Manifold

For those of you who have a 991 (base or S), are experienced on the track (at least intermediate level), and have tracked your 991, how do you like it on track? How does it compare with other Porsches you've tracked? Any durability issues? If you have PDK, how do you like it on track (in both manual and automated modes)?

I'm considering replacing our Cayman R with a 991S. The 991 GT3 is also a possibility, but it may be too expensive (hopefully prices will be known soon).

kosmo

i have about 20mins of track time in an S w/ PDK, and No PDCC. That car is FAST. Its soo much easier to drive than the 997, kinda felt like a fast cayman. I didnt time myself but it felt faster than my 08 TT. The only thing that gave me pause is the outward visibility. W/ a deeper dash the car felt bigger to me. It was harder to point the nose at the corners which is what I love about the 997. Other than that its really good. I cant imagine what the GT3 will be like.

simsgw

I have instructed a couple of advanced drivers who own the Cayman R, and I have taken my 991S to the track.

First, the 991S. On track, it is essentially neutral but with the power to create oversteer at will. Much more pleasant to drive fast than a 997 and much more exhilarating than a Cayman R. I can't speak to durability of course, since I've only done 5600 miles total and only a few hours of that on track, but it shows no sign of being fragile.

The PDK works fine when shifted 'manually', that is by driver command, but it is superb when left to its own choices. I can't say for sure that this description is correct, but it completely describes the experience: It feels like they recorded telemetry of the best race drivers to learn which control movements and car dynamics precede each shift and then programmed the PDK to recognize those combinations and shift appropriately. Every time I think it's time for a gear change, my reflex is to reach for the shift lever. Before I get there (and there's no 'there' there anymore) the computer instigates the shift for itself. It's like having Walter Rohrl riding inside the transmission, wearing a blindfold, and shifting when he senses the need. The only time I have to shift early or hold a gear is when I see a track situation that a blindfold expert could not know about.

The first track day was within three hundred miles of brand new. The car was still in break-in, which I always respect, so I put the PDK in manual mode to prevent high-rpm shifts. Works fine that way and I still would do it if I had reason to think I'd lose a tenth letting the PDK shift up and then back down at some point on a track. The truth is I doubt that situation arises even for Walter, and certainly not for me. I'm too used up to be able to find a tenth the PDK cannot, so I'll continue using it in Sports Plus full 'automatic' mode on track.

I do shift for myself occasionally on public roads while leaving it in automatic mode. The brief transition to manual mode and the return to automatic work just fine and again they seem well chosen, with timing based on extensive research into the patterns of highly trained drivers. I use full manual mode only when I'm on a tour with other Porsches and need to use the gears to avoid overrunning them.

You will love a 991, and incidentally, it seems faster than a GT3 from the last generation and I suspect that only a TT from the 997 era can beat it. The GT3 of this generation will be faster yet of course.

Fresh out of the box, the 991S is faster than a Lamborghini Countach, whose stats I noticed in R&T last month. Yet it is so well integrated that it's hard to believe. No rough edges, if you know what I mean. I am used to growing accustomed to fast road cars quicker than most, because none of them is as fast as dedicated race cars. But this car is something else altogether. Let me try to describe what I mean.

When I am alone on a road or on track, I sometime feel frustrated that it seems like I need another 200 hp. Yet in the presence of other cars for context, it's clear that I'm blowing off cars well known and praised for their performance. Lambos for example.

The 'need' for another 200 hp is nonsense of course. No racing driver ever feels otherwise and no car ever needed it less than the 991S. What it always means is "this chassis has enormous potential and would be stable and predictable even if it had another 200 hp." That is the 991S in spades. At times when I think that, particularly certain deserted roads I've driven in the 997 and this car as well, I look down and notice the numbers. Typically, I'll be ten or even twenty mph faster than the 997S in the same spot.

The 997S was willing to go fast, but needed persuading that I knew what I was doing before it would respond. This car is eager to go fast. The GT3 and Turbo versions of this generation will be brilliant. Completely unnecessary on public roads, since no sane driver could responsibly drive any faster on public roads than the 991 3.4 Coupe manages with ease, but the faster versions will be delightful for those of us able to use tracks to explore their potential.

If you do plan to track your car, then the 991S will be all the fun you'll ever need until you have had extensive advanced training. Truly using the power that the Turbo version will provide would cause heart arrhythmia in most people.

I look forward to it.

