Locking the fronts only with my Big Red's
 

1988 Carrera, with a Big Red Kit from Race Technologies circa 2004 with a price tag of about $5700 Race Technologies I have attached the invoice because when I tried to figure out exactly what I had it never came up as the part number is no longer any good. I think its a kit that is no longer available. The rotors are cross drilled which seems really blingy to me,so I don't get that part either.

The P.O installed these about 10 years and 30K miles before selling me the car.

I have never driven a Carrera with stock brakes hard,so I don't have a baseline to compare to. For a few different reasons I have on more than one occasion considered removing the kit and re installing the stock brakes(came with car)

If you look at the invoice attached it looks as though a 23.81mm 930 Master has also been installed.

I am looking for some advice on dealing with front wheel lock up that has me nervous about pushing the car. I have always driven a heavy sedan, and like the feel of lots of rear brake so it may just be me adjusting to a rear engine car. Either way,I want more rear brake and less lock.

Tires on the car admittedly are not the best,so i think I could hide some of the problem with new rubber, however I believe that the car should have a more neutral or rear biased brake feel than it currently does.

How can I get there?? OK, I couldn't upload the invoice so here are the Part #'s 1B16001R2 BREMBO BIG RED GTP2 FRONT KIT 2C16002R2 BREMBO BIG RED GTP2 REAR KIT 93035501103 MASTER CYLINDER






http://forums.pelicanparts.com/uploa...1390706137.jpg

If it has an adjustable bias valve (it should) you can try opening it a bit to increase the rear bite. If they are so unbalanced that you can't find a satisfactory setting, you will need rear calipers with more piston area or fronts with less.

regards,
Phil

Too much front brake. Adjustable bias or more aggressive rear brakes......

Bias is not adjustable

Great looking car...you need to have a rear bias installed in the rear.. If it were me, I would call Race Tech for advice. They are great to deal with. If your in Van, you might look up a Porsche shop in Langley - I think the owner is Jeff ...as he post here.
Good luck

I have been in contact with Jeff to ask about my cars set up. The PO had given me a ledge file of records and notes with the car,but couldn't remember much about the set up, Jeff's name was mentioned several times.

Its come to my attention that maybe some info on the suspension might be helpful in solving this issue.

As far as I know, the car has upgraded torsion bars ( 21/27mm )

Shocks are Bilstein sports, I believe the car is sitting at Euro ride height. Has 17x8 and 17x9 wheels with 225/45 and 255/40 tires. Im still getting used to the car, and the tires are done.

Fronts seem to lock under any panic stop and at anything over about 60%. From 60MPH if you jump on the pedal, rather than haul the car down the fronts just lock. I can modulate around it,but it feels like a recipe for me to crash at my first track day with the car.

I'm no expert, but fronts should not lock up while stopping from 60 mph. You really need to get a brake bias to control the rears...that's what I did when I put the same set- up on my 914.

I think brake bias control is so you can limit the amount of rear brake,seems to me we need the opposite here.

I need less front brake and more rear.

Correct. I adjusted the rear until it would slightly lock up.

One other possibilities is that the fronts have more aggressive compound pads.

You really need to get a brake bias to control the rears-

With no rear bias control in place,I currently have full rear braking capacity. Adding a bias adjuster to the rear circuit will not increase the amount of rear brake,it will simply allow me to dial more rear brake out,which would make the current condition worse.

Agreed that pad selection may also have an effect, I believe the pads in place currently are stock OEM front and rear

You can install a valve in the brake line....

Interesting problem, how much rear brake do you really want? Having the fronts lockup before the rears is more preferable to having the rears lockup first on a 911, but it sounds like you have way too much capability up front. Do both fronts lock evenly and does the car stay straight? Time to try harder pads up front and softer pads at the rear if you want to reduce that front lockup, otherwise stock brakes will restore the factory setup and balance. Having said that, a call to Race Technologies might solve the problem quicker, maybe there was a rear bias valve fitted with the kit and its seized or otherwise faulty, have you eyeballed the entire brake system? A rear bias valve could be tucked away out of sight above the gearbox somewhere. Got to get it fixed, great brakes are a big part of the Porsche experience.

You have the GTP brakes, NOT the Big Red (aka 993TT brakes). These have a fair amount of built-in front bias and along with the front Sport Bilsteins (too much front bump valving), aggravate the propensity for front lockup.

