Jean

While we keep upgrading our cars for the latest and “bestest” engine mods to achieve our fastest lap time while keeping the 991TT shape and a streetable car, I will struggle this season keeping up with tuned 720s and 991GT2RS in Time Trials. After having done all sorts of upgrades to the engine and chassis, I am somewhat limited now.

Upgrades to a full braking system (AP Racing, Brembo racing and other) start at 10K USD on average, yet surprisingly there is no hard data from manufacturers about performance in terms of lap time improvements. Many claims about temperature related topics, or lighter rotors, which truth is, have very limited impact on rotational inertia and unsprung mass at high speeds. My braking distance is consistent after few laps at max speeds by the time I take a cooling lap 10 minutes into a session. The professional racing brake systems are in the $17-20k, I do see 1.8-2.1 Gs on the 997RSR, but a lot of it is due to aerodynamics and weight.

I run often with stock 991TT brakes and brake pads (coefficient of friction Mu~0.27) and race oil, never faded. I tested braking through my VBOX HD2 on a challenging long straight reaching 177mph, then you have to drop to 50mph with a right hairpin at lateral 1.4Gs in 2nd. gear.

Results: 177mph to 60mph: Distance 243 meters (797ft) and Time 4.6 seconds. Peak G (Grey): -1.55, Average G (Orange line): -1.16. It is pretty clear from the grey line below that despite me feeling that I have pushed very hard the pedal to the limits, I am still leaving quite a bit on the table in terms of initial reaction as well as foot firmness.




The geek in me took a considerable amount of time building a braking simulation model, and my results were 249 meters and 4.7 seconds on the model, pretty close to above data which gave me comfort using it and therefore a change in a certain parameter should quite accurately estimate the final results. Here is simulated data from 177-60mph by changing brake pads:

1- Simulated with pads of 0.30Mu: 224 meters (734ft) Time: 4.2 seconds. Average G: -1.26

2- Simulated with pads of 0.35Mu: 192 meters (630 ft) Time:3.6 seconds. Average G: -1.45

A snapshot from the results page is shown below..



So with a simple change of pads to 0.30, you can get an improvement of 0.5 seconds and ~60 feet shorter braking distance in a single straight, hard to beat bang for the buck! Going to a harsher compound you can actually reduce a full 1 second in a single straight and reduce your braking distance by 160 feet.

Where are the limits? Seeing peaks of -1.5G under braking, I guess one can say that a 0.30 pad will not overcome the tires. I think most of us can achieve 1.3-1.4G peak under very hard braking with Trofeo or Cup2.

The stock 991TT tested back in the day achieved 186mph-0 in 7 seconds and 279 meters, that is an average of -1.2G approximately, from 124mph-0 in 122 meters, again an average G of -1.2 on stock Pirelli tires.

Anyone here has similar data, or what’s your opinion on the braking topic? I believe most of us are leaving a lot on the table in terms of braking like professionals do. I know for a fact I will not be buying a 10K system with all the bells and whistles before maximizing my left foot control and maximum braking force on my race pads.

Kinetic1

Jean

Great analysis and data.

I’m wondering how this effects heat and heat transfer in rotors and pads though. Obviously you would be creating more energy transfer in a shorter amount of time over and over.

pfl

So it is interesting that I find someone else into data as well. I too have VBOX HD2 (and before that other VBOXes when HD2 wasn't available) and have data on all my cars. In terms of 991.1TT (which I have) I have data on P Zero stock tires, Trofeo R, and Cup 2 N1s. Here is where mine stands:

Max braking g Force:
P Zero N0: 1.02 g (Lap time: 1:49.40) - All stock including stock rotors and fluid. These tires that same day delaminated so I believe they may have been too old. P Zero usually does 1-2 good laps before it falls off massively.
Cup 2 N1: 1.29 g (Lap time: 1:46.25) - Car had Brembo Type V front rotors and Castrol SRF brake fluid, all else stock except tires. These tires by far lasted the longest time. More consistent than P Zeros but not as consistent as Trofeo Rs.
Trofeo R: 1.330 g (Lap time: 1:45.99) - Car had Brembo Type V front rotors and Castrol SRF brake fluid, all else stock except tires. The best track tires if every hundredth of second counts. Very consistent down to the cord. For the price, now the Goodyear Eagle F1 Supercar 3R is a better buy IMO.

From my data, I too can gain from initial application even though I feel that I go very rapidly to brake (I left foot brake).

In my opinion, the Turbo (At lease in the trim of mine which is without PDCC) in stock form severely lacks the in suspension area (Damping, Rolling, Rebound).

MaxLTV

In short, it will not change much if anything at all. Turbo brakes are so strongly boosted that the coefficient of friction of the pads matters very little - that pad is pushed into the rotor strongly enough that "slipperiness" of the pad can be compensated for with more pressure. Basically, if the pad is more slippery, just push the pedal harder. Stopping force is pressing force multiplied by mu. So 10% higher mu will only allow pressing on the pedal 10% less. Even GT cars with much less brake boost are still not very sensitive to mu. I could slow down the car in almost the same distance with pads crumbling and only the backing plate left. The week spot would be ABS programming and tires. The mu of the pads will improve the braking distance only if you cannot get ABS to engage or to lock up tires on non-abs car. OEM pads have more than enough force to engage ABS even with full race slicks. The jumps you see on the graph are transients - basically, a combination of sensor noise and ABS pulsing and/or unevenness of the tarmac.

In general, any brake upgrades do not improve best possible lap time but rather the longevity of how many laps you can do before brakes start to overheat.

