simsgw

We'll drop a note to the Formula One teams about that. I'm sure they'll file it with the one from the guy who advised them that serpentines on the parade lap are silly because you can't put heat in the tires that way.

I don't feel like writing a note about suspension set-up tonight, but for those actually paying attention, the rail-hanger talk about "stiffer is better" is not the way to learn it. Those are the same guys who tell you to be sure to bleed air when the tires warm up.

Bad day. Some other night.

If your description is correct, I'd say your car has a problem. None of those were true of the 997.1 that I test drove back-to-back with the 997.2 that we bought. I liked the dot two better, but none of those were factors. And they certainly are not true of all 997's, which I'm sure you didn't mean to imply. If they were, all the other 997 owners in this forum would be complaining loudly. What you call 'lag' would be enough by itself to junk a car for me. I wouldn't drive an Accord with that behavior.

Did you or some previous owner put an aftermarket chip in the car? Have you let a good Porsche mechanic test drive it and diagnose what's going on?

It does take time to learn how to drive a different type of car, but I think the save-your-*** function of PSM is valuable even when -- maybe especially when -- you're still learning the knack. As to whether you think it would have been faster, hang on to that thought. Some day we'll meet on a track.

Gary

ADias

Stated like a pro, by a pro.

MagnusB

simsgw, I was trying to say that the non-standard suspension on my 996 was better for autox than the standard suspension on the 997.1. And you know better and make fun on me? Nothing wrong with being confident but arrogant and rude I have a problem with.
Stiffer than the standard 997.1 suspension would most certainly help. As an example the body movement in an autox slalom was much more than in my 996 and affected the speed I could carry.
Your F1 comparison is irrelevant, I hope you understand that.

As for the choking, I wrote down my observations, using my car. Nowhere did I state this applied for all 997.1 but certainly for mine. I was quite surprised and kind of hoped for some constructive comments.
No aftermarket chip. The previous owners drove it on Sundays :-)
Since it happened last weekend I haven't had time to deal with it.

Mike in CA

Gary, the autocross hot target pressure as recommended by Michelin for my Pilot Sport Cups is in the range of 32-36 psi hot, front and rear respectively. Of course it's a balancing act, and one wouldn't want to release too much air since the tires obviously do lose pressure as they cool. That said, once the tires have exceeded their design pressure after a couple of runs, can you explain why I shouldn't bleed off enough air to keep them in that optimum pressure range?

Yomi

At the risk of coming in the middle of a tangential conversation...

I can give no opinions on the 997, but the stock 996 suspension is softer than ideal for autocross. Remembering that autocross != track, and you often have to radically shift the direction of the car with no time to let the car take a set. Of course there is such a thing as too stiff, and it's possible to successfully drive the stock car (duh on both). But I agree with MagnusB in that some fairly straightforward changes can improve the autocross handling. Most of them also bump you into a faster class (a correlation that should make you go hmmm).

The comment on tire temps and air pressure just confuses me. Water vapor in the air inside tires expands when heated, making the pressure rise. This changes the profile. It's pretty normal seeing people adjusting pressures as the day goes on, keeping the tires at the pressures they believe are best. I'm just not getting which part of this process you don't believe in: tires heating up with use, pressure rising with heat, pyrometer readings showing contact patch changes with pressure, grip relation with contact patch. All of these are fairly well documented. Unless you're just commenting on how some people would blindly lower pressures without measuring the current pressure and rollover and without understanding what the pressures are doing to their grip.

On PSM: smoothness is important in autocross. Sometimes it doesn't look that way, and some cars / drivers are more sudden than others (back to F1, Alonso often jerks at the wheel from the camera's view, but it works for him). I found in my 996 on the first few courses that PSM never came on, and on later courses it was typically because I'd done something boneheaded (e.g. got late and had to turn sharper than I should have). It's pretty lenient. Admittedly I have street tires and R compounds will probably annoy PSM more. I have been turning it off on my runs these days because I want the predictability.

That said, I'd run PSM on the track -- partly because if I'm at an HPDE and get into a position where PSM is activating, I'm far past where I want to be. That's an indication of my skill level and goals however. I can't imagine why I'd turn it off on the street.

simsgw

Now that is a very good question. Cindy was taking a nap, so I typed. Then I typed some more. The first effort ran 12k bytes, the second 11k, and I stopped myself on the last effort at 4kb. All that typing to discuss tire design and reasons why we do adjust pressures at the race track -- sometimes -- and when we would not.

