1988 Porsche 911 race car brake problem

[Charles A. T.]

Hi guys,

I have a brake problem to solve the 911 race car I'm going to race this summer (not my car - a friend drives it in DE's and I will race it).

First, let me explain what's the car:

1988 911, 2200lbs widebody, 3.6L with PMO carbs. G50 5 speed. Still on torsion bars with RSR shocks from Elephant and other bits (bushings, bump steer kit...). Used to run on 18 inches wheels. Will now run on 15 inches Fuchs copies (Braid) (245-40 and 275-35 Hoosiers R7).

The brake set-up is stock rears and 2005 Boxster calipers at the front. All the rest or the set-up is stock.

The problem: when required to brake hard coming from a high speed straight or high speed turn to a slow corner, the interior wheel just want to keep on locking (badly).

What I'm doing first: changing the brake fluid, the brake pads, discs if necessary and will have him install steel braided lines.

The other thing I'm tempted to do is to change the factory brake proportionning valve like I did on my 944 (in that case to prevent the rears to lock up in the same situation).

Any idea which one would fit best for this brake set-up? I know that Bill Verberg might have an answer for that...

Thanks

c.

[Sboxin]

Nice car . . .

A little more information on the tire brand/type and brake pad brand, etc would be helpful ??

We run Raybestos AP45 ST-43 front pads SC Calipers and Pagid Black rear pads
Carrera calipers on slotted rotors with Hoosier R/A7s 245/40/17 front and 315/35/17
rear. The '78 911 has power assisted brakes with stock master cylinder and proportions.
Car is 2550 gross race weight. When tires/brakes are cold always have an inside tire
lock up problem . . . Son/driver is just extra careful on pedal pressure during the warm
up period . . .

Personally, I think you have way too much front brake for this light weight car.
Hopefully Bill V. will provide you with some helpful information and analysis.

Regards,

[Charles A. T.]

Hi Roy,

I'm waiting for the brake pad info... Will be racing on Hoosiers R7.

Stock master and brake proportionning valve.

The problem is not only when it's cold, it's always.

I too I think we should just go back to stock front calipers. But if there's a way to cure this problem with a valve, it would just be easier ad cheaper.

Thanks

c.

[911 Rod]

Are you sure the problem isn't that the wheel that isn't locking up isn't working properly?

[stownsen914]

You don't say anything about spinning or squirreliness, so I assume that it's the front inner wheel that's locking and not the rear. Since you put substantially larger calipers on the front with stock rears, I'd guess you've thrown off the bias and it's to front biased now. Other than getting larger rear calipers, or a dual master cylinder setup to allow proper bias adjustment, others may have suggestions for fixes like replacing whatever proportioning valve you have or running a grippier pad compound on the rear than in the front.

Also, it happens the same on both sides of the car, right?

Scott

[Eagledriver]

Those of us that race Spec911, have found that running stock Carrera brakes or even stock 930 brakes without the stock proportioning valve is a good solution. Race cars are so much lower than Street Cars that they can run much more rear bias. I think you will have a good balance with your Boxster front, Carrera rear if you remove the proportion valve.

-Andy

[roblav]

I'm biased, but a track car ought to have two master cylinders with a driver adjustable bias knob on the dash.

And if you added front calipers that have a different cylinder/piston size than stock, you messed up the bias.

[Imcoz]

I second the removal of the stock P valve... seems to have worked well on my 88 DE car.

[Bill Verburg]

Originally Posted by Charles A. T.

Hi Roy,

I'm waiting for the brake pad info... Will be racing on Hoosiers R7.

Stock master and brake proportionning valve.

The problem is not only when it's cold, it's always.

I too I think we should just go back to stock front calipers. But if there's a way to cure this problem with a valve, it would just be easier ad cheaper.

Thanks

c.

I generally dislike this setup as it aggravates the only true issue w/ 911 brakes and that is heat in the front. The larger Boxster caliper pistons put more torque through the same rotor, ie the same rotor which was already overworked now has even more heat to deal w/

That said w/ 3.2 rear and no p/v the bias is pretty good, I like more rear but where you are still good

You also really want a 23.8mm m/c

After those 2 issues are addressed, be sure that the car is corner balanced and that all the pistons are free to do their jobs if any one of them is rozen up it throws the whole system out of whack.

[freeform911]

I had the same issue, turned out my bump steer was not set properly and corner balance was off. Once i had those two things sort, no more lock up into a turn.
Btw i also have boxster front and stock Carrara rear, with proportioning valve gutted.
Steve

[Trackrash]

Originally Posted by Bill Verburg

After those 2 issues are addressed, be sure that the car is corner balanced and that all the pistons are free to do their jobs if any one of them is rozen up it throws the whole system out of whack.