Gary

96redLT4

Any thoughts on the normally asprated,but just a bit more factory tuned horsepower, Gary, ie the X51 pckg?
J

simsgw

Only personal reaction. Had it been available, I'd have considered it overkill and not checked that box. And my current reaction is that if I had ordered it, if my 991 produced 430 hp instead of 400 hp, I would consider the car wanted another 200 horsepower. No satisfying a race driver you know.

As for recommending it to others, I'm of two minds. People who don't track and don't care for the other features of the S package don't need anything but the 3.4 model. Its 350 hp is more than the S model produced not long in the past and as an overall package, it would be in the running for the finest car in the world if we didn't have the S model to consider.

On the other hand, the X51 package sounds very well done. So if someone is buying the S anyway, and plans to track their car, and the extra $15k is neither here nor there, then why not check that box? I probably would have considered long and hard about the alternatives of natural versus 'full' leather, and Burmester versus Bose, and however many other choices I'd have had to change to pick X51. But I suspect I'd have picked the nicer leather and nicer stereo. Thirty just isn't enough to tempt me seriously.

Bottom line for me is that nothing ever will feel like the compression in your chest from a dedicated race car that asks each horsepower to move less than five pounds. I really don't think I would have spent the extra fifteen thousand, because at my age I'm looking for fun, not absolute acceleration. And even for someone younger, I'd suggest spending the money on one of the courses that use outright race cars like Skip Barber. You can't use that extra power on public roads, so why not go for the entire track experience and drive open-wheeled race cars?

Gary, just personal opinion of course

CS-Eng

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Precision Auto

Waiting for the 991 GT3.....

Mike in CA

Me too....

Dr.Bill

I should know in a couple weeks. I just got a 991S and have a couple track days coming up. If you see my Cayman R listed for sale, you will know the answer.

Anyone have suggestions for starting tire pressures? The 20" tires seem to have higher recommended pressures than the 18" or 19" ones.

FORENN

I can't even imagine how sick the 991 GT3 is going to be.

simsgw

I would start with the factory recommendation for 'low' speed North American driving with partial loads. 31/34 cold, that is 68F, and compensated for the actual temperature in your garage of course. Don't adjust at the track.

I haven't had a track day with weather that was nice enough to ask someone to crawl around taking pyrometer readings half a dozen times, but those pressures are within a pound of what we found doing that for the 997, so I have no reason to think Porsche got it wrong with the 991. Furthermore, unlike my 997 when we started tire testing, this 991 feels balanced and very neutral with those settings.

Gary

jdsc

Gary,

I always enjoy your comments about the 911 on track. It's nice to hear what an experienced hand has to say. I 've got a 991 S that I am going to be tracking in a few weeks and have heard various concerns about the brakes and fading when pushed hard. Also one or two posts about warped rotors. Having a lot of experience with Porsches on track. and particularly the 991, have you experienced this and what would you recommend. I'm thinking about Motul brake fluid vs factory and possibly stainless brake lines. any words of wisdom? Thanks.

simsgw

Long answer, so let me give you the quick version:
Don't worry until you start posting times around the best of the day. Work on your own driving skills first. The car is much better than you are unless you've had a lot of that training already.
Once you reach those speeds (and you may never decide you care enough to watch walls approaching at the speeds that entails), then consider three inexpensive changes to the brakes:

• Use high-temp brake fluid;
• Change to stainless steel brake lines;
• Add more air flow with bigger brake ducts.

That's the short version. I'll explain for those with time to read it all.

Where to start... Let's define some terms I think. Fading is a traditional term that applied first to drum brakes. Their shape encouraged heat retention and at a certain temperature the brake shoe no longer fit tightly against the inside of the drum. The reduced contact heated the shoe compound beyond all reason and the parts that did contact the drum achieved nothing. Brake power would literally fade away from normal to nothing in the final useful application. I had it happen once and it was scary. Felt like a failed brake line.

The mechanism of disk brake fading is more subtle. Friction compounds in tires, clutches, and brake pads have a range of operating temperatures. You can push the brake disks and thus the pads beyond the temperature range for which the pad is designed. That causes chemical changes of the compound and with determination you reach the point of ignition for the volatile compounds used when molding the pad. I've done that too. Not scary, but Cindy was impressed: "Whoa. Like a little bunsen burner on each wheel!" Not much braking power, but nothing like as bad as those earlier drum brakes.

