how can the factory recs for tire pressure be so far off? porsche appears to me to

be very careful-almost compulsive with their recommendations. for example, what other manufacturer goes to the trouble of researching specific tire models and brands every year? i have tremendous respect for viken's knowledge but such an egregious mistake by the porsche factory appears to me to be very unlikely! how could they be so wrong about something so simple and so crucial? <img src="confused.gif" border="0">

Their recommendations may be based on factors that are weighted differently than an enthusiast might weigh them. For instance, they may be weighting tire life higher than most of us would because we may be more interested in performance. I don't know for sure, but perhaps inflating to a higher pressure increases tire life, or traction in wet conditions, or decreases the chance of the tire coming off the rim in an accident, etc. Who knows. Any tire experts in the house? Ken

I assume there's no mistake on their part. I assume that they know what they're doing, and that they also know what we're doing; not running at their recommended pressure. I can only speculate on the rest.

Alright, now I'm REALLY confused!! Lets keep this simple. What are the best pressures for the C4S tires, Fr=225/40-18 and Rr=295/30-18 PZeros? Here's what I know (All cold temp pressures). Street use (Porsche recc): 36 Fr, 44 Rr. Track use. Tires run at higher temp and are subject to more forces in all directions, so run softer?? 32 Fr, 36 Rr?? How about posting your tires and the pressures you all run them at.

[quote]Originally posted by Ken Kammerer: <strong>I don't know for sure, but perhaps inflating to a higher pressure increases tire life, or traction in wet conditions, or decreases the chance of the tire coming off the rim in an accident, etc. Who knows. </strong><hr></blockquote> Higher pressure will decrease rolling resistance which helps fuel economy. That is my guess as to Porsche's recommendations. It will not help wet traction or increase tire life, the opposite is actually true. Greg

Remember, too, that we're talking about a car with 170mph+ capability that is manufactured in a country where people actually drive at those speeds. The higher inflation pressures are mandatory for extended high speed autobahn driving where tire stress is extreme. Also, the high rear pressure relative to the front provides more rear grip to preclude oversteer in a car with a significant rear weight bias. Both of these points relate to safety (read liability). Here in the States I feel OK about using lower pressures for better handling and tire wear. I typically run 32F and 36R cold pressure on my 225/285x18 Bridgestone S03's.

[quote]It will not help wet traction or increase tire life, the opposite is actually true. <hr></blockquote> Greg, I respectfuly disagree with that statement. Overinflation is not going to help with either one but slightly higher pressures such as 36/44 do increase traction and therefore improve wear. This applies on a rainy day and with non-agressive driving when the hot pressures are going to be close to cold pressures. However, if the car is run agressively, the hot pressure in the rear can easily exceed 50 and more and some traction will be lost. I dare say that between 42 and 46 HOT is a pretty good range, at least for PZeros 295/30 I run on Turbo. Am I way off? <img src="graemlins/icon501.gif" border="0" alt="[icon501]" />

VS, I am not a tire engineer so I can't totally defend my statements or refute yours. In my experience I have found that the higher tire pressure will cause the tire to wear faster. I lowered mine within a month or so of having the car when my tire dealer showed me another 993's rear tire that was excessively worn down the middle with only 4K miles or so. I got a little over 15K out of my Pirellis running 34/36 or so. For track use raising your pressures during a wet day will decrease grip because you are effectively increasing your spring rates which is the opposite of what you want to do in wet conditions. I can't say I have done it but have you checked your hot pressures even with in town, non-aggressive driving. My guess is they would be in the low 50's, to get 42-46 hot you would probably need to start in the 36-38 range. Greg

palting,greg,ken,and others, i am also totally confused by this issue i can't believe that porsche ag uses econobox tactics of artificially raising pressure to improve average fuel economy. customers who buy such expensive cars simply don't pay much attention to a guzzler tax <img src="graemlins/burnout.gif" border="0" alt="[burnout]" /> ! has anyone systematically measured cornering forces or tire wear at factory recommended pressures lower pressures recommended by some? i would suspect the factory has but i've never seen the actual data. absent such data all we have is anecdotal opinions of questionable validity. hasn't panorama ever approached this issue in a systematic way? if not, they should! then we can stop beating each other up over an issue that should be easily resolved! <img src="graemlins/c.gif" border="0" alt="[ouch]" /> <img src="graemlins/c.gif" border="0" alt="[ouch]" /> <img src="graemlins/c.gif" border="0" alt="[ouch]" /> viken--do you know if panorama or someone else has studied this issue in a systematic way?

lower pressure - higher traction but lower grip. higher pressure - higher grip but lower traction. Bear in mind that the max inflation rating for most tyres is around 50psi. If you are already running say 46psi cold, then you will be getting close to that just cruising on a warm day.

[quote]Originally posted by ignacio: <strong>viken--do you know if panorama or someone else has studied this issue in a systematic way?</strong><hr></blockquote> No, I am not aware of any publication with any tests. However, I started experimenting with tire pressures over twenty-five years ago when I did some autocross and went on to road racing. In discussing with some very experienced people and trying out the different pressures under different conditions, these were my findings. I am pretty sure Greg has enough experience with this as we both agree. The bottom line is if you can't feel a difference and/or are unwilling to experiment, stick with the factory recommendations and suffer the consequences. I for one do not believe that any 911 version from the 964 on should have the same pressures (17": 36/36 - 18": 36/44) given the fact that there are varying weights and weight distributions (C2 vs. C4S) not to mention varying chassis dynamic. Lastly, I might add that other high-performance automobile manufacturers have much more logical tire pressure recommendations which seem to agree with my experience.

