jscott82

Bill, You got me thinking.... I think we were saying the same thing only differently ( and you are more accurate).

These are limiter valves (ie: at low brake pressures there is no diff). I read somewhere that 993 c2 begins limiting at 40 bar (580psi), but never knew what are normal brake pressures under race conditions. I assumed it was less than 40 bar, I could not have been more wrong….. A quick post on the racing forum to ask the folks who log their brake pressures, they are saying normal race pressure may be 1200 PSI going up as high s 1900psi. So yes to my surprise, the limiters are in full effect under race conditions….

Bill Verburg

993C4, C4S, RS and C2 all use 1 or 2 40bar .46 reduction factor p/v

993tt uses 55bar .46 x2, there is some confusion in the parts books, some references say turbo and awd use 2 others 1. It would be interesting to see what owners observe in their cars. same thing w/ the C4S rear calipers, some references say they are 28/30 others 28/28.

the term regulator that Joel used is right out of the parts book, that's what the p/v part is listed as.

jscott82

Good info from the racing forum...
https://rennlist.com/forums/showthre...ferrerid=45114

Bill Verburg

I'm curious as to the reason you seem to believe anyone but me?
the person that wrote that is correct as to the knees of 33, 40, 45, 55, 60 which were used on one model 964/993 or another but the reduction factor for all of them is 46%

don't know about the 18 one, that is very powerfull

Stoptech has a nice piece on the way these work.

trophy

That Stoptech article is a good one, thanks Bill.

ninjabones

All very interesting stuff... with the OEM ABS programmed for the masses, favoring vehicle stability over braking performance (and set-up for a car with cushy suspension, street tires, street braking compounds, etc)... and rear p/v's set-up to optimize front/rear bias under basically street conditions... I wonder what would turn out to be a better set-up for a race prepped 993 (assuming full-on race set-up with stiff springs, aero, slicks, etc )

1) stock system

or

2) dual master cylinder set-up, adjustable bias bar, and eliminate the ABS and rear p/v's ?????

I wonder if the ability to optimize front/rear bias under varying conditions (to compensate for fuel load, track conditions, tire grip levels, etc) with the dual master/bias bar setup would offset the benefits of a less-than-optimal ABS system.

jscott82

You need more facts and figures..... just kidding

I believe you.... just confused on the diff (or lack thereof) between proportioning valve vs. limiter/regulator. I alway thought there was such thing as a proportioning valve that begings its curve at 0psi.

Great article..... thank you

jscott82

My money would be on the dual MC setup.... At least on our cars, the only thing ABS brings to the table is unlocking the wheel for you. If you over-brake and are into ABS then you are not stopping as fast as you could have had not over-braked in the first place.
With the dual MC setup you can dial maximum braking into any conditions. But without ABS you need to develop the skill to unlock the tire otherwise our tire budget goes through the roof....

Just my 2 cents.

ninjabones

If that were the case, then none of the top-level racing cars would use ABS. There are a lot of advantages to abs, most notably allowing max braking effort in turns. Without abs, your braking performance is limited to the point when the inside/unweighted wheel starts to slip. Without ABS, the driver has to limit braking pressure to all wheels equaly, despite the fact that only one wheel might be slipping. That leaves a lot of potential braking performance unused and on the table. With ABS preventing lock-up of that single inside/unweighted wheel, the driver can continue to exhert braking effort to the remaining wheels, thus increasing overall braking performance. However, as I mentioned, a street oriented ABS is not optimized for racing conditions (being designed for a 3200 lb car, with soft suspension, street tires and for drivers that might not be able to handle a bit of instability). I'm really not sure how much benefit we get from the stock ABS once we've modded our cars to this level... but I would surely appreciate the input of anyone who has high level knowledge/experience in this.

Bill Verburg

There are p/v's that have a knee at 0, the knee is determined by a spring in the p/v, the reduction by varying orifice areas, an adjustable p/v just varies the preload on the spring which changes the knee pressure.

just as a fer instance all the late Cup cars have eschewed abs in favor of twin master setups, some owners are retrofitting 996 abs.

Personally at my level I like abs, the thing I don't like is the need to be more concerned about the front vs rear tire diameter and grip, guys w/o abs can more freely choose tires, some even go so far as to use different brands front and rear.

The guys that have had some real problems are the Cayman owners, ever heard of their 'black ice' issues?

Martin S.

I concur, Pagid Yellows on Front, Blacks on rear, as well as the 993RS rear calipers. These calipers have larger pistons as I understand it. Steve Weiner is too much of a gentleman to mention that he sells these calipers, but I am certain he can get them for you.

Note, I have been tracking "Baby" since 2001. I have never experienced a momentary lock up before the ABS kicked in...also note, things would be much worse if you didn't have the "Weissach" rear suspension we see on all 993 cars.

aribop

This is all great information. I appreciate everyone's input!

TheOtherEric

We definitely got different answers after going thru the math. We'd have to both show our work to determine why the discrepancy. If I get a chance I'll pull up the spreadsheet later.

But looking at your brake info page, I already see some discrepancies. My Big Red rears have 30mm and 36mm pistons, larger than what you have listed. Weird.

Bill Verburg

I doubt that you will find a mistake on my brake page, but if you do please do let me know about it, I also doubt that there is a mistake in my calculations, but again....

One problem that you are having is correctly identifying RS and tt calipers

tt rear are 28/28, RS rear are 30/36 other wise they are the same caliper and use the same pad as the normal 993 front.

TheOtherEric

Yeah, that's my point. My TT/4S rears are 30/36 not 28/28. I measured them myself and rebuilt them with those parts. It's *feasible* that when I bought the 4 calipers from LAPD for $1200 that they somehow mistakenly sent me RS rears, but it seems highly unlikely.

How sure are you about TT rears being 28/28?