Thanks. Found the answer:
“As derived above, the bulk modulus is directly related the interatomic potential and volume per atoms. We can further evaluate the interatomic potential to connect K with other properties. Usually, the interatomic potential can be expressed as a function of distance that has two terms, one term for attraction and another term for repulsion.”

Are you suggesting adding a bulk modulus column to the brake fluid comparison charts?

Count me in. 😐

Wow, that is some light reading. :rolleyes:

Everything will compress, even diamonds, just not very much and it takes a lot of pressure. So yea, liquid does compress, but for a brake system, it is unnoticeable for the driver.

Just ordered one bottle. Hopefully I didn’t wait too long.

Oh, so you're the person who read the blog... ;)

Car looks great!

I did not document my track day stuff on the blog. I've only had the car on track two or three times. Funny though, I cooked the brakes, but it was pad selection (Hawk HPS) not fluid issues. Car was a real sweetheart on the track, I went with the intentions to just putt around at 85%, ended up getting comfortable enough to push it pretty hard. Hard enough to, uh, cook the brakes. Car felt really understressed and in its element out there, like it could lap for days on end if the consumables would hold up.

Did you change the R&P or the actual gears? In SCCA STR which I was staying legal for we can't regear, so I've switched to 15" wheels to get a similar effect.

Haha yeah that was me :)

I did actually change 2nd and 3rd gears. I run with our local club and we are pretty (actually, very) isolated. Our classes are based on SCCA street classes broken into 5 groups, and various modifications (including ranges of treadwear) are assigned points that will then bump you up a class if you accumulate points. My car started in B class but I have enough mods that I am running A class so I can do whatever I want and stay legal. You can imagine the temptation..... I'm running on RE71R's so I'm giving up a bit on tires but we often have cold(ish) weather and the one guy running Hoosiers seems to struggle on those days. I managed to win the class in 2019, main competition is a Lotus Elise (on hoosiers) and a 500+ hp Subie STI.

(DISCLAIMER: I realize you are generally knowledgeable and probably know 95% of the below info but for the benefit of others who might not know I've included some basic info)

Your experience with brake pads is pretty typical for first time track users, right down to the "I'll take is easy" part. I'm not surprised that you cooked the HPS pads, I'm running HP+ on the Rot Rod, if you check the compound chart against the HPS you'll see they have a better cF at every temperature and when they fall off at high temp they don't fall off completely:

http://forums.pelicanparts.com/uploa...1580479739.jpg

Not that those temps do not equate to fluid temp, damn near any decent Dot 4 fluid is fine. Talking about brake fluid is tantamount to motor oil. Everyone has a favorite....

There is another huge factor at work and that's the Carrera brake setup - my apologies if you've already addressed this, but the pressure limiting valve in the 3.2 brake system screws up the performance of the car!!! Very un-Porsche like if you ask me but they increased the size of the rear caliper pistons in 84, so that the Doctor and Lawyer crowd would get more life out of front pads and be more likely to have to change fronts and rear at the same time (or after the warranty period?).

Of course the pressure limiting valve was chosen to conservatively limit rear brake pressure at the limit and maintains brake bias to the front under high brake force. I found my car was highly prone to front lockup at auto-x until I removed the pressure limiter and swapped to earlier calipers. Huge improvement in brake bias, and no more front lockup. With this change alone you might find the HPS pads are fine on track, because the rear pads will actually be doing more work at the limit.

Sorry for the rant! What 15" setup did you end up with? Are there STR legal tires that work for you in the small sizes? Did you ever upgrade the front dampers?

Oh man we've derailed this thread.

I had my OE Bilsteins revalved by Elephant, then swapped the rears to Hotbits double adjustables. I run 15x7/15x9 fake Fuchs with 225/245 Rival S. I much prefer the car on the Rival - the tire is more tolerant of slip angle and the car seems to really wake up with rear slip angle. I have the car set up pretty loose right now - OE front torsions / 33 rears and it's the most fun it has ever been.