Basically, your question is a tough one. We're talking about a tire that was designed to be a competition tire while meeting the standards for a DOT-approved road tire. Between the physics stuff, everything I tried to say ended up discussing how racing teams pick pressures and adjust them. Well, this isn't an engineering seminar in the first place, and besides that we're really talking about fun events. Or so I assume. If you're going to the SCCA Nationals with those tires, then we can talk about this at more length. In fact, on the clear understanding that I took no time to edit my typing as I went, I'm willing to dump those first efforts on anybody who really wants to read enormous detail about tire design and pressure adjustments.

But I don't want to go too far the other way. For most drivers and most tires, the answer would be "just trust me, don't do it." They have neither time nor inclination to read a 27k introduction to the subject, let alone the whole books available on each substructure of tires. For someone serious enough to spend the money on a street-competition tire, your situation is different. You deserve more detail. So let's try a quick version. [Ahem. Checking the result, let's say relatively quick.]

Without getting into tire design too far, it is the epitome of "engineering is a compromise". The compounds alone have probably fueled a dozen doctoral theses in the last ten years. No single tire pressure is optimum for every corner of every track, nor all the straights connecting them. Moreover, when they are working (and also absorbing heat from the other car systems like brakes and exhaust), tire temperatures go up and down constantly. Tire pressures both follow those temperatures and mitigate them to some extent. Someone else said you need higher pressures to keep the profile and that's one of those sorta almost kinda true statements. The tire profile is dynamic and it was designed to be that way. You cannot "keep the profile" with a pneumatic tire. And you don't want to. What you can do is control the way the tire profile adjusts to the different combinations of cornering and acceleration around a track. Each track. Differently.

In the older cars I began in, it was easy to see why the "stiffer is better" philosophy arose. We added stiffer shocks, extra springs, and bumped tire pressures up to where the working pressures were near the manufacturer's safe-pressure rating. We were putting a gauze pad on a broken femur. Basically, the suspension of those cars was terrible. The interaction between body roll and tire/wheel geometry was not really designed so much as welded up on a "let's see what happens" basis. The early attempts to really design a suspension mathematically were so rare the names still resonate. People like DeDion and Chapman.

Now, they use supercomputers. And our suspension comes with one of those names attached: Porsche. Hell, in Formula One they even design a specific blend of gasoline for every track. The rules limit the combustion efficiency of the fuel, so they design for weight vs volume vs vapor pressure and probably a dozen factors a spacecraft engineer would not think of. Fuel is simple compared to the way tires and suspension interact. The Michelin Sports Cup tire has at least three different compounds of synthetic rubber that I personally know about from hearsay. I'm sure it really uses a couple more. They pick the operating temperatures of each of those compounds based on computer simulations of the mission profile. That is, how will an owner of a GT3 use this sort of tire? He won't be in Le Mans, but he might take a run at the Nordschleiffe. He won't run Monaco on Sunday, but he will autocross and might enter street races (mostly in Europe, as I did), and he certainly will want good performance on "track day" events. Then, assuming the really top drivers with a life to devote to practice will be running full competition tires, they factor in a little bit of forgiveness for "the rest of us." (I'll skip how you do that.) Not the level of forgiveness in Aunt Sadie's tires of course. Just something more than they would give Tony Stewart or Michael Schumacher. Then they use computer modeling to project the heat transfers across each thermal path of the target car model. A Porsche GT3 is not the same as BMW M-3 in that regard. What about the suspension? Is it adjustable? Of course, in cars of this class. So next, they model the effects of suspension set-up. Find the optimum for tracks and then back down to something the car designers will countenance for road/track mixed usage models. And then...

Then they pick a tire pressure recommendation.

And still... All those factors (and the 27k I did not post) cause so many imponderables that even the recommended value has an outrageous range for an engineering parameter. We often specify a part to the nearest ten thousandth of an inch. And those are cheap, well cheap-ish, commercial parts for a car. You should see the tolerances we specify in spacecraft structure, and don't get me started on computer chips.

So with all that granted, why should you or why should you not bleed air from your tires at an event? Well, with your choice of tires, I will not assert it's a no-no. Like racing tires, with the Michelin Sports Cup tires we have a target working temperature provided by the tire manufacturer. That is a very unusual situation that is essentially the first step we have to reach in race cars.