But in general, the formula is as follows:
If pedal goes soft, go to high-temperature fluid or add more cooling
If after a few laps the stopping distance increases and ABS does not engage - need higher temperature pads or more cooling (insufficient long-term heat capacity or cooling)
If after a few laps the stopping distance increases and ABS DOES engage - overheated front tires
If you cannot engage ABS or lock the wheels at the END of a braking zone - need bigger pads and/or rotors and/or higher temperature pads (insufficient short-term heat capacity)
If you cannot engage ABS or lock wheels from at the beginning of a braking zone, especially after along straight, from lap one - need higher mu pads and/or lower-temp pads

Jean

Glad you enjoy it I agree concerning the Trofeos, they could benefit from some additional life though.

I might be wrong but I believe the impact of a higher Mu on thermal energy transfer for the two different pads will be offset by the resulting shorter braking distance, assuming the theory is correct while applying same pedal pressure. I doubt at high speeds where airflow has an exponential cooling effect, the braking system will generate high temperatures exceeding the operating temps that discs or calipers are designed for.

Purely theoretical :
- Thermal Energy (Mu 0.30)= Breaking Force per corner x breaking distance = 6.630 N x 224 m = 1485 KJ
- Temperature increase = Thermal E / (Iron specific heat capacity x disc mass)= 340 C = 644F temp increase due to this braking zone alone, all other parameters being disregarded (airflow, cooling vanes, etc.)..
If your discs are already carrying 300F before braking, your braking temp would reach 944F, well within any system limits.

That's some great data and pretty convincing tire test Would be also interesting to see how a brake pad change impacts the data. And fully agree about suspension, first upgrade I did...

Jean

Great way to explain it and spot on MaxLTV

My personal experience is that I cannot trigger ABS from the get go at >170mph.. It comes quite a bit later at slower speeds, could be that I don't have the b.lls, or that it just takes time for max Gs to build in. Your assessment about the erratic long Gs seems correct, and what I am seeing is the ABS modulating and doing its thing even when I do not feel it.

While everything you say makes perfect sense and is the general wisdom, I do believe that higher friction pads will reduce braking distance, especially the initial bite, and unless your system reaches temps that are beyond the system's tolerance, or the brake pad optimal Mu, or maximum tire grip, there is a substantial amount to gain before reaching those levels. Moving to a pad with claimed 0.4 Mu between 300-1000 C, should give more clamping force and better stopping distance through a certain phase of the braking cycle .

Pdtp#16

Jean, just curious. What has been your ware like? I’ve been running girodiscs with pagids rst3’s instead of yellows. With the yellows I was going through rotors pretty quickly. On the reds I’m running through front pads every 10 to 12 sessions. Sebring it’s pretty rough on breaks.

Going to add some direct air ducts via the hood and front bumper to help keep them cooler and see if this helps. I’ve been debating upgrading to PFC’s but only if I can get my consumables down by having a system that runs cooler and last longer. Not sure if that’s the case. I don’t have any data of before and after I switch the brakes setups other than feel. In terms of feel, once I went to girodiscs with red pads it was much more consistent which meant I was more confident which meant I could brake later. On the yellows I had a better initial bite but the system got hotter quicker and brake pedal got a bit soft after a few sessions. This was exacerbated on the stock discs.

either way going to try getting more air in it first. But two days of pad life seems too low. That’s essentially $300 a day just in pads.

Jean

Frankly I have never counted the number of sessions, but this topic is very subjective to the type of track you drive on, driving style and speed, as you mention..

I always used stock discs on the 991 and for track days I use stock pads even for spirited runs, I find the braking power and consumption to be very reasonable and keeps costs down. This includes the occasional fastest lap, which is always one try, and next lap is cooling at a slower pace. On continuous laps I manage the temps, when the last 0.2 seconds gain with hard braking doesn’t matter, I simply brake a bit earlier. These are heavy cars.

You know all this, but I should mention that hard short braking will also help rather than longer braking distance at 80%-90% for example. You want to be on the brakes the shortest time possible to let heat dissipate.

The RST pad you’re using is a sprint pad I believe. I never used a sprint pad I always go with enduro pads such as the Endless or RSL1 even in Time attack. My times are 5-6 seconds off pro 991.2 cup cars on a 3.4 mile, 2:10 minute track with those pads on a F1 track known to be super harsh on brakes and tires.

Improving cooling with ducts will help, but mostly cooling laps in between sprints, and never go to pits without a cooling lap first.

I would use less abrasive pads and check lap times before going through big expenses.

if you still have issues you might want to try one of these or similar, the data you gather for 300USD will be of great use:

http://www.izzeracing.com/products/b...e-sensors.html

Here is a snapshot of one lap with stock brake discs and pads, this is a 2:11 lap, if you see the braking Gs, it is quite hard, reaching often -1.2 to -1.4G from high speeds.

Pdtp#16

thanks for the link. I’m definitely going to add as it will help in determining how much and where to apply more air / cooling. regarding the pads makes sense. After speaking to my guys I decided to try the ferodo’s DS1.11 which actually Randy has used as well in combination with Girodiscs.

I don’t have access to my data right but I’ll try to replicate the table above and see how it looks. Another thing that I know I was doing this weekend was pushing (at least 85%) pretty much for 20 minutes straight with very little cooling laps in between other than cooling lap at the end. I usually do a couple of laps and alternate, however with traffic and a few cars chasing just couldn’t get myself to get into that pattern! Ha.

will dig into more data and see how it goes with the ferodos before doing anything else. Thanks for the help and advise