These are really nice brakes that may simply need the addition of a Tilton manual bias adjuster (lever style) into the front brake circuit to provide the adjustability you need.

Before you do ANYTHING, make sure the factory rear brake pressure limiter thats standard on all Carrera's has been removed. :) :)

There are other things to do as well, however I cannot offer anything further without knowing something about your driving skills and experience.

This is great Steve- can you tell me te difference between the GTP and the 993 TT?

Where is the factory brake limiter located? I think you may have hit on something previously overlooked.

My experience has been in solo 1 and 2 since 1999, but all of that was done in a car I built myself, a 600HP early second gen Fbody. A very challenging car to be quick in,but easy for me to control, and totally different from a 911.

I have no skill or experience in a 911, but love the car so far. I miss the power oversteer of my old car, but the 911 feels much more stable at higher speeds.

I'm used to left foot braking in my old car to help it rotate, so I like having substantial rear brake. That being said I like all 4 wheels to lock if that going to happen at all and with this car it's very clearly just the fronts long before the rears come to the party. Time to look a little closer for a hidden bias valve.

Two suggestions. Tires that have little grip will make this worse, as they lock before you transfer much weight to the front. New tires would help.

Secondly, you may be able to change your technique a little. Be more progressive in getting on the brakes. Jumping on them hard is not what you want to do with those.

JR

the carrera bias valve is under the triangle cardboard piece between the master cylinder and the smuggler's box.

I've looked at 3 different GTP setups not counting the GTP/L which was pretty nice

Most likely what you have is the Kit w/ F40 front calipers, and Lotus rear calipers on 993tt 322mm rotors f/r w/ a 930 m/c

I have feedback from multiple owners, all of it along the lines of what the OP has reported, some have led to serious accidents

torque bias for the F40/Lotus GTP kit is very much front 2.03, for referencea ll 911 before Porsche started messing w/ bias in '84 were 1.491

You should never use a p/v on the front

and w/ your current setup be sure to remove the stock one on the rear circuit

there are 2 things that you can do to alleviate the issue
1) lower the front bias w/ a caliper w/ smaller pistons, a 993RS/tt front(aka Big Red) bolts on w/the correct adapter and lowers torque bias to 1.856
2) raise rear bias, a 993 rear caliper can be adapted w/ a little work, a 965 rear can be adapted w/ a little less work, either of these will lower the bias w/ 993RS/tt front to 1.520. A 993RS rear can also be adapted and is far more difficult to do but that will lower the bias to 1.426

I don't understand why RaceTech sold that kit, the previous version was Big Red(993tt/RS) all around and was fantastic as long as you got the 993RS rear calipers, form what I understand some came w/ 993 tt rears which would be as bad as the GTP kit

I'm going to make a second call to race tech on Monday and see what they have to say.
Bill, from a performance perspective, would I be better served re installing the OEM Carrera brakes? Are there some simple mods that could make the stock pieces more effective? They did come with the car as well.

Unless you are tracking the car yes, stock Carrera are going to be fine, the bigger brakes mostly give you thermal capacity that is never used on the street, you do get more brake torque but since stock brakes can lock the wheels and it's the tires that are stopping the car it's not really useable w/o all the other track car mods

The 993RS big red kit from Racetech sure looks cool though and they are great for a track car

my car
http://forums.pelicanparts.com/uploa...1390754855.jpg

same brakes on my other car
http://forums.pelicanparts.com/uploa...1390754995.jpg

Bill, I will be tracking and autocrossing the car.

For A/X I'd want Carrera maybe even w/o the p/v to get the rear bias to help rotate the car

for bi track use heat is always an issue w/ Carrera brakes but lots of people manage to deal w/ it

what you have, IMO, is dangerous

Sounds like we are on the same page here, and that maybe backdating to the original brakes will yield the best result. I would need new rotors, what would you recommend in that department?

For 3.2 Carrera rotors, stock or Zimmerman coated

So people aren't confused (or chose to ignore his advice) I want to layout WHY you never put a proportioning valve in the front brake circuit of a car.

A proportioning valve works by reducing the pressure that is downstream of the valve. It can be by a set percentage of pressure, a set psi of pressure, or a combination of the two. The important part is that it is reducing the pressure and one of the ramifications is that the pressure to the caliper can not be released without releasing the upstream pressure to the point where it is less then the downstream pressure.

