Tire pressure for plus 2 upgrade; confused?
09-26-2017, 04:13 AM
#16
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Technically, racers measure the temperture of the tire's core. Not the tread. As they pit, the tread starts to cool, while the core's stay accurate for a longer period of time, and that's what the tire engineers use. I learned this from the Michelin staff this year at the WeatherTech IMSA races at Road America. Michelin gave a technical session to us in the Porscheplatz tent.
Tyres cool with alacrity and ideally need to be monitored in real time as they do in F1 given the temps change more or less instantly depending on whether the car is braking, cornering or running straight. In F1 about 10 years ago they came up with wide range IR scanners and before long they were broadcasting the output showing how the temperature was changing across the tread surface for each transition. Interestingly, nowadays they do not tend to show these during races- probably because the novelty has worn off..
Sticking a probe into the tyre to measure the body temperature is a comparatively "low tech small budget" compromise but at the end of the day what one needs to know is the optimal temperature at the point of contact in real time as overheating will kill the tyre life dramatically.
In real world practical motoring terms there is no "one size fits all" when it comes to tyre pressures -it is more a question of what works best across the board for the specific conditions one is asking the car to accomodate
09-26-2017, 04:45 AM
#17
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On my GTS with 18in wheels I found the most even heating across the tire autocrossing is 34F 34R HOT. Higher and tires get hotter in the center of the tread and will eventually blister. Cooler and the edges of the tread get and you are not using the whole foot print. Ideally you want the tread across the tire getting heated evenly. 34 34 provides the quickest autocross times.
After they have cooled completely (next morning) they are at 28F 30R. Front tires work harder and gain more heat autocrossing.
Since tires are not worked nearly as much daily driving I set my tire pressures at 32F 34R cold for daily driving. With these cold pressures the temperatures measured across the tread are even for the most contact patch. They gain enough heat for the pressures to increase to near 34F 36R.
If going to be driving at speeds in excess of 100 mph for extended periods I increase the cold pressures to 34F 36R this will gain to near 38 psi. The important part is the temperatures measure even across the tread. Lower pressures and the tires actually get even hotter maintaining the higher speeds. Higher pressures and they actually don't get hot enough.
I have monkeyed with the pressures and measured the temperatures across the tread a LOT. This has been my experience.
After they have cooled completely (next morning) they are at 28F 30R. Front tires work harder and gain more heat autocrossing.
Since tires are not worked nearly as much daily driving I set my tire pressures at 32F 34R cold for daily driving. With these cold pressures the temperatures measured across the tread are even for the most contact patch. They gain enough heat for the pressures to increase to near 34F 36R.
If going to be driving at speeds in excess of 100 mph for extended periods I increase the cold pressures to 34F 36R this will gain to near 38 psi. The important part is the temperatures measure even across the tread. Lower pressures and the tires actually get even hotter maintaining the higher speeds. Higher pressures and they actually don't get hot enough.
I have monkeyed with the pressures and measured the temperatures across the tread a LOT. This has been my experience.
Last edited by RKD in OKC; 09-26-2017 at 05:08 AM.
09-26-2017, 07:50 AM
#18
Three Wheelin'
Thread Starter
Okay...beginning to get a slight understanding:
Clearly, the factory numbers are antiquated and the explanations make the case.
what I am having trouble with are the front to rear variances:
my late model, newer cars, have similar low profile performance tires
and all of them have lower pressures at the front and higher at the rear.
for instance, our Mercedes, with "similar" tires are 29/35.
most of the responses here have the rear wider tires with a slightly lower
value than the front.
can someone explain the reverse numbers, (front/rear)? not trying to split hairs,
just trying to comprehend like Spock; logic
Clearly, the factory numbers are antiquated and the explanations make the case.
what I am having trouble with are the front to rear variances:
my late model, newer cars, have similar low profile performance tires
and all of them have lower pressures at the front and higher at the rear.
for instance, our Mercedes, with "similar" tires are 29/35.
most of the responses here have the rear wider tires with a slightly lower
value than the front.
