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Old 03-11-2006, 01:25 AM   #1
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Tyre Pressure for Boxster ??? (POLL)

Porsche recommends 2.0 bar front, 2.5 bar rear (29 and 36 psi)
I use this pressure.

My tires are 225/40 ZR18 and 265/35 ZR18 (pzero rosso)

Well, the rear tyres are worn away just in center side, so it could be a overpressure.

I would like to make a poll to know which pressures you use in your boxster 986!

Just collaborate! all of you!


Best regards.

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Old 03-11-2006, 07:34 AM   #2
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valcax:

1) How often do you check your tire pressures, and when?
2) Is your tire pressure gage accurate? I recommend a digital readout.
3) You may want to switch to nitrogen fill on your tires, mainly because you will get far less pressure changes due to outside air temperature changes. I think tire pressures change about 2 pounds per 10 degrees of outside air temperature change. Nitrogen filled tires won't change as much, and stay more accurate. There are other advantages of nitrogen, but I think this is the best one, IMHO.
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Old 03-11-2006, 08:59 AM   #3
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Hi,

Always check your Tires when they are Cold. Your Pressures will be more consistent Tire-to-Tire, but this isn't why you check it Cold. You check them Cold because the Car and Tire Manufacturer determined their recommendation under this condition.

Tire Pressure can vary widely based on both Ambient and Internal Temps. The Mathematical equation would be:
  • p2/p1 = (t2 + 459.69)/(t1 + 459.69)

    Where:
    p2 = ending pressure in psi at temperature T2
    p1 = starting pressure in psi at temperature T1
    t2 = ending temperature in deg. F.
    t1 = starting temperature in deg. F.
    Note: the 459.69 factor converts the °F. to absolute temp in
    ° Rankine

    Rearranging the equation:

    p2 = p1 * (t2 + 459.69) / (t1 + 459.69)

This allows you to determine the approximate pressure resulting from a
temperature change.

Suppose you check your tires cold after they've been sitting out overnight at 0°F. The Tire pressures all measure 30 psi. You then proceed to drive spiritedly to a location, where the Ambient temperature is 100° F, the Air in your Tires has heated up an additional 100°F over the ambient due to the driving/speed frictional heat buildup, leaving the air temperature in the Tires at 200°F. You check the Tires immediately after stopping driving (Hot). Your Tire pressure would be 43 psi because:

p2 = 30 * (200 + 459.69)/(0 + 459.69) = 43 psi, or a rise of 13 psi.

If you check your pressure Cold at, say 30°F and find them at 30 psi and, then again Cold at 70°F, your pressure would be 32 psi because.:

p2 = 30 * (70 + 459.69) / (30 + 459.69) = 32 psi, or a rise of 2 psi

As a Rule, Overinflation is better than Underinflation, though neither is ideal. Underinflated Tires allow a rapid build-up of heat which can cause the Tires to delaminate and their integrity to fail. Remember the Ford Explorer/B.F. Goodrich controversy? All the Rollovers were due to Underinflated Tires. The Pressure stamped onto the Sidewall of the Tire is the Manufacturers MAXIMUM Tire Pressure for ANY condition and should not be used.

So far as using Nitrogen, this is really unnecessary and DOES NOT eliminate Pressure changes with Temperature. This would violate a Basic Law of Physics – Charles’s Law. Charles's Law states that the volume of a given amount of Dry Gas is directly proportional to the Kelvin Temperature provided the amount of gas and the pressure remain fixed.

A good Rule of Thumb for setting your Tire Pressures was presented at a Tech Seminar I attended at a past Lotus National Convention by a Guest Speaker who was a Tire Engineer for BF Goodrich, Mr. Oscar Pereda.

He called it a Realistic Starting Point, saying it has never been just right, but is a good place to start. The rule is:
  • (Vehicle Weight in lb/100) + 2 psi at the Heaviest end (Rear) + 2 psi all around if Suspension and Alignment are OEM Stock.

    Example: Stock 986, 2,822 lb.
    (2,822/100) = 28 psi
    Add 2 psi all around = 30 psi
    Add 2 psi to heavy end = 32 psi at rear
    With modified suspension, the result is 28 psi Front, 30 psi Rear.

"There is no Ideal tire pressure," Pereda said. The optimal setting depends on the individual Driver and his preferences.