If you have the proper pads, and I consider the factory pads for the Carrera S to be fine, then the next point of failure for disks in general is those volatile compounds I mentioned. They can outgas violently enough to create a fluid layer that lifts the pad from the disk. Never had this happen to me, but early disks without ventilation holes or grooves were susceptible and I heard reports. Can't verify them, but I suppose it's been duplicated in labs or we woudn't see Porsche disks designed the way they are today. Again, no problem for the Carrera S.

We're moving through the weak points of road brakes in order. Now we get to the big issue for track work. Overall heat capacity. Truth is, this one depends on your driving experience and it's kind of a bathtub curve, like depreciation in nice cars and that economic theory from Reagan's day. Using the brakes badly will cause excess surface heating of the pad and glaze formation. Done on track, that requires higher pressures and increases the heating without increasing the efficiency.. yada yada yada. Vicious cycle. I'll spare you the mechanism because it probably isn't going to happen in track work. Using the brakes like it's a race car is a problem as well. Somewhere in between is a sweet spot. Carrera brakes used as we do in aggressive road driving will be just fine if you learn how to use them properly.

But what happens to cause those reports? Well, let me be civil, and presume that all reports on the forums are by very competent drivers who wouldn't make the mistakes that overheat any brake. Make your assessment of individual posters, but we can be charitable and assume that while continuing our discussion.

I have explored the limits of Carrera S brakes and then discussed the results I saw with Cass Whitehead from the Porsche Driving School. The quick version is this isn't a race car. It's a damned fast road car. That's the way they teach people to drive at PSDS and it's just fine in that usage. Drive it like a race car and you need to upgrade the system to come closer to race equipment.

I don't mean to imply any of this surprised me. Nor will it surprise anyone who races. We always upgrade the brakes along with other equipment. Let me exemplify this issue. Reputable magazines like Road & Track and Car and Driver do brake tests on every car for which they report a full road test. Their procedure is to accelerate to a target speed which has risen over the years. I think it's 80 mph now. They accelerate to that speed and brake as fast as the car can manage down to zero. Then without pause they accelerate again to the target speed. Repeat and rinse. They do this until the brakes fail completely or they reach ten stops. They haven't published this data in at least ten years, maybe twenty, because modern brakes are too good. They never had failures and even the fade measurement (subjective or in terms of pounds pressure required on the brake pedal) became meaningless. Cars really are better than ever and the brake system relative to the overall performance is well matched in modern cars. Notice that acceleration to target speed as fast as possible. That makes the test more stringent according to the car's performance.

Now we get to cars as good as ours. Cars that not only face daily drive emergencies, but track use. Let's compare the trials we impose. [Damn. I can't find a spreadsheet I used when working on the brakes of my 997. I'll have to rely on memory.]

Working from memory, that test the mags use dumps 1.5 megajoules of heat into the brake system. With a C2S, the acceleration to 80 mph and braking to zero takes perhaps 12 seconds and that's generous. Ten repeats will mean you've dumped 1.5 mj into the brakes in two minutes. Then the road test editor turns away, the driver might do a cool down lap out of respect for the eventual owner of the car, but that's it. Compare that to a lap at the track.

Lost the damned spreadsheet, but we're coming down from 100-140 mph to 40-50 mph on every fast section followed by a tight corner. At our Fairplex track, I was doing that five times on each lap. The energy equations are ruthless. 120 down to 50 is something like twice the heat involved in going from 80 mph to zero. (Wish I had time to repeat the calculation, but I'm close.) We only do that five times in one lap, but then we repeat. We don't pull into the pit after two laps, which would be ten intense braking tests by magazine standards. I seem to remember it worked out to 1.5 laps being equivalent to the entire magazine test. Then I repeated of course. Laps unending. Well, ten or fifteen laps anyway, but enough time for the system to stabilize. And the brakes quit working. At least like Porsche brakes. Felt like I had an aneurysm in one of the brake lines.

Lapping is a different type of test in engineering terms. The magazine test is what we call an impulse test. We subject something - the brakes -- to a stress test. Then we stop and measure the effect. A continual service test is different. We speak in terms of duty cycles and more terms that aren't worth explaining, but the important point is we evaluate continual use. The system has time to stabilize at whatever condition the equipment is able to sustain. That's what happened to my 997, and it stabilized at a point outside its design range.