can someone explain the reverse numbers, (front/rear)? not trying to split hairs,
just trying to comprehend like Spock; logic
09-26-2017, 12:32 PM
#19
Okay...beginning to get a slight understanding:
Clearly, the factory numbers are antiquated and the explanations make the case.
what I am having trouble with are the front to rear variances:
my late model, newer cars, have similar low profile performance tires
and all of them have lower pressures at the front and higher at the rear.
for instance, our Mercedes, with "similar" tires are 29/35.
most of the responses here have the rear wider tires with a slightly lower
value than the front.
can someone explain the reverse numbers, (front/rear)? not trying to split hairs,
just trying to comprehend like Spock; logic
Clearly, the factory numbers are antiquated and the explanations make the case.
what I am having trouble with are the front to rear variances:
my late model, newer cars, have similar low profile performance tires
and all of them have lower pressures at the front and higher at the rear.
for instance, our Mercedes, with "similar" tires are 29/35.
most of the responses here have the rear wider tires with a slightly lower
value than the front.
can someone explain the reverse numbers, (front/rear)? not trying to split hairs,
just trying to comprehend like Spock; logic
Read the PDF I linked to. It's not vendor specific and it shows the method used to determine inflation pressure based on vehicle weight (per tire) and tire load factor.
Consider the numbers you get as a starting point and then adjust as desired.
If you're not using factory rim and tire sizes, you can pretty much ignore what the manual says.
Hugo
09-26-2017, 12:50 PM
#20
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Hang on.
Long-Timers: About 10-15 years ago some 928 long-timer wrote a multi-paragraph explanation of how to think about weight/load on each tire, contact patch shape, tire pressure, speed, PV=nRT and the basic dynamics of changes.
Might have been Jim Bailey, Wally Plumby. Heck, might have been Ed Ruiz or Walt Koneckny<sic?>.
Does this ring a bell?
I'd rather link to the original than plagiarize from whole cloth and dim memory.
Long-Timers: About 10-15 years ago some 928 long-timer wrote a multi-paragraph explanation of how to think about weight/load on each tire, contact patch shape, tire pressure, speed, PV=nRT and the basic dynamics of changes.
Might have been Jim Bailey, Wally Plumby. Heck, might have been Ed Ruiz or Walt Koneckny<sic?>.
Does this ring a bell?
I'd rather link to the original than plagiarize from whole cloth and dim memory.
09-26-2017, 01:47 PM
#21
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The physics of tires haven't changed since ... the invention of the tire. But, materials, design and manufacturing technology have undergone significant changes in the last 30 years.
Modern tires are able to operate at higher temperatures and dissipate sidewall heat faster than the old-tech tires that were extant at the time Porsche recommended pressures for 928s(*).
Bottom line: for modern tires the 44 PSI recommendation is nuts.
Modern tires are able to operate at higher temperatures and dissipate sidewall heat faster than the old-tech tires that were extant at the time Porsche recommended pressures for 928s(*).
Bottom line: for modern tires the 44 PSI recommendation is nuts.
09-26-2017, 02:18 PM
#22
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You said in your response that racers measure the core temperature and indeed they do this- by use of a tyre pyrometer which has a needle that they literally stick into the core of the tyre to give a more representative measure of the temperature the rubber had been heated up to as the outer skin will have long since cooled [with said alacrity] but to some extent acts as an insulating barrier to then keep the heat in. One can purchase tools to do this starting at around $150 a pop if not cheaper. This is the low tech low cost method to determine tyre temperature and works best if you can do the measure by the side of the track. If of interest you can find out a bit more here:
http://www.longacreracing.com/techni...rometer%20Tips
If you know of any other method to measure such feel free to share as we are always keen to learn.
A well known racer friend of mine came up with the alacrity bit many years ago - I also had to look it up!
09-26-2017, 02:46 PM
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Lex,
You said in your response that racers measure the core temperature and indeed they do this- by use of a tyre pyrometer which has a needle that they literally stick into the core of the tyre to give a more representative measure of the temperature the rubber had been heated up to as the outer skin will have long since cooled [with said alacrity] but to some extent acts as an insulating barrier to then keep the heat in. One can purchase tools to do this starting at around $150 a pop if not cheaper. This is the low tech low cost method to determine tyre temperature and works best if you can do the measure by the side of the track. If of interest you can find out a bit more here:
http://www.longacreracing.com/techni...rometer%20Tips
If you know of any other method to measure such feel free to share as we are always keen to learn.