On the other hand, despite all that, Michelin is still designing a tire for mixed use. For road and track. They use the same techniques in design and that limits at the same time it helps them. What Michelin actually says is to use the cold temperatures recommended by the car's manufacturer for normal road use. On an event day, use the cold pressures they recommend for competition set-up. Then if the temps are not high enough at the events that you run, you should adjust those cold temp settings up or down for the next event. That's what I did also. After the first DE day in this car, I moved the front pressures up two pounds. I'm still within the Porsche recommendation range, but I reduced the differential between front and rear. I don't think Porsche is likely to suggest that to an average customer, but I liked the handling better on the second DE day with that adjustment, so I left them that way on the road. We'll see if the tire wear pattern is acceptable. Looks alright so far.

That sort of thing is just what Michelin has in mind. Adjusting tires while 'hot' does not simply 'beg' a question, it demands it: What is hot?

That's why the general answer is "trust me, don't do it." The person who asks won't consider 'hot' to be the same thing as I would. For people who care enough to run competition *** DOT tires, let me go further, just as Michelin did when they provided that working pressure spec.

What you will do in the optimum case is set your tires before the event according to Porsche's recommended range, because you're driving there on the same tires. If you're reasonably close to the event site, you can adjust to the lower cold settings of Michelin Cups. Something eight to ten pounds under road setttings if I remember correctly, in the mid twenties. Pick your point in that range based on past experience at similar events as I did. Then you must adjust that number up or down according to the temperature in your presumably closed garage before setting off in the morning. With a TPMS-equipped model, Porsche does that calculation for you, but it is slightly more complicated because we're not picking simple summer/winter, heavy-load/light-load settings. What I do is think in terms of so many pounds high or low compared to the computer setting in the TPMS. (That's light-load, summer for us.) That is, in my case, the fronts have to read out as being +2 and the rears have to be spot on. If you don't have TPMS, just check the temperature and adjust your target pressure by 0.7 psi for each ten degrees deviation from 68F.

Now we get to the track and measure the pressures. Let's say they were nine pounds under the TPMS setting in the morning. What do they read now? Five pounds low? Whatever it is, make that much adjustment. Otherwise, go through the whole temperature adjustment thing from scratch and set the tire pressures after letting them cool from the drive to the track. (See how much help TPMS can be? Without an adoring pit crew, dealing with things while you go to the drivers' meeting, all this is a pain in the ***.)

Now we have a repeatable measurement, and repeatable is very important if you really care this much. I don't use the TPMS reading to set the actual pressures incidentally. I use a racing pressure gauge with barometric compensation. I don't bother with the calculation now that I've cross-checked the TPMS when I bought the car. I just see what it says as a rough indication and then set each tire to the more precisely measured pressure using that gauge. (Alright, I cheat and do cross-check with a mental calculation because I'm an engineer. But it's not really necessary, just a professional habit.)

Next, hot air. Uh... not me. The kind in your tires at the track. What do you do if the pressures rise above the Michelin target temps? Well, letting the hobbyist speak for a minute, if I set the tires as described, I would wonder what I"m doing wrong that has the pressures rising like that. Maybe nothing. Maybe we're checking them at the wrong time. Or maybe we forgot to set them to the Michelin cold pressures in lieu of Porsche's over-the-road recommendation. You need to read Michelin's owners manual for these tires closely. How do they tell us to check the pressure? Right after we leave the track while sitting in a sunny pit? Or move it to the shade, wait fifteen minutes and then check pressures? I don't know. But we need to know. You need a repeatable measurement. Now it's still high? Why? Again, with the decision to make adjustments like that comes a responsibility (to your times if nothing else) to think about what you're trying to 'fix'. Figure it out. Clearly, if everything else is right, this driver and this suspension set-up requires lower cold pressures before we begin the runs.

Besides repeatability, you need to remember what really matters is the tire temperatures. (Well, mostly. I'll leave the rest to that 27k we're skipping.) Michelin has target temps for these tires, and for a non-race car, they must be read at the tread surface with a contact pyrometer. Their targets will be something like 190 to 200 F shortly after leaving the track. Read dynamically in a high-load corner with telemetry they might be as high as 450 F, so you see the problem here. The tread surface cools rapidly between stress events. What you need to do, if you really plan to adjust pressures according to temps at the track, is establish a routine so you get repeatability. It will be different at each track because of pit configuration, but so long as it's repeatable we're okay.