Nope, going out today.
Thanks!

As a few answers have said...it kinda depends. but here is some real world usage and what we and our customers experience.

If you're going to go on the track, use a quality DOT4 fluid. SRF, Motul, ATE, AP, Willwood, etc. All are good.

A Higher wet boiling point (WBP) allows you to not have to FLUSH the system very often - just a quick bleed after a weekend or two. As the post from the Wilwood info indicates, all fluids will get some moisture in them over time. Sure, you can flush the system every time just before a track weekend and you'll be fine, but there's no need. Higher wbp means that even though some moisture has gotten into the fluid, the fluid can handle it for longer and at higher temps. Many (most) PCA racers I know that use SRF only flush the system once a year. Quick bleed before each race weekend is it.

So while SRF is certainly more expensive...it effectively lasts longer so it may actually be cheaper in the long run.

Again, this is going from what many of us do in PCA racing, and what I see in BMW and SCCA racing.

If you actually WANT to flush the fluid out more often, go with the highest DRY boiling point, since the fluid will never really have time to get much moisture in it.

Answer is, It depends on the thermal stress levels seen in use. Back in the '70s before miracle pads, & brake fluid iused to go through a set of pads and need to flush the brakes after a single day at LRP, the car was a Carrera 3.0 so similar to what VFR has, these brakes are stressed especially so when the driver is ham footed about using them. Flash forward to The 21st century, track car is a well set up 993, I can use ATE200 all season w/o a flush or bleed at any track I've been to, w/ a single exception, the group was black flagged at WGI after ~40min in an hour session on a 95F day, sitting on pit road waiting to go back out w/ foot lightly on the brakes to prevent rolling, the fluid boiled and I had a very soft pedal to the end of the session, reason was the pads were worn way below where I usually change them out. I switched to Motul 600 as a precaution.

So for me the first column in the table I posted which is wet boiling point, is the more important one, for those that flush and bleed regularly the next column dry boiling pt might be more important, the thing to remember is that all the b/fss here aggressively pull water from atmosphere and a week or 2 after a flush wet is the state of the fluid, especially so during eastern summers where humidity is high.

Each car and driver and use is going to be different and will require a solution that may be different from others. If A/x is your thing I'd be surprised if you needed any exotic b/f the brakes just don't need to process the thermal loads seen at at a track in a long session.

while I'm sure that there is some variation in b/f compressibility I've never been able to detect a difference even when I used Prospeed 683 b/f which goes out of it's way to tout it's compressibility advantage

A mushy feeling usually comes from hysteresis in the system, calipers flex, lines flex mounts flex, any gas in the fluid. Some guys w/ big brake upgrades and small oe lines experience it and of course pad knock back

Castrol SRF
 

Hello all,

There are a few stores that have them. It appears they have limited supply in the U.S. Please PM me for further info, I can't post the names.

Best to all

Jose

Thanks for the chart/list Bill.
Where does ATE fit into this mix?
(Says on the can boiling point is 500 deg F, (wet - 329 deg F)
Have I been using an inferior fluid for the last 30 years in my street and track cars?
Bill K

Never mind - I see the answer above. Makes sense.

http://forums.pelicanparts.com/uploa...1580671951.gif

Why no Brembo HTC64T on the list?

It's wet/dry numbers of 536/635 put it at the top of the list....

https://www.hrpworld.com/bre-htc64-brembo-htc64-brake-fluid-1-2-liter-bottl.html

Yeah I guess we have - apologies!!

I find your front/rear spring rates interesting, they make sense to me. I've always been perplexed by the fact that everyone seems to up the front rates by a proportionally higher amount than the rear, when it seems like the last thing the car needs. I'm seriously considering 29's or 30's for rear, and then using the 28's in the project car.

Thanks for sharing the details. Are you still running STR?

No, I don't think there's any point into running anything other than a 2019+ Miata in STR anymore. I do run the car at locals, but drive a boring but effective GS car at national events now.