mark kibort

its kind a good thing in checking only one variable at a time.. its how you do things scientifically. the results were more than obvious.. i have the video to show the rear locking up, plus the theory behind why. not many have gone through the strokes of doing the math... its shame. thats whats nice about physics... it all has to happen for a reason!

i wouldn't go so far as to say expert.. Just relevant experience.

mark kibort

cayenne rotors can be used with the early S brake calipers, but thats a cludge as its the single piston caliper.

why not round spacers and the stock S4 or GTS calipers? it looks pretty clear that you can go to 355mm with either. (or at least to 330mm with the S4 with some grinding , and no grinding with the C2 turbo or GTs calipers)


Great question. the math is pretty clear on that one as well. the full street car is going to be much easier on brakes... counter intuitive, but if you think about it.... whats the top speed diff going to be if you loose 500lbs? since KE is more dominantly speed factored (KE=1/2mv^2) if we loose 10% of our weight we can reasonably expect a 3% speed change at the top of the straight.** That coupled with a few MPH of cornering advantages coming off of any turn, and the lesser capability of the street tires vs race tires, for keeping opimal slip % during decel, you have a situation where a stock car is much easier on brakes than a race car heat dissipation wise.

**using 3500lb car loosing 500lbs to 3000lb race car, and saying it will perform with at least a 4-5mph greater speed down the main straight most all tracks with a slow end-of-straight turn entrance, where the KE demands will be the greatest.

actually, there is.... using the s4 calipers and using GTS rotors is the simplest and cheapest braking upgrade around. there is a reason the S4 calipers are so widely used on race cars and exotic supercars like the ferarri F40/50... because it works. use it as anderson did up to 14" rotors, however, the pads are the same so the caliper is radiused more, so you need that C2 turbo caliper to go real big and just use spacers and longer bolts. if you look at the C2turbo calipers (same as F50) you see that the "spacers" are built in to the castings... so using a .25" spacer allows for 13" rotors (330mm) and a .75" spacer for 14" rotors. I dont know if that is safe though, so i wonder what the "experts" say about that... maybe find a 13.5" rotor (340mm ish if that is available) then you would only need what ive used with the S4 calipers.... a sub .5" spacer .
i dont know what the GTS calipers are capable of handling as far as rotor size, but im sure its the same or greater than the C2turbo caliper that both use the same rotor.( offset too)

Wheel Dynamics

Currently running 19" however the plan is to put it back to 18"..... so wheel size is not the biggest concern at the moment.

I may have to just try a few things and report as I have time... may take several months or years...

Thanks again,

The Forgotten On

I run 993 C2 calipers in my 1981. The fronts are about the same as the S4 brakes, but with the updated seal design.

The rears however are much larger, I think about 30% larger than the rears on an S4 or somewhere about. It's a cheap, bolt on upgrade for S4+ 928s as you can get a used set of rears for about $200 on ebay.

The car stops extremely well. However, with the 33 bar bias valves the car has stock the rears heat up much more than the fronts do. Both are wearing about the same though.

They fill the 18's I have much better than the single piston OB calipers I had. Plus they have the updated smoother look that the S4 calipers lack.

I would look at them first as the GTS calipers aren't cheap like they used to be.

Petza914

On my '79 I run GTS Big blacks on my fronts and the S4/GTS rears. Fronts use a slotted 2-piece Girodisc rotor from a 993TT and calipers are mounted using the adapter kit from 928MS. Rears use a slotted rotor from Powerslot with the black e-coating.

davek9

Petza914 those look great, how do you keep from locking up the fronts and flat spotting your tires?

I ask because I've done it on my '85 w/ the stock brakes and non anti-lock.

Petza914

The smart-*** answer is with proper pedal modulation

But this is a street car, not a track car. So far I've not experienced any type of front wheel lockup under braking, and that includes some driving in wet conditions. From a safety perspective I'd want the fronts to lock-up before the rears anyway to avoid an end-swapping scenario in a corner.

I'll just have to remain aware of your experience and will let you know if I encounter a lock-up of any sort under threshold braking.

GlenL

I track my car pretty hard. Yes, "flat out." I don't see a pull for bigger rotors or multi-piston calipers as I get all the stopping force the tires can handle by upgrading to more aggressive pads.

The earliest cars that don't have the "S" calipers in front do need help there. Otherwise we're talking about "feel" and "modulation" and it's a whole pile of money to upgrade the brakes when I'd rather spend the money on, say, tires.

UpFixenDerPorsche

Arrhh gee guys - unless you're racing, or into very competitive track day performance - pls tell us how your stock brakes are leaving you short of braking performance?

:-)

GlenL

Performance. Phhhhththththt!

I might be convinced to do this as my brake rotors look tiny inside wide-open 18" wheels.

ThetaTau87

What are you looking to improve, stopping distance or fade resistance? The best/cheapest way to reduce stopping distance is to get better tires. Tires are always the limiting factor. Lowering the ride height and installing stiffer springs/shocks reduces weight transfer under braking and improves stopping distance.

The cheapest way to increase fade resistance is with higher temp pads. For a street/track car a dedicated set of track pads avoids the trade-offs of using noisy high temp pads on the street or quiet lower temp pads on the track.

Once you start out driving the best tires and pads then consider upgrading to bigger calipers/rotors.

MGW-Fla

The 330mm Cayenne rotors will work on the front with the Wilwood 4 piston Superlite calipers(#120-11133). I used Rob(xschop)'s adapters when he had them available. Stock Wilwood stainless brake lines that are made for Dodge were the perfect fittings and length. Rob no longer had any centering rings, so I got those from Ed(OBehave) (http://perfexmfg.com/brakes.php). Ed also has his big brake adapters available to upgrade an S model to either S4 or GTS brakes.

Here are a few pics of mine which I posted previously in other threads on the subject. It was a very economical upgrade(less than $700); one that even a plebe like me could do; I did notice a difference on track & I'm very happy with it. I get a lot of comments and questions as they are noticed quite often.

Wheel Dynamics

Pete, wow, great job, those look great! Any issues with braking, how do they perform?

mark kibort

the bigger rotors give a better feel for stopping and slowing, to fight the locking up potential. plus, more inertia helps too!

ive raced 'flat out " at laguna with the stock calipers on the "S" model. with good pads, it works. dicy, but works. they cook as they are near half the mass of the 13" rotors

exactly.... there is no difference on the street. its threshold braking from 130 to 40mph that would require the larger rotor, or repeated track type stops for 30 to 60 mins. otherwise, the stock brakes can do the job just fine

exactly... the drawback of a pad only approach is rotor wear for slowing demands and or cold stopping performance, which i havent noticed with any race pad on the street .
tires are what is is all about, or repeated slow downs at the track, where the greater thermal mass helps.