A well known racer friend of mine came up with the alacrity bit many years ago - I also had to look it up!
You said in your response that racers measure the core temperature and indeed they do this- by use of a tyre pyrometer which has a needle that they literally stick into the core of the tyre to give a more representative measure of the temperature the rubber had been heated up to as the outer skin will have long since cooled [with said alacrity] but to some extent acts as an insulating barrier to then keep the heat in. One can purchase tools to do this starting at around $150 a pop if not cheaper. This is the low tech low cost method to determine tyre temperature and works best if you can do the measure by the side of the track. If of interest you can find out a bit more here:
http://www.longacreracing.com/techni...rometer%20Tips
If you know of any other method to measure such feel free to share as we are always keen to learn.
A well known racer friend of mine came up with the alacrity bit many years ago - I also had to look it up!
This is on the new 911 RSR. I'll try to find out how they monitor core tire temps. Might be a while.
09-26-2017, 05:50 PM
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Let me make it more precise:
Bottom line: for modern tires on a 928 the 44 PSI recommendation is nuts for normal driving on North American streets.
Porsche's recommended rear tire pressures for 911s varies from 31 PSI to 44 PSI depending upon tire type, tire size, 911 flavor, cargo/passenger load, and comfort vs. standard.
I run 31/36 on the 991: part-load comfort pressure (only good to (IIRC) 165 mph.) And those pressures are quite a bit too high for track work and too low for A-X.
Tire pressures between 911s and 928s are not comparable. Tire sizes (width and sidewall) are different, weight balance is different, top speeds are different.
Originally Posted by LexVan
So it's not just old vs new tire debate.
Though here's an example of old versus new:
Pre-GTS (old-tire) 928s required 44 PSI to safely sustain top speed (160-170-ish) with 11 cm of sidewall. Whereas my far-a$$-heavier 911 can run that speed at 36 PSI with 9 cm of sidewall.
Maximum speed rating at a particular tire load and pressure is all about the ability of the sidewall to dissipate heat. Less sidewall means less surface area for dissipation. Higher speed less means time for the sidewall to radiate before it comes around again to absorb flexion heat when it's near the ground.
Edit: sidewall and the center of the tire. Both heat from flexion and both dissipate heat. But the reinforced treaded section is lots tougher than the sidewalls.
09-26-2017, 08:01 PM
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Let's use some round numbers and squares.
Your car weighs 3500 pounds. How much tire and air pressure do you need to hold it off the ground?
Well, 1 square inch of tire at 3500 pounds will do it. The other extreme is 3500 square inches at 1 PSI. Neither works in the real world but they are illustrative.
Let's use a rounded-off 928 at 3500 pounds and 36 PSI of tire pressure. These means you've got roughly 97 square inches of tire in contact with the ground. Divide that by four (tires) and take the square root:
Each tire - assuming equal car weight distribution, the same wheels and tires, and assuming a square contact patch - has a contact patch approximately 4.9 inches square.
I think you'll agree that that figure makes some intuitive sense.
But a 928 is nose heavy (and a 911 is a$$-heavy.) So - again, all other things (tire sizes, etc.) equal - on a 928 you're going to have a bigger contact patch on the front than the rear and the a$$-heavy 911's going to have a monster rear patch and a little tiny front patch.
Let's assume we want balanced contact patches. Why? Well, we want balanced handling right? (*)
What do we do? We increase the pressure on the heavy end and decrease the pressure on the light end. So, with equal sized tires a 928 wants a bit more pressure on the front than the rear and on a 911 more on the rear and less on the front (all other considerations set aside for the moment.)
(*) Wait. You say that Coulomb's investigation of friction in the 1700s demonstrated that "the force of friction is directly proportional to the applied load and is independent of the apparent area of contact."
