Bill Verburg

thats correct the 964t rear is functionally the same as a 993rear, they are the same caliper except that the 964t is wider for a 28mm rotor vs 24mm for 964/993 and w/ a slightly different o/s

swept area has no effect on bias, it reduces the point mechanical and thermal loads and will increase longevity as a result

brake torque at a wheel is a function of first and most importantly piston size, large always generates more brake torque at the same line pressure.

secondly effective rotor diameter, for instance a 964t/RS uses the same rotor diameter as a 993tt/RS but the effective rotor diameter of the 964 is greater and generates more torque,2381nm vs 2306nm.

third pad friction coefficient you can raise rear brake torque by using a higher mu pad in back, Boxster/Cayman have similar bias issues and often use Pagid yellow front w/ Pagid black in back, There can be unexpected side effects of this, I experimented w/ it on my car either I got too much rear or something else was going on but the ABS didn't like it at all and would kick in way more intrusively than I care for, I quickly went back to the same pads f/r.

You want as much rear bias as you can handle, some guys go to close to 1:1, this works best when you aren't riding the edge of braking traction in back which is possible much of the time, depends on the track and driver
it removes thermal and mechanical load from the stressed front to the less stressed rear, this raises the thermal envelope of the cars brakes as whole and lowers wear on rotors and pads
you involve the big rear tires more, after all the tires are critical point, each can only do so much and getting those big rear to do more work is a big +

Bill Verburg

Here's an idea if you know a good machinist

split the 993tt rears
bore the 28/28 to either 30/34 or 30/36 and use oe pistons/seals or purchase some custom SS or Ti pistons/seals to suit.

cobalt

I have dozens of spare pistons and seals in 44/36/34/30mm sizes this might be the way to go. Although I have my mechanic informing me on the other side saying he has turned a lot of 993TT's into rwd track cars and never changed or modified the rear brakes and does not see the the problem. He claims he discussed it in detail with Brembo and they said they didn't see any real issues doing so. So now I have a dilemma. I guess I can try it for a DE and see how it feels. I have no doubt it will be a year before I have all the bugs worked out.

TrackDays247.com

The GiroDisc ( we have em!) 2 piece rotors ARE and exact duplicate of 993TT rotors, if that helps at all...

Bill Verburg

It's definitely safer to have more front bias but it's also definitely faster to move as much bias to the back as you can stand

imagine moving 50% of the heat from the front to the back during a track event
imagine moving 50% more stopping torque from the front to the rear

that's the difference between 993tt and 993RS


stock 4/4 964 is 1.734
stock 964 4/2 is 1.5048
964RS is 1.713

stock 993 is 1.604
993RS is 1.426
stock 993tt 1.997

early 993cup is 1.659
late 993Cup is 1.426
993 RSR base is 1.426

TrackDays247.com

Very popular to have: 996 GT3, 996, 996TT, and Cayman/Boxster S to have HIGHER rear friction pads. Thanks again Bill for the always pro advice...

Bill Verburg

996GT3, Cayman, Boxster all have front bias issues, 2.084, 1.607, 1.811(S & 996)

cobalt

I tracked my 14 boxster for a few events last year and I was very happy with its braking but the car was not very fast. Interesting that the 996GT3 is at 2.084 that was one car I never felt comfortable driving and did not like it under hard braking. So nothing I can do to change the proportioning valve to help the 993TT rears.

I'm on a tight budget but the most important parts of my build are safety 1st then braking & suspension the rest is all secondary. I can source the rs rears they are more than I want to spend but can have them here in a week. I have the 90 C4 hydraulic system and 55 bar valve. What else would you do to improve the rear bias should I swap out the 55 with a 40 like the RS or stay with the 55. having moved my weight distribution towards the rear with a TT motor I figure an additional 75-100 pounds moved aft I defiantly don't want my back end lifting under hard braking and this car will be carrying considerably more speed into the braking zones.

Anyone interested in 993TT rears installed but not used?

Bill Verburg

My best advice is to use the 993RS rears, and take baby steps w/ the p/v
leave the 55 for now, once you are comfortable w/ the car go to an adjustable

you can run w/o a p/v but you might want to sneak up on that as your comfort level increases,

w/ the C4 system use a 964RS 25.4mm m/c to improve feel and damp over braking tendencies, This should be a bolt on.

The more you can involve the rears the faster you can go up to the point that the rears lock first.

cobalt

Not a big fan of that much rear bias. I was at an F1 race back in the 70's and I sat over the pit's back in the day when they had pit roof seats. Andretti in his JPS was arguing with his pit chief about brake bias and he said it had too much to the rears. He went out came back and as he went to brake in the pits the backs locked and the car just did a 180 in a fraction of a second. So there is a point of taking it too far.

BTW on a side note my wife drives as well and this car will be a monster for her. Considering her safety over my speed which would you do if you were in my situation and it was your significant other?

Bill Verburg

993RS all the way, we are not talking absurd levels of rear bias like the guys w/ 964 fronts in back

as i said if you have concerns leave you p/v and then after you have more confidence then go to a less intrusive or adjustable p/v

There are loats of 993RS braked 911/964/993 being tracked daily, I have 2 of them, my 993 and my 911 both have 993RS brakes w/ no p/v and no issues or qualms, these cars brake like nothing else out there except 996/7 Cup cars and late 997RS which have a load of rear bias too.

cobalt

Done should have them by Monday. Engine is in but still a ton of work before it can be driven.

Thanks for your help.

Bill Verburg

one last data point
Every 911 w/ A or S front and M rear made from '69 thru '83 has a brake torque bias of 1.491 w/ no p/v

cobalt

Interesting. My 74 3.0 IROC tribute had the aluminum (S?) calipers up front with M calipers and IIRC a 23.81mm MC vs the stock 21. At 2390 pounds 272 HP I always found it to be a little heavy on the rear bias when trail braking. I would have a tendency to lock up the rear outside brake back in the days of threshold braking and lucky to still have brakes after the second 30 min run of the day. Always a down side to that car I cooked brakes like no tomorrow. This will take a lot of sorting before it is safe to unleash my wife in it.

ThomasC2

FWIW I've been running without a p/v for years and like it a lot. I have big red and small red in the rear (964 3,6T). The rear is much more stable under braking than with the turbo p/v I had before.