For example, at the Streets of Willow, the pit road is long and relatively slow. If I go all the way to my pit, the time delay will be different each time I come in from the track. What I would do is pull off through the skid pad as usual, but using the back-most exit path. Next to the wall there is a parking place they use for early-morning tech inspections. I'd pull somewhere around there, get out and check the tire temps. Three places for each tire, starting with the wheel on the outside for most turns. Running the Streets clockwise, that would be the left rear. Then go to the right rear. Or start with the fronts if you prefer.

It's best to do one axle each test run because the tires are cooling fast. And do multiple laps between each adjustment because tires have to stabilize in working condition for all this to matter at all.

All that is what it takes to make meaningful changes in tire pressures. Even with all that, the real problem may be wheel alignment. Maybe you're running too much camber on the outside for the long straight at Track FiddleBush and that is causing low temps on the center reading because the pit entrance is right afterward. Maybe you need more negative camber on the drive wheels when you're accelerating as you must after Turn 43 right before leaving the track? Or maybe you don't want that because this isn't a drag race and you're skewing your answer by letting one corner dominate the full track question. Maybe a lot of things.

Now we're back to the simple question: Do I need two pounds less in the right front? Or two more? Damfino is probably the only honest answer. Even a Michelin engineer would very likely give you the same answer. I know what Michelin Corporate would say:

• Stay within the range recommended by the car design team in the first place.
• Try to pick cold temp settings within that range that result in working temps at the event within our recommended range. (32-36 psi, did you say? Whatever the owners manual says anyway.)
• If you must adjust, do not exceed the safe-pressure value on the sidewall when adding air. When bleeding air, do not let the pressure fall below the equivalent of a cold twenty pounds or the bead may pop off the tire.

That's what I estimate Michelin will say if you dig around in the manual. Those are kind of "don't get dead or seriously mangled" recommendations and I honor them for that. For practical purposes at a fun event, what I would say is that -- again unless you're planning to take this outfit to the Nationals -- we don't have at amateur events the equipment, the manning, or the rigorous procedures possible to use tire pressures as an effective tool along with suspension adjustments in the pits and all the other ways we shoot for best qualifying time or TTOD at a Nationals autocross. So don't bother trying. Without the precision measurements and the fixed repeatable procedures, you just end up "chasing your set-up" we used to say.

If the tire pressure you measure right after a run is higher than Michelin recommends, I would first consider whether the last series of manuevers could have put more heat in the tires than is representative of the rest of the track. If you finish with a handbrake turn, the pressures in the rear tires will be higher than elsewhere. You don't want that to affect your overall performance. First, wait ten minutes and see what the pressures read. Then try a practice run backing down from that corner and check the pressures again. You can do that sort of thing, but it's damned hard to avoid "chasing your set-up" when you don't have a full day of practice and people making notes and taking temps and everything else.

I've never done autocross since the really serious version came into popularity. I don't know if we can duplicate those rigorous race track procedures at an autocross. I don't say I wouldn't try if I wanted to get to the Nationals, but I do say it's tougher to be precise about set-up compared to full race events.

A simple intuitive approach is probably just as effective as spurious precision. Michelin is very helpful in offering a target working pressure, so I would take advantage of it:

1. Set the pressures cold to some specific offset from Porsche's light-load recommendation (with ambient temp adjustment) before you leave home. You're looking for that Michelin recommended competition cold setting, but with your past adjustments taken into account.
2. Check the working pressures after you've driven enough to stabilize the tires. After a few laps is what I mean if that's possible. At least five I'd say. Do it after the first practice if you don't get multple chances.
3. Now adjust for handling purposes. You might want a little extra in the front, so add two pounds and make some more runs if allowed. Otherwise, take your best guess.
4. Take notes if you expect to return to this venue with a track configuration that is at all similar.

If you do find that pressures have gotten outside the high limit Michelin prescribes for a working pressure, then bleed a specific amount you decided in advance from those measurements. Remember that result and next time use that many pounds less as a cold setting before starting out in the morning. But I only say that because I assume you're the kind of person who buys such tires and you already did the rest of that stuff. A complete novice can make other mistakes that create a spurious reading. They "bleed a little air", take the car back out, and blow a tire on the second straight. And stuff an XKE into a concrete berm. Or a new Morgan Plus 4. Been there, seen both.

Bottom line. If you're the kind of person who read this full answer, you aren't going to get killed bleeding air. But don't relay the advice to some novice. Remember the short answer and give that: "just trust me, don't do it."

In a long life, you'll know enough dead people in the natural course of events. Feeling guilty about one more is not something that adds to retirement peace of mind.