That is where I would start..... A sticky or corroded piston and yea, hold on....

[Dan J]

With a semi trailing arm rear suspension the more rear brake the better
The more rear brake will use the lever of the arm to add more weight
to the rear tire making more grip for rear brake
As Bill states this car goes totally the wrong way ( a very common error)
As a quick fix you could install Pagid R29 frt R14 rear and remove or gut the proportioning valve
Hopefully you have a LSD a 60/40 at least
Obviously the corner weights should be pefect

[Cory M]

In addition to the good advice above you might also consider your alignment. If you are running a lot of front camber you could try reducing it a bit to increase the size of the contact patch.

[Charles A. T.]

Hi Guys,

When we bought the car, it already had the Boxster calipers. I'm not fond of that kind of changes, but we had to live with it, as the previous owner had sold all the original parts.

I've read all your comments and I will remove the p/v. Mr. Verburg, any idea of the 23.8mm m/c part number? Will go through each calipers to make sure they work properly.

The car is pretty well aligned and corner balanced. I will have it realigned anyway. Front camber does not seem to be too much, still, we'll put it on the alignment platform to make sure it's up to specs.

Any recommendations for brake pads (front and rear) with this brake set-up?

Thanks a lot!!!

c.

[GaryR]

Originally Posted by Bill Verburg

I generally dislike this setup as it aggravates the only true issue w/ 911 brakes and that is heat in the front. The larger Boxster caliper pistons put more torque through the same rotor, ie the same rotor which was already overworked now has even more heat to deal w/

That said w/ 3.2 rear and no p/v the bias is pretty good, I like more rear but where you are still good

You also really want a 23.8mm m/c

After those 2 issues are addressed, be sure that the car is corner balanced and that all the pistons are free to do their jobs if any one of them is frozen up it throws the whole system out of whack.

I agree with Bill and Dan (the two most knowledgeable guys I ever met!), and a bad corner balance will cause this very symptom alone. Also, 275/245 is pretty small for a widebody, I would at least go 295's on the rear if not 315's and possibly up to 275's up front... the rear rims look way too wide for whats on them in that pic to me.. and -3 camber in front, -2 in rear +/- if you can get it.. If you want less front initial bite I would run Pagid Blacks in front, PFC08's in rear. I did just the opposite to counter some rear lockup that occurred during late trail braking on my 911 and it worked great.. and even that turned out to be a sticking caliper. Calipers are cheap, can't you just put on two new front Carrera calipers?

[Bill Verburg]

Originally Posted by Charles A. T.

Hi Guys,

When we bought the car, it already had the Boxster calipers. I'm not fond of that kind of changes, but we had to live with it, as the previous owner had sold all the original parts.

I've read all your comments and I will remove the p/v. Mr. Verburg, any idea of the 23.8mm m/c part number? Will go through each calipers to make sure they work properly.

The car is pretty well aligned and corner balanced. I will have it realigned anyway. Front camber does not seem to be too much, still, we'll put it on the alignment platform to make sure it's up to specs.

Any recommendations for brake pads (front and rear) with this brake set-up?

Thanks a lot!!!

c.

turbo m/c

You can do as Dan J suggests(always a good idea!) the Pagid 14 has a higher coefficient of friction than 29, so when 29 f/14r is used bias is moved to the back a bit.

The p/v has a much larger influence on bias, w/ it is place the line pressure to the back is cut almost in half.

As Dan suggests the more rear bias the more you will want an effective lsd, The new asymetrics like GT sells are the best way to go.

And as Cory says lots of camber while it increases grip in the corners, reduces the tires grip in braking.

[JackMan]

FWIW, I have an 89 911 Carrera that I did DE's in for a few years then I had to retire it for a disposable 944. I too put Boxster front brakes on my 911 and they put too much braking bias on the front of the car. My bottom line is go back to stock calipers, get good cooling on the rotors and calipers, get a dual master cylinder so you can adjust bias and go race!

[3rd_gear_Ted]

The 951 front calipers are a better suited mod than the Boxster caliper.
Get some stock Carrera rear calipers and a 930 M/C.
In lieu of 930 brakes, this is a proven good combo if given some decent cooling air flow up front.
BTW, brakes are overrated.

[aston@ultrasw.c]

Originally Posted by Charles A. T.

The car is pretty well aligned and corner balanced.

Did you balance across the corners or along the front axle?

Made a difference for me.

[Charles A. T.]

Originally Posted by aston@ultrasw.c

Did you balance across the corners or along the front axle?

Made a difference for me.

Cross weights were done.