compact. And the rotor is 340mm, which is not massive. For a BBK, I'm assuming it's got to be one of the lighter offerings out there for the rear. You can check out the kit here:

https://www.essexparts.com/essex-des...49-34028-e90m3

[jritt@essex]

Originally Posted by peter de la mare

I’m due to refurbish my CP5555 calipers next month and I’m curious if anyone has swapped the standard aluminium pistons for stainless steel ones and if any improvement in heat management has been noticed?

I have cooling ducts already and run 710 fluid, but sometimes I think more could be done.

Stainless pistons are always going to work better on the track than aluminum, as stainless conducts heat much more slowly than aluminum. As dogbone mentioned, the CP5555 (road) and CP5060 (competition) are essentially the same core caliper, although are many variants of each. AP Racing used to offer stainless pistons that would work in the CP5555, but I honestly don't know if they still do. We haven't sold the 5555's in quite a few years. If you'd like me to look into that for you, shoot me a PM. I'll see if I can dig something up. If you'd rather upgrade to one of our newer systems, please let me know what questions you have. AP's new Radi-CAL Pro5000R line replaces the older Pro5000 line, and the CP9660/9668 use the same pad shape as your CP5555 caliper. Thanks!

ps FYI....all of our Competition Brake Kits that use AP Racing Radi-CAL's will increase in price by $300 per axle starting on March 1st. We were hit with a sizeable cost increase on the Pro5000R range, and we unfortunately have to raise our prices a bit. If you're on the fence, now is a great time to buy. Thanks again.

[jritt@essex]

Originally Posted by peter de la mare

Thanks, I'll check them out. I believe that stainless is less conductive than titanium, but a doubly whammy of stainless piston and ti shim might be even better?

We pretty much never have problems with fluid boiling with the ventilated stainless steel pistons in our Pro5000R Radi-CAL range. Sure, if you put them on a small enough disc on the right car, it could be done. But with our standard caliper/disc combos we use on various applications, it just doesn't happen. We typically make sure that the discs in our system are large enough to handle the heat without pesky ducts. Ti shims are pretty much pointless with our Competition Kits. Ti shims have a much greater impact when used in a kit with aluminum pistons like a StopTech Kit or Brembo GT kit.

The calipers we provide to pro race teams typically have titanium pistons, but the primary benefit of those is weight. They aren't terribly different than stainless in terms of heat rejection. They are much lighter though, hence their use in professional motorsports. The cost on ti is huge compared to stainless though. For reference, each titanium piston in an IMSA or NASCAR caliper costs about $200...that's each piston. That's about four times more expensive than a stainless piston.

Finally, we always recommend starting without ducts when installing our system, and rarely do people need them. In many cases, home-brew or aftermarket duct systems hurt our components more than help them because of poor implementation.

[peter de la mare]

Thanks for the replies and info

I'm in the UK, I've sourced a set of s/s pistons, sadly AP don't offer a vented version so these will have to do for this year. As I've invested in these calipers for 2018 by way of a change of colour and new seals I shall have to stick with them until next winter.

Maybe the exchange rate will have gone back to the $2 per £1 level I shan't hold my breath however.

I hear you about the s/s and Ti shims, I'll abandon that idea.