Yeah. Not for pneumatic tires on asphalt. Or indeed, only for rigid bodies on perfectly flat surfaces with uniform coefficients of friction.
Next up: contact patches.
Your car weighs 3500 pounds. How much tire and air pressure do you need to hold it off the ground?
Well, 1 square inch of tire at 3500 pounds will do it. The other extreme is 3500 square inches at 1 PSI. Neither works in the real world but they are illustrative.
Let's use a rounded-off 928 at 3500 pounds and 36 PSI of tire pressure. These means you've got roughly 97 square inches of tire in contact with the ground. Divide that by four (tires) and take the square root:
Each tire - assuming equal car weight distribution, the same wheels and tires, and assuming a square contact patch - has a contact patch approximately 4.9 inches square.
I think you'll agree that that figure makes some intuitive sense.
But a 928 is nose heavy (and a 911 is a$$-heavy.) So - again, all other things (tire sizes, etc.) equal - on a 928 you're going to have a bigger contact patch on the front than the rear and the a$$-heavy 911's going to have a monster rear patch and a little tiny front patch.
Let's assume we want balanced contact patches. Why? Well, we want balanced handling right? (*)
What do we do? We increase the pressure on the heavy end and decrease the pressure on the light end. So, with equal sized tires a 928 wants a bit more pressure on the front than the rear and on a 911 more on the rear and less on the front (all other considerations set aside for the moment.)
(*) Wait. You say that Coulomb's investigation of friction in the 1700s demonstrated that "the force of friction is directly proportional to the applied load and is independent of the apparent area of contact."
Yeah. Not for pneumatic tires on asphalt. Or indeed, only for rigid bodies on perfectly flat surfaces with uniform coefficients of friction.
Next up: contact patches.
09-26-2017, 08:10 PM
#26
Three Wheelin'
Thread Starter
Let's use a rounded-off 928 at 3500 pounds and 36 PSI of tire pressure. These means you've got roughly 97 square inches of tire in contact with the ground. Divide that by four (tires) and take the square rootDave: that makes good sense, but what about the fact that I am running 235 up front and 295 at the rears. So how would you advise to compensate?
09-26-2017, 09:32 PM
#27
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By the way, this is what it looks like when you correctly quote a previous post.
Note the use of the open and close square brackets with QUOTE and /QUOTE inside.
09-26-2017, 10:34 PM
#28
This is what I get when I work through the PDF I linked. I'll be happy to edit or delete if it's inaccurate or just plain wrong.
First, from the owner's manual, original tires for an 87 S4:
Front: 225/50/16 @ 36 PSI minimum load index 88
Rear: 245/45/16 @ 44 PSI minimum load index 88
Using the ISO ETRTO standard load inflation table:
For the fronts, load index 88 at 36 PSI corresponds to 1235 lbs
For the rears, I used the reinforced load inflation table because it goes up to 42 PSI. For load index 88 at 42 PSI, the rating is 1235 lbs..
If you're looking at replacing the original tires with Pilot Sport PS2's:
Front: 235/40/18 Load rating 95
Rear: 295/30/18 Load rating 98
Based on the ETRTO load inflation table:
Front: minimum 29 PSI for 1235 lb load rating for load index of 95 using the standard load inflation table
Rear: minimum 26 PSI for 1235 load rating for load index 98 using the standard inflation table or 30 PSI using the reinforced inflation table
Check this link for another take on the subject: http://jenniskens.livedsl.nl/Technic...3/MyTip392.htm
Hugo
First, from the owner's manual, original tires for an 87 S4:
Front: 225/50/16 @ 36 PSI minimum load index 88
Rear: 245/45/16 @ 44 PSI minimum load index 88
Using the ISO ETRTO standard load inflation table:
For the fronts, load index 88 at 36 PSI corresponds to 1235 lbs
For the rears, I used the reinforced load inflation table because it goes up to 42 PSI. For load index 88 at 42 PSI, the rating is 1235 lbs..