Tires and the correct tire pressure could just be one of the most confusing things ever. I actually work for Porsche's #1 tire supplier (I am speaking by myself and not as a representative for my company so take what I say as my personal opinion only.) while I am not an engineer or tire expert I do know a few tidbits that might be of use in this discussion. First off it is ALWAYS safer to have a tire a bit over inflated rather than under inflated (keep in mind all extremes are dangerous). The reason for this is heat, the less pressure in a tire = more friction = more heat = tire failure. Take a paper clip for example, when you keep bending it the paperclip heats up and breaks, the same is with the steel cables in a tire, the more sidewall flex you have the more heat and eventually it will break and cause tire failure. Tire manufacturers engineer the tires to take quite a bit more pressure than the max amount they list for the tire, this is a safety measure. There is a reason for the recommended pressure, and it is safety. Be very cautious about going underneath the recommended pressure by any significant amount. Also keep in mind that a tire will lose on average 1-3 PSI a month so if you start with a lower pressure and don't check your pressure you can have a very dangerous situation on the freeway. As for the handling issues, it is all up to the individual, a higher pressure will overall give better handling, but also make for a harsher ride. Everyone will have a different opinion on what they like the best. Please keep in mind that anytime you significantly waver from the recommended tire pressure listed for the weight that you carry in the car you take a chance, but heat is a tires enemy and low inflation = heat. Heat was the major factor in the tire failure cases of 2000. A4EP 02 C4S co-waiter, June production

a4ep- thanks for your thoughtful post. have you decided what pressures you will use in your c4s next month? will you follow the PAG rec of 36/44? i have 6500 miles right now on on my p-0 assyms and i see no sign of excessive wear in the center yet. about 1/2 of tread remains on the back-slightly more on the front. the ride quality is good and cornering is superb especially on dry roads. my prejudice is to follow PAG recs for now and report back the results. my c4s is a daily driver-- about 60% hwy miles. daily temptation to take the long way home! <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

A4EP: thanks for a clearly stated case <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" /> I think there are two really different issues here: 1. What is the absolute pressure that maximizes grip? 2. What is the differential between front and rear that optimizes handling. Maybe Greg and Viken can comment on each of these separately. IMHO, hot pressure of 42 - 46 for front/rears is pretty good for the grip. More than 55 for the rear is really bad for the grip As to the f/r pressures differential, less of it helps to reduce understeer bias because rear tires loose grip due to sub-optimal pressure. I use adjustable swaybars to sort out the handling balance of the car -- this is a "cleaner" solution.

[quote]Originally posted by VS: <strong>1. What is the absolute pressure that maximizes grip? 2. What is the differential between front and rear that optimizes handling. IMHO, hot pressure of 42 - 46 for front/rears is pretty good for the grip. More than 55 for the rear is really bad for the grip As to the f/r pressures differential, less of it helps to reduce understeer bias because rear tires loose grip due to sub-optimal pressure. I use adjustable swaybars to sort out the handling balance of the car -- this is a "cleaner" solution.</strong><hr></blockquote> Each tire will have an optimal hot pressure (track use). What you listed seems reasonable for a street tire. When you start raising or lowering to affect the balance of the car you may not make the car any faster. When I go to dial in my car I work with the shocks and sway bars, not the front to rear tire pressure. I sometimes end up with four different pressures on my car depending on the track. Found this interview with a racer/engineer, looks like some good stuff. <a href="http://www.theracesite.com/index.cfm?pagetype=2&form_article=2719" target="_blank">http://www.theracesite.com/index.cfm?pagetype=2&form_article=2719</a> A couple quotes "To achieve handling balance on a car that has an understeer/oversteer condition there are two things you can do: 1) Make the sliding end stick more 2) Make the sticking end slide more Since the ultimate goal is to go around the racetrack faster, the best way to achieve balanced handling is #1. Unfortunately, #1 is not always possible and we sometimes have to settle for #2 to have a more balanced car that can be driven a little faster" Another bit which contradicts what VS states about lowering rear pressure to make car grip less. "To make the sticking (rear) end slide more: 1) Raise the rear tire pressures. 2) Increase rear spring rate or ARB stiffness. If you are already lifting the inside rear, this will have little or no effect on mid-corner handling. 3) Adjust rear static camber more positive. Be warned that this is generally a bad idea and can cause unpredictable rear end grip. 4) Raise the rear ride height. "

ignacio - Unfortunately I won't get to monitor the tire pressure on the C4S. I am just a "co-waiter", my Dad is the one getting the car and he is in S. Florida while I work in Denver But since I recommended this car to him, did all the research (thanks everyone on rennlist!!!) and helped him choose the best color and options and then flew in to place the order with him I consider myself a co-waiter ) I will recommend that he use the factory specs for tire pressure but if he still has the little nobbies on the tire sidewalls a month after delivery we will have to change a few things around to get rid of them Good luck finding the correct pressure. A4EP 2002 C4S co-waiter