It is very important that you be able to modulate your front brakes. They do most of the stopping. As an easy example let's say you have front brakes that lock up with 400 PSI of brake pressure and you have a proportioning valve that cuts off 20% of the line pressure.
You start braking hard and lock the fronts by applying 400 PSI to the calipers. Normally you would just reduce the pedal force by a slight amount but with a proportioning valve the upstream line feeding the fronts was at 500 PSI and until it is reduced to 400 PSI none of the pressure to the fronts will be released. And when it does start releasing the release will be 20% less then you would expect from the pedal feel.
After you have released the pressure and you want to increase the pressure you first have to build the upstream pressure by 20% before you get any increase in pressure to the caliper. In other words there isn't any real modulation of the front brakes.

A proportioning valve is an obvious kludge to balance the brakes. Most cars have a much higher percentage of weight on front end. Add that to the weight transfer to the front end and it usually works out to 70% or more of the brakes are on the front. This means that such a small percentage of the braking is done by the rears that the bad characteristics of a rear proportioning valve isn't noticeable.

One of the really nice things about the brakes on an early 911 is there is no proportioning valve. This gives you wonderful brake modulation and great feel right out of the box. Messing with that balance really takes a lot of thought. (and a calculator!)

Westy

Use the search function here with something like "Carrera brake bias valve." You should find some pictures, and a graph. What this amazingly small unit does is allow the pressure your foot produces coming out of the master cylinder to climb in a straight line on a graph as if no valve were there, so front and rear calipers get the same pressure, up to a set pressure. After that, the front pressure climbs just as it did before, but the line to the rear flattens. I don't recall if it is just limited and never gets higher, or if it just rises at a lesser rate. There is an orfice in this valve, and it can be changed. I believe Porsche used different orfices on some later cars, so guys with the stock setup can play around a little.

If I understand Wayne's post, it sounds like this approach (and the aftermarket bias valves) works for the rear, because you use it to prevent rear lockup at threshold braking. Which means you never have to modulate your braking to avoid lockup in the rear.

The proper race car setup involves an adjustable bias bar on a dual master cylinder system, but you don't want to go there.

In fact, the advice you have been given about reverting to stock (at least for now) is very good:

For autocross, stock unmolested brakes are just fine.

For beginner track use, the same is true. Stock is fine. As you get quicker, you would switch to a better brake fluid - say one with a 600 degree F dry boiling temperature, or even better. As you get quicker yet, you remove the brake dust shields, duct air to the front brakes and install blockoff plates so the air can't just blow out the spokes of the hub. If you do this right, you can race this way. PCA Club Racing Stock class rules require the stock brakes, and guys do just fine in the 3.2s in the brake department as long as they pay careful attention to cooling and really good fluid.

Now if you have pumped this engine up to 320 HP, that's a different matter. More speed = more heat when you have to brake.

Bill has never "steered" me in the wrong direction.
: )

The proper race car setup involves an adjustable bias bar on a dual master cylinder system, but you don't want to go there.

I'm just wondering if that is the ultimate solution here? While I can remove the current set up and revert to stock, if ultimately I will need to upgrade perhaps just using a dual master now would set things right.

I will be calling RaceTech tomorrow to see what they have to say.

As always, I really appreciate all the wisdom shared here.

Unless I miss my guess, you'll lose your power braking (vacuum boost) with a dual MC, although with the MC up in the trunk it has to be easier to install than on earlier cars with the MC down by the steering. But perhaps there is some other kind of power brake assist which will boost a dual MC and bias bar setup. All of which may be more work and expense than just going back to stock?

As an another data point, I installed '88 Carrera brakes in my '74 15 years ago and had to install a stock bias valve in the rear line. After many years of track use, I upgraded to 993TT front/rear with the proper 23mm MC. I then had to remove the rear bias valve and my brakes work just fine under track use. The front pads wear slightly more than the rears but the rears are definitely working. I agree, the first thing to try is to remove the stock bias valve, if it's still there.