If you're looking at replacing the original tires with Pilot Sport PS2's:
Front: 235/40/18 Load rating 95
Rear: 295/30/18 Load rating 98
Based on the ETRTO load inflation table:
Front: minimum 29 PSI for 1235 lb load rating for load index of 95 using the standard load inflation table
Rear: minimum 26 PSI for 1235 load rating for load index 98 using the standard inflation table or 30 PSI using the reinforced inflation table
Check this link for another take on the subject: http://jenniskens.livedsl.nl/Technic...3/MyTip392.htm
Hugo
09-26-2017, 11:50 PM
#29
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So, contact patches.
(And a note: All the math and assumed numbers are purely for purposes of illustrating the dynamics as the various characteristics change. Don't go all slide-rule on me...)
Think about a bicycle tire. It's super thin and has a short side wall. Not a lot of contact patch is possible. Let's call it one square inch. Two tires, two inches of contact patch total. Add 30? pounds of cycle and 170 pounds of rider.
200 pounds, two square inches of patch. So, about 100 PSI per tire to hold up the bike.
What happens if we double the pressure (and make the math easy)? 200 PSI per tire, 400 total, 200 pounds of load. So, we've now cut our contact patch in half.
What happens if we lower the pressure to 50? Do we get 4 inches of patch? Maybe. Probably not though, as the super thin side walls will probably have bulged-out like a balloon and we'll be riding on rim. (As in, a flat tire...)
What can we do about this? Stiffer sidewall. Sure. More - taller - sidewall? That'll help too. But, since we're dealing with a narrow tire we can only go so far. At some point a really tall sidewall won't stay seated on the rim or it'll bend into a U-shape. A super-stiff side wall will be like riding on the rim anyway.
Let's go back to the 200 PSI example: we doubled the pressure and cut the contact patch in half. What does that contact patch look like? If it was square before, what does it look like now? Is it still going to be square? Probably not. The added pressure will raise the rim further from the ground, the side walls will become less 'bulgey' and take pressure off the edges of the contact patch: the patch will become narrower. There will be some effect on the length of the contact patch too but it will be less than on the sides. The center of the tire will be in contact with the ground and the edges less so.
This, incidentally, is why gross over-inflation of a tire will cause the center of the tread to wear more than the edges: the pressure is bulging the tire in the center of the tread. The contact patch has been made narrower. The extreme edges of the tread are no longer in contact with the pavement.
For under-inflation the reverse is true. As you lower the pressure, eventually most of the load is born by the edges of the tread. The center of the tread bulges-up and doesn't contact the road as much as the edges do. Thus the edges of the tread wear much faster than the center.
What's all this mean? Outside of the extremes, adding a bit of pressure narrows the contact patch. Reducing pressure a bit makes the patch wider.
Now let's reverse all of this.
Take a narrow tire and swap it for a wider tire but keep the same pressure and same load. What happens? The contact patch is going to be about the same size (for non-ludicrous changes in tire width of course). 600 pounds on a tire at 30 PSI is 20 inches of patch regardless of whether it's a 215, 225, or 235 width tire.
What does this mean when you go from 225 to 235 and don't change the load? It means that your 235 tire is slightly-bulged in the middle and that you've got some 'unused' tread on the edges. If you lower the pressure slightly you can make use of those edges and get a bit more contact patch.
But, wait, didn't someone write that with a wider tire, with a lower profile (so that you can +N the wheel size) you're supposed to have higher pressure? Maybe. Lower profile for a given tread width means a shorter sidewall. What does that have to do with it?
Next: sidewalls.
(And a note: All the math and assumed numbers are purely for purposes of illustrating the dynamics as the various characteristics change. Don't go all slide-rule on me...)
Think about a bicycle tire. It's super thin and has a short side wall. Not a lot of contact patch is possible. Let's call it one square inch. Two tires, two inches of contact patch total. Add 30? pounds of cycle and 170 pounds of rider.
200 pounds, two square inches of patch. So, about 100 PSI per tire to hold up the bike.
What happens if we double the pressure (and make the math easy)? 200 PSI per tire, 400 total, 200 pounds of load. So, we've now cut our contact patch in half.
What happens if we lower the pressure to 50? Do we get 4 inches of patch? Maybe. Probably not though, as the super thin side walls will probably have bulged-out like a balloon and we'll be riding on rim. (As in, a flat tire...)