The Porsche is a very high performance car. Because it reaches higher speeds in shorter times, it has more energy to dissipate when it slows for the next corner group. At Fairplex, I was dumping energy into the brakes at max rate for about twenty seconds out of every lap. Add in trail braking and call it thirty seconds. That left only 70 seconds or so for the air flow to cool the brakes and remove that couple of megajoules I had just pumped into the disks and pads.

As laps continue, the temperature rises steadily until the system is hot enough to radiate all that heat away in those seventy seconds. I won't explain radiation theory, but the hotter the system becomes the more efficiently it dumps heat into the cooling air flow. Eventually it reaches a balance between the two effects. Heat goes out as fast as it goes in. That temperature is the stable temperature -- assuming nothing fails within the length of the test.

Good as they are, the Porsche road brakes are not designed to operate at the temperature where that stable flow of heat is reached that I required. We'll consider what parts to improve in a minute. Let's review though.

I cooked my brakes. Race cars can be driven faster longer than road cars. End of story.

Now is that an issue for most people? No. Let me re-phrase that second statement: Race drivers are faster than road drivers.j None of this should be a surprise of course, but consider what it means for the brake system. The heat to be dissipated in braking rises as the square of speeds reached. Every time I'm faster through a corner, that puts me on the next straight at a higher speed. Every time I use a more efficient line for the same cornering g-load, I begin to accelerate earlier. That puts me at the end of the straight going that much faster. It all adds up.

Killing speed from 140 mph down to 60 mph involves a lot more energy than doing it from 80 to zero. [Trust me on this. 'A lot' is fuzzy I know. Damned spreadsheet.] At every braking opportunity, I'm dumping more energy than a highly skilled road driver in the same section. A lot more energy. Cass chuckled when I described my results. "Yep. That's why we don't teach 'em to drive your way."

I use the brakes in ways that put the least strain on the system, but energy is energy. Road cars are not designed for that extreme continual use. I could drive the mountains of California all day and not push the brakes to the temps I reach in three laps on a race track when I'm driving like a race driver.

I cooked my brakes. Too much energy, too little cooling.

I haven't done it yet, but it isn't even a question to be explored: I can do the same thing to a 991S. It's a road car, not a race car.

This is one of those design 'corners' I described the other day. Brakes designed for racing impose burdens most owners won't tolerate, so the system from the factory is a compromise. Like everything else in engineering. But we can shift the design over a couple of corners away from convenience and toward higher performance. Hell, you can rebuild the car as a race car if you have the budget and a trailer to haul it to tracks. But let's stick with simple stuff that we can tolerate in a daily driver. Less convenience, more performance is our objective.

The first goal is to make the system more able to tolerate high temperatures. Sustained fast driving [in track terms] dumps energy into the brakes that always will raise the temperatures to levels the magazines never see. Always. I changed two things first, and I consider them bargains in terms of performance while causing no problem in daily driving:

• High temp brake fluid. I used Motul 660. Opinions vary. The CDI in the adjacent region is an active racer and considers this overkill, but it's what I used. The '660' is the temperature rating before the fluid boils. I was boiling the factory fluid, so it probably is rated at 550 or 575. The fluid boils sooner as the water content rises. All brake fluids suck up moisture like a sponge. They are hygroscopic fluids. The routine fluids must be changed every couple of years because a certain amount of moisture is always present to be absorbed. The high-temp fluids are more hygroscopic than factory fill. (Compromise again.) They must be changed routinely. I was planning to do it three times a year before I sold that 997S. With this 991, I may or may not drive hard enough to feel the need for Motul. (Personal health problems.) If I do use Motul, then I'll expect to change it twice during the season, and then replace it with factory fill for the Winter. Full flush each time of course.
• Stainless steel brake lines. The real point of this is to protect the fluid from heating so badly during that impulse of heat at each braking section. SS lines are better at keeping the fluid near the calipers from spot heating and subsequent boiling. They also help retain the brake feel. When the temperature climbs, the usual brake lines get more flexible and we can feel that in the pedal. Actually, I presume they still do. Factory lines used to be like overcooked pasta in racing conditions, but I honestly haven't tested modern ones. I quit racing 19 years ago. Still, it's cheap to replace with stainless steel lines. Suncoast sells complete sets and my dealer did the replacement with a complete flush for a couple of hundred.