The dual master setup provides the ultimate in adjustability

it adjusts 2 ways
1) bias bar for small changes
2) different m/c for the front and rear circuits so that each can be sized as needed

Oh and for all the ones I've seen for 911 installation you do lose the vacuum assist

Update,

So I called RaceTech today and they were very interested in being helpful. I explained the situation, and gave them all the part numbers.

The Front Calipers are as Bill suggested, F40 Calipers AKA "B" callipers as noted by the B in the part number. Originally designed for use on the Ferrari F40.

The B calliper comes in both small and large bore, 3640mm is the application in this case, and is considered the small bore.

The rear Kit is known as the Mini Jag, or "C" calliper and originally designed for Jaguar.

The C calliper for the rear has a 2840mm bore.

I found the OEM bias valve in place under the hood as some may have suspected.

We are assuming at this point that the pads are the standard issue Ferrodo front and rear.

According to RaceTech they sold about 100 of these kits and never had one complaint of this type. That being said they were happy to look into it further. We agreed that the first thing would be to check and confirm that I did receive the small bore callipers in my kit as they are easily mixed up. I took some pictures and sent them off so they could compare.

A few hours later, I had a call back from race tech looking for some more detailed info about whats been happening, and a more detailed description of my car and its modifications. I was really surprised, and delighted by this level of concern on their behalf. The tech took all of my info and will be speaking with the owner of the company about it who is reportedly very experienced with Porsche/Brembo braking and has been personally racing for many years.

I'm very happy to see they are giving you good customer service. Seems to be a huge exception to the rule these days, good for them.

Question for Bill (and some of these other guys who know this set up well), are you guys thinking that this just too much of a mis-match to fix with changing pads, aggressive in the rear, and sort of minimally aggressive in the front?

Obviously, correction from the supplier w/o too much $ outlay would be best but, when I think of the cost of some of the changes that have been mentioned, it sounds SERIOUS!

I too have massive Brembos installed by PO w/o much info but, my brake balance seems very good. One thing I've noticed is that you really do need self control when you have this much brake. If someone's used to abs or maybe just having a big v8 sitting right over the front wheels, there's no doubt it's a serious adjustment mentally. Looking forward to resolution!

cd
'82 Targa

The one way to do it while retaining the power steering is use the Tilton 72-610 mount. I thought about going this way back when I was upgrading my system. It would require that you add a bellcrank so you could mount it 90° to the side of the booster.
http://forums.pelicanparts.com/uploa...1390905191.jpg

In my case I ultimately really wanted to lose the power booster as it eats some of the sensitivity you get from the dual master setup. I ended up putting in the Smart Racing setup and have been loving it for more then 10 years.

Ok that changes things a lot as I said there were several different versions of that setup

I suspect that the rear bore is 28/30, I have never heard of a 28/40, please double check that.

assuming 40/36 front and 28/30 rear on 322mm rotors torque@70bar is is 2067nm 1241nm and bias is 1.666

that is much closer to where you want to be, you definitely need to remove the p/v as that cuts in at 33bar and makes the front bias much higher from that point up.

you can further trim bias w/ pad selection
for example Pagid RS 19 fronts and 29 or 14 rear
or 29 front 14 rear

Hi Bill,

My mistake, just checked my notes and yes its 28/30 for the rears. With regards to pad selection, which of your recommendations could be considered street able if any?

For the p/v removal, would you suggest a simple union, or something adjustable?


Cheers!

well I use Pagid RS29 f/r on the 993 and don't swp them out for street use, I have no issues w/ them, but I don't drive in temps below 40F

On the C3 I have used Pagid sort blue, but that's not really a track pad

if native bias is ~ 1.6 then I think that 19 front and 29 rear would move a little bias to the rear w/ issue.

I've found that the trick to living w/ track pads on the street is to keep them clean and never put them away wet

The forum strikes again, or at least BV.

Wayne - that Tilton part looks like it is for a non-powered dual MC setup. Wouldn't the big pie plate vacuum boosters bump into each other if you tried to mount two MCs more or less side by side? Mounting a dual MC on an SC or 3.2 is pretty easy, what with the adapters out there. But power seems harder with the older vacuum systems.

I agree with you that power brakes can be suboptimal. On my SC, to which I added the stock power brake setup several years ago (long story) no longer allows me to just kiss the brakes where a fast sweeper requires something (or the driver does for confidence), but not the loss of much speed. I always seem to get a bit more than I want in those situations.