What can we do about this? Stiffer sidewall. Sure. More - taller - sidewall? That'll help too. But, since we're dealing with a narrow tire we can only go so far. At some point a really tall sidewall won't stay seated on the rim or it'll bend into a U-shape. A super-stiff side wall will be like riding on the rim anyway.
Let's go back to the 200 PSI example: we doubled the pressure and cut the contact patch in half. What does that contact patch look like? If it was square before, what does it look like now? Is it still going to be square? Probably not. The added pressure will raise the rim further from the ground, the side walls will become less 'bulgey' and take pressure off the edges of the contact patch: the patch will become narrower. There will be some effect on the length of the contact patch too but it will be less than on the sides. The center of the tire will be in contact with the ground and the edges less so.
This, incidentally, is why gross over-inflation of a tire will cause the center of the tread to wear more than the edges: the pressure is bulging the tire in the center of the tread. The contact patch has been made narrower. The extreme edges of the tread are no longer in contact with the pavement.
For under-inflation the reverse is true. As you lower the pressure, eventually most of the load is born by the edges of the tread. The center of the tread bulges-up and doesn't contact the road as much as the edges do. Thus the edges of the tread wear much faster than the center.
What's all this mean? Outside of the extremes, adding a bit of pressure narrows the contact patch. Reducing pressure a bit makes the patch wider.
Now let's reverse all of this.
Take a narrow tire and swap it for a wider tire but keep the same pressure and same load. What happens? The contact patch is going to be about the same size (for non-ludicrous changes in tire width of course). 600 pounds on a tire at 30 PSI is 20 inches of patch regardless of whether it's a 215, 225, or 235 width tire.
What does this mean when you go from 225 to 235 and don't change the load? It means that your 235 tire is slightly-bulged in the middle and that you've got some 'unused' tread on the edges. If you lower the pressure slightly you can make use of those edges and get a bit more contact patch.
But, wait, didn't someone write that with a wider tire, with a lower profile (so that you can +N the wheel size) you're supposed to have higher pressure? Maybe. Lower profile for a given tread width means a shorter sidewall. What does that have to do with it?
Next: sidewalls.
09-27-2017, 09:52 AM
#30
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You can use the calculations and tread width, section, and sidewall height stuff all you want and while it may get you close to the optimum tire pressures it is NOT optimal. Sure that may work okay if there are a couple of things assumed.
First highway, track, street, and autocross all work the tires differently. Not just average and top speeds but also cornering, radius, and cornering speeds.
Besides just tire width and sidewall height there is also the rim width. The pressure to provide the tire works evenly across the tread is different for the same tire width for different rim widths due to the shape of the tire. It also can create more or less pressure by how much the sidewall bulges or doesn't bulge due to that rim width tire width combination. The effect is more pronounced with lower profile tires because pressure range is smaller to maintain the center tread contact.
The next thing to consider is the average number of curves and the radius and speed of those curves. Turning the front wheels changes the castor and camber, ie the angle of the rim with respect to the road surface. The camber and toe on the rear of the 928 changes, but not nearly as much as the fronts. These changes, besides the side loading with work the tires MORE causing the temperatures of the tires to rise and the pressures along with them. This is typically why front tires COLD pressures are lower than rear tire pressures.
Then there is also the spring rates of the tires pressures and what that does to the handling characteristics. Smaller radius turns and their slower speeds make a vehicle tend to push or understeer, while with the same exact setup larger radius turns and higher speed tend to oversteer. Often rear tire pressures are increase to help remove oversteer at higher speeds by adding spring rate to the rear of the car. This makes the vehicle safer.
The last thing to consider is that what is being quoted are COLD pressures. As a tire is worked it heats up. The general rule the pressure increase 4% between cold at hot. But that is affected by the pressure and how much the tire is used. Generally with higher pressures the center of the tire heats the tire less than the 4% because the edges are not being used. Lower pressures increase the temperature and tire pressures more because the outside tread is used and the sidewalls flex more.