Gary

AYHSMB

Magnus, you make some good points, so I'll try to get this back on topic.

I don't know about the 996 (I hear the M030 suspension for the 996 is very good) but 997 base and PASM suspension are both definitely not right for auto-x. PASM is over-sprung and under-damped. The optional S-PASM in the 997.2 is supposed to be much better. In general the cars don't have enough rear sway.

Another way to look at it is to look at what was done to the Cayman R - lower,stiffer, more rear sway, LSD - these are basically the same things you would like in a 997.

Yes, the 997 has a decent amount of throttle lag. I suspect part of it is the ECU timing advance, which a chip might fix. Another part of it is the heavy sprung dual-mass fly wheel, which you can fix by going to a light single mass unit.

It is definitely a bit non-linear , there are two peaks because of variocam, the first is around 3000 rpm, then there's a funny dip, then in comes back.

You absolutely should be faster in an auto-x with PSM off and saying otherwise is just being dense. Obviously your goal is to drive smoothly so that PSM never comes on, but any time it comes on, you would be faster with it off.

The 997 is a good autocrosser stock, but there are certainly things you'd like to change if you could. It's a damn shame that porsche didn't make a "sport pack" option that we could use in a stock class.

sjfehr

I've found the exact opposite to be true. I am generally faster with PSM on than off. Perhaps it's because I've learned to drive with it and take advantage of its 1-wheel braking and other magic, but it frequently turns a large error into a really small one and only on rare occasions does it ever interfere and slow me down. Makes it much easier to drive very close to the edge of traction.

simsgw

Not at all personal. I was making fun of your post, not you. Don't believe we've ever met, so how would I know? Forgive me if I sounded a little brittle, but my wife is sick, it's the first really hot week this year and I'm working on the failed cooler between posting these notes, and I'm not as healthy as I was -- so it's a pain in several ways. I apologize if I made it sound like a personal attack. It was not meant so.

I did suggest that perhaps you didn't mean what you actually said, and my intention was to give you a chance to back out of it. If you did in fact mean what you said about "the 997" and especially about "it doesn't really matter how much magic electronics you put into the car" then you're just wrong. I don't know how to soften that.

You can consider that arrogant if you like, but this is my profession. Would you expect me to nod respectfully if you complained of the carburetor in your car?

Porsche Stability Management operates as a save-your-*** system if you drive it in the gormless fashion I did when I first took mine on track. I kept taking it into corners like a Formula Ford, expecting that classic Porsche tail-wagging behavior and instead I got John Deere. Driven that way, the PSM assumes you're in over your head and throws out the anchor in a couple of ways. But driving the 997 the way I drove my Formula Ford or the way you drove your 996 is not good driving. You have to suit your style to the car.

When you do that, and admittedly it took me a couple of track hours to figure out, you find out the car is faster than any other Porsche we drove before buying. I can't compare it to your modified 996 without driving them both, but of course we could find out if we were on the same track. I believe I mentioned that traditional way of settling debates about cars.

Now, as for body roll... The stiffness of the suspension determines the natural rhythm of the car. That much is always true. As a rule of thumb, in a proper modern suspension, increasing stiffness decreases mechanical grip and softening it increases grip. That's why we increase the stiffness at the rear or soften it up front if the car is understeering excessively. And so forth and so on. Jounce, rebound, camber, lots of other stuff in there. Whole books have been written, as I mentioned elsewhere. But as far as stiffness itself is concerned, that's the rule you have to remember.

As to whether a go-kart the size of a 911 is the fastest way round a gymkhana... well, I'd be likely to agree. With many very tight turns and reversals you need a very rapid response -- a hyperfast rhythm to the suspension. But I gathered that autocrosses have grown up since those days when we'd borrow a parking lot at Toys-R-Us, throw down some pylons and a little basketball hoop for the "shoot and reverse out" obstacle and set Alice down with a stopwatch. The last two I saw took place on an abandoned airport and the Streets of Willow. Neither was an especially high speed event, it took a car like the 997's or the Lamborghini to reach three-digit speeds, but they certainly were not twitchy little games either. They were proper speed events, with the simple restriction that speeds were limited by the set-up and you only got one lap.