Mike, that other racer, runs an independent shop emphasizing performance, and the reason he doesn't recommend Motul 660 is that his customers don't flush the system regularly and the fluid absorbs enough moisture to be more temp sensitive than the factory fluids. We're trading convenience for performance in this note, and I'm specifying a mild schedule. In racing, we flush the brakes after every event. Assess your own tolerance level for fluid flushes.

These steps raise the temperature the system can tolerate. With these changes, the system can operate properly at 600 to 650 F. But if you drive as fast as I do, the heat input may drive the stabilizing temp even higher. So let's address that next:

• Bigger brake ducts. We need to extract more heat from the system. We need to get more of it out before the next lap begins. That's the only way to lower the stable temperature. I considered GT3 brake ducts which are a direct swap for the C2S. At least they were on the 997. But they don't add much opening. They just improve the airflow smoothness I suspect, but they are not much bigger. (Again on the 997. No data on the 991 GT3.) I wanted to get that temp down to the brake fluid operating range and I suspect that isn't enough improvement. I used GT2 brake ducts. They are significantly larger, gather more air and dump a much higher air mass onto the brakes. They need mild trimming to fit, but it isn't a big deal. The shop manager at my dealer did it himself to make sure it was done right, but afterward he said it was quite simple. He added some rivets to the usual two-sided adhesive pads to make sure everything would stay put on a race track.

Those three changes were all my 997S needed and I was pushing it pretty hard. I'm not as young as I was when racing, but the car does the hard work. I was running down Turbos and GT3's driven with less experience, so the brakes were getting a work-out.

Other changes. Disks. Well, I've never warped a disk on a Porsche, but I have on 'sport' sedans not really up to the job. I'm sure that someone driving a Porsche into corners as fast as I do, but less skilled at using those brakes, might accomplish the same thing. I wouldn't worry about it if I were you. Learn to brake properly and let concerns about the brake disk wait until you see symptoms. As I remember, Porsche upgraded these brakes even from the high capacity system on the 997, so I don't expect you to have trouble.

Pads. I saw no problem with the pads on the 997 at the higher stable operating temperature I was achieving. Certainly that wasn't the weak point in the system in race work. It was the heat capacity that caused my 997 brakes to fail. (Alright, technically they didn't fail. They just got floopy.) Conversely, a driver I'm coaching in a Cayman uses the Pagid pads. I've forgotten which ones. After I taught him race braking, he managed to cook his brakes by the third session. Different pads have different temp ranges and different levels of volatiles. I don't disapprove of aftermarket pads. I would just say they are not the first part that needs changing. Had I kept that 997 I might have found tracks that put different types of heat cycles into the system and made me want to change pads.

For now, I'd stick with the changes already described:

• High temp fluid.
• Stainless steel braided brake lines.
• Bigger ducting.

I haven't asked your experience level. Honestly, until you start turning lap times down close to the fast time of the day, I doubt the need for these first three changes. Cass assures me they do none of those things to the PSDS cars and they rarely have problems. Between us, I suspect the reason is they don't get many race drivers willing to spend the cost of two races getting instruction in subjects they already understand just fine. Some will, just to get Porsche specific training because their race car isn't a road car and they want to see what might need to be adapted.

I'm sure the instructors in the Master class have special guidelines for such drivers. One tip I'll mention in passing without trying to expand it into teaching driving over the Internet: don't push the system into full ABS regularly. That slows you down and imposes a limit on the heat you can put into the system.

To illustrate the significance, when I'm driving in anger so to speak, when I'm in full race mode, I brake at least one second and often two seconds deeper than 'performance' drivers around me. Then I go into full ABS on all four wheels and trail brake all the way to the apex of the corner. The trail braking is surely taught in PSDS, but I'll bet they encourage drivers to keep the car just short of ABS engagement as they do it.

This is a wonderful road car, but is designed to be wonderful at that, not at racing. Drive it like a road car when you go to tracks and you won't have trouble once you learn the skills that keep you from abusing the brakes and tires. If you want to sacrifice some convenience for additional speed, wait until you get yourself trained to reach those limits, then start making the changes I've discussed.

You'll have a great time,

Gary

Dr.Bill

Thank you sir! I thought the 36/44 on the door card was way high!

STALKER99

I'd add a set of Pagid RS29 pads along with the SS lines and fluid.
Then you are good to go.

Easier to add track pads then doing extra cooling in most situations, but this might not be true on the 991 since I have not looked into brake ducts yet.