All that being said there is a single optimum pressure a tire wheel combo runs at for the best contact and handling HOT. The cold pressures vary widely according to the driving being done to achieve that hot pressure. Once I find what the optimum handling pressure is by measuring tread temperatures I can then maintain the best HOT pressures and only need to pull over and measure tire pressure to determine if the tires are heating properly and not really mess with the tire temps. I also note the cold pressures after the car has cooled significantly for a particulare driving type so I now where to start from depending on the type of driving I am planning to do,
With all that the best way do determine what tire pressures are optimum is to drive the car the way you are intending to drive it and measure the temperatures across the tread to see if the edges and center are all being worked. This is best done with a tyre pyrometer ie with the needle to go a little into the tread to get the temp. It is used because the surface of the tire tread cools very quickly effecting readings. You can also tell if you have too much or too little camber be comparing the inside egde temps to the outside edge temps.
It is also recommended that novices run higher pressures while autocrossing. This does two things. It insures the tires will stay cooler (more pressure) and will provide less cornering traction slowing novices down. A novice is not going to pay enough attention to tire inflation and scrub to keep the side loading from pushing the tires off the rims. If you autocross with pressures too high with more advanced capabilities (pushing the car faster) you run the risk of overheating the center of the tire tread and blistering the tires.
Now I'm tired.
First highway, track, street, and autocross all work the tires differently. Not just average and top speeds but also cornering, radius, and cornering speeds.
Besides just tire width and sidewall height there is also the rim width. The pressure to provide the tire works evenly across the tread is different for the same tire width for different rim widths due to the shape of the tire. It also can create more or less pressure by how much the sidewall bulges or doesn't bulge due to that rim width tire width combination. The effect is more pronounced with lower profile tires because pressure range is smaller to maintain the center tread contact.
The next thing to consider is the average number of curves and the radius and speed of those curves. Turning the front wheels changes the castor and camber, ie the angle of the rim with respect to the road surface. The camber and toe on the rear of the 928 changes, but not nearly as much as the fronts. These changes, besides the side loading with work the tires MORE causing the temperatures of the tires to rise and the pressures along with them. This is typically why front tires COLD pressures are lower than rear tire pressures.
Then there is also the spring rates of the tires pressures and what that does to the handling characteristics. Smaller radius turns and their slower speeds make a vehicle tend to push or understeer, while with the same exact setup larger radius turns and higher speed tend to oversteer. Often rear tire pressures are increase to help remove oversteer at higher speeds by adding spring rate to the rear of the car. This makes the vehicle safer.
The last thing to consider is that what is being quoted are COLD pressures. As a tire is worked it heats up. The general rule the pressure increase 4% between cold at hot. But that is affected by the pressure and how much the tire is used. Generally with higher pressures the center of the tire heats the tire less than the 4% because the edges are not being used. Lower pressures increase the temperature and tire pressures more because the outside tread is used and the sidewalls flex more.
All that being said there is a single optimum pressure a tire wheel combo runs at for the best contact and handling HOT. The cold pressures vary widely according to the driving being done to achieve that hot pressure. Once I find what the optimum handling pressure is by measuring tread temperatures I can then maintain the best HOT pressures and only need to pull over and measure tire pressure to determine if the tires are heating properly and not really mess with the tire temps. I also note the cold pressures after the car has cooled significantly for a particulare driving type so I now where to start from depending on the type of driving I am planning to do,
With all that the best way do determine what tire pressures are optimum is to drive the car the way you are intending to drive it and measure the temperatures across the tread to see if the edges and center are all being worked. This is best done with a tyre pyrometer ie with the needle to go a little into the tread to get the temp. It is used because the surface of the tire tread cools very quickly effecting readings. You can also tell if you have too much or too little camber be comparing the inside egde temps to the outside edge temps.
It is also recommended that novices run higher pressures while autocrossing. This does two things. It insures the tires will stay cooler (more pressure) and will provide less cornering traction slowing novices down. A novice is not going to pay enough attention to tire inflation and scrub to keep the side loading from pushing the tires off the rims. If you autocross with pressures too high with more advanced capabilities (pushing the car faster) you run the risk of overheating the center of the tire tread and blistering the tires.
Now I'm tired.