At those events, the 997.2 we drive would have been just fine. The natural rhythm of a 997 with PASM in sport mode is about as high as an SCCA Spec Racer I'd say. A little higher if memory serves, but I won't swear without better data. In any case, more than ample for those two events I saw being run, and certainly enough at those DE days to make me wish for five-point belts and a racing seat. I do not assert that all autocrosses are as open as those two, but at those events the rate of change of direction was roughly the same as any race track that we might call a 'technical' track.

The misconception about PSM is easy to reach because the PC way to describe such things is like that stupid Mercedes commercial with a bunch of old people bragging about falling asleep at the wheel. (That actually caused us to cancel our appointment at a Mercedes dealer once we recognized their design direction.) With the Green movement in Germany so important, I can't see Porsche going too far in the opposite direction either, so they do let you assume that PSM is there to 'catch' the car when you do something stupid. It works for that, as I mentioned a few days ago. But you also could feed a rope through the open windows and use the car to pull a cow out of a mudhole. That doesn't mean towing cows is the real value of a Carrera. A stuck cow might have a different opinion of course.

What PSM does is blend braking and cornering seamlessly so you optimize the number of seconds the tires are delivering their full potential on each lap. I still don't propose to teach race driving via e-mail, but if you can't see the number of seconds that saves, consider that an important guideline in racing school is "drive the car, but race the tires." You've got four little thrusters on this car and if they're not doing their best, nothing else matters. We can't really take them to their limits without full thrust vectoring in an all-wheel drive package like the Turbo S, but PSM comes darned close.

Mind you, I don't object to people modifying their cars for the fun of it. Some of the guys in high school used to hang fuzzy dice on their mirror too. Whether it's an aftermarket supercharger or fuzzy dice, it's all fun. If they weren't toys in many ways, we'd all buy a soccer van or a pick-up truck. If you think you can make the 997 faster by modifying the suspension, you may be right. That's what opinions are for, but if you do think that I'd love to meet you at a track sometime. Just not a Toys-R-Us...

Professor Gary, retired

simsgw

Fascinating. Do you also make sure you brake gently enough to avoid ABS?

G

Mike in CA

Wow, Gary. A lot of really good, thoughtful information there and well presented. Thank you. I must admit though, that I wasn't looking for direction so much, as I've been autoxing for about 35 years now and am reasonably familiar with the issues and techniques around managing cold and hot tire pressures for best results.

Instead, my question to you about why I shouldn't adjust tire pressures to keep them in the optimum range, was a polite way of asking what the heck you were talking about when you wrote, "Those are the same guys who tell you to be sure to bleed air when the tires warm up ". To me, it seemed to be a blanket pejorative statement but perhaps I misunderstood your intent.

Having read your latest post I see clarification of your position. As you have pointed out there are many variables to consider when adjusting tire pressure. FWIW, I start with cold pressures that experience has shown get me in the ballbark. Then using an accurate pressure guage and a pyrometer, I chalk my sidewalls, and taking into account how warm the day is and where I am in the sequence of runs and the F/R pressure balance that is best for my car and driving style, I can zero in on the optimum pressure as my tires heat up. Usually, for the last couple of runs I'm able to find a sweet spot with minimal additional adjustments and achieve my best times.

But yes, I do "bleed air when the tires warm up".

simsgw

Well, yes, but not because you heard somebody hanging on the rail say you should. You're obviously doing just what I was talking about, Mike. As I said in another note, I'm having a rough couple of days here. Sorry if I sound offensive. It isn't intentional at all, just old and creaky showing through.

Gary

Mike in CA

Not offensive at all, Gary. I always enjoy your comments. And I can relate to old and creaky......

Mike in CA

There are two possible outcomes when PSM intervenes; it's either preventing you from making some highly skilled maneuver that is outside the normal parameters of the system's programming and would otherwise make you faster, or it is preventing you from making a big mistake, turning a large error into a really small one as sjfehr so nicely put it, that would slow you down at best or spin you out of control at worst.

Your statement above assumes that only one of those two outcomes is possible, which on the face of it is incorrect. In fact, given the sophistication of the programming and the way the system implements it, the second result is much more likely than the first. Maybe Vettel or Hamilton would benefit from PSM being turned off but for the vast majority of us, keeping PSM engaged usually leads to faster times.

ADias

Mike: Note that Gary means 'cold temp' to be referred to 68F as an absolute reference. That 's what PAG recommends too. What this means is that 33PSI cold means 33PSI if the that day cold temp is 68F, but if the the day temp is 78F it should actually be set 0.7/0.8PSI higher (lower if lower temp). I have been following this absolute temp ref and I like it.