For those inclined to find a setup for Track use, Pereda provided additional instruction. Drive in a circle, first one direction, then the other. Measure and Record Tire Temperature distributions with a probe-type Pyrometer that actually penetrates the rubber, not the Aim and Click infrared type. You want bulk temperature, not surface temperature, because the surface cools rapidly while the inner temperature is more stable. When measuring Tire Temperatures, if the Center is hotter or cooler than Shoulders, there is too much or too little Pressure, respectively. But, remember, this is for a Track setting. Hope this helps…

Happy Motoring!... Jim’99
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Old 03-11-2006, 12:27 PM   #4
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Quote:
Originally Posted by MNBoxster
Hi,
Charles's Law states that the volume of a given amount of Dry Gas is directly proportional to the Kelvin Temperature provided the amount of gas
Air from a normal pump isn't dry gas though. It contains water molecules which expand when heated.... Thats why nitrogen is better at maintaining consistant pressure.
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Old 03-11-2006, 01:50 PM   #5
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Jim, good science lesson. Thanks

I have a few questions:

1) In this formula; p2 = p1 * (t2 + 459.69) / (t1 + 459.69) where does it take into account the "vessel" holding the air is rubber? Do all "vessels" follow this rule?

2) In your example about 30 psi, at 0 degrees F, then you arrive to a place that is 100 degreess F...how do you get to the tires being now 200 degrees?

3) Jim, in this example; "If you check your pressure Cold at, say 30°F and find them at 30 psi and, then again Cold at 70°F, your pressure would be 32 psi because.:

p2 = 30 * (70 + 459.69) / (30 + 459.69) = 32 psi, or a rise of 2 psi"

So if your tires we at 30 psi when it was 30 degress F outside, and then drove up to Duluth and it was -10 degrees F...the tire pressure would be 28?? My personal driving experience in the Midwest does not show this to be true. My tires would have lost more than just 2 psi!

Help me out!
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Old 03-11-2006, 03:43 PM   #6
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I use digital and analog gauge. They are accurate.
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Old 03-11-2006, 03:47 PM   #7
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Quote:
Originally Posted by ephidaboost
Air from a normal pump isn't dry gas though. It contains water molecules which expand when heated.... Thats why nitrogen is better at maintaining consistant pressure.
Hi,

You're right, but this isn't significant enough to make an appreciable difference...

Happy Motoring!... Jim'99
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Old 03-11-2006, 04:11 PM   #8
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Quote:
Originally Posted by bmussatti
Jim, good science lesson. Thanks

I have a few questions:

1) In this formula; p2 = p1 * (t2 + 459.69) / (t1 + 459.69) where does it take into account the "vessel" holding the air is rubber? Do all "vessels" follow this rule?

Hi, it assumes that the expansion of the Rubber Tire is minimal, which it is.

2) In your example about 30 psi, at 0 degrees F, then you arrive to a place that is 100 degreess F...how do you get to the tires being now 200 degrees?

100°F is the Ambient Temperature, the other 100°F is due to the driving/speed frictional heat buildup created by the Tire itself. Now, Temperatures do not compound, if you have 1 Qt. of 100° water and add it to another Qt. of 100° water you do not get 200° water, but for the Air, the friction and flexing of the Tire raise it's temperature X° above Ambient Temp which is 100°, so you end up with 200° inside the Tire, if the Ambient Temp were less, the added heat remaining the same, you'd end up with less overall temp.

3) Jim, in this example; "If you check your pressure Cold at, say 30°F and find them at 30 psi and, then again Cold at 70°F, your pressure would be 32 psi because.:

p2 = 30 * (70 + 459.69) / (30 + 459.69) = 32 psi, or a rise of 2 psi"

So if your tires we at 30 psi when it was 30 degress F outside, and then drove up to Duluth and it was -10 degrees F...the tire pressure would be 28?? My personal driving experience in the Midwest does not show this to be true. My tires would have lost more than just 2 psi!

The Formula works, check your math. Given the scenario you describe, you'd have 30*0.9183156 (489.69 / 449.69) which equals 27.549 psi.

Help me out!
Hi,

If you experienced a different drop in pressure, I suspect you were either mistaken, or some other variable was not considered such as a leaky valve or Rims (alloy Rims are notorious for leaking or seepage. This is a pretty widely used formula by Racing Teams and Tire Companies...

Happy Motoring!... Jim'99
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Old 03-14-2006, 12:34 PM   #9
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Its very interesting technical exposure, but I would like to know tyre pressures you wear.

Best regards.
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Old 03-14-2006, 02:10 PM   #10
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Quote:
Originally Posted by valcax
Its very interesting technical exposure, but I would like to know tyre pressures you wear.

Best regards.
Hi,

Start out with the simple formula: Curb weight / 100 + 2 (+ 2 on heavy end), and then experiment to what suits your individual Driving Style, Regional Roads, mix of Expressway/Street Driving, and Typical Load. There is no Right or Wrong Pressure beyond applying this formula. What may be correct for one, may feel off to another. Don't Poll and go by what others say.

Significantly Higher Pressures in the Rear will tend to induce some Understeer (Understeer = Safety for the Average Driver), while higher Front Pressures will tend toward Oversteer.

One thing people don’t usually take into account is their Load Factor. If you keep your Fuel Tank topped up rather than allow it to drain, you’ll want more pressure. If you have more Junk in the Trunk (both literally and metaphorically.. ), likewise, you’ll want a few extra pounds. Same if you more often than not have a Passenger.

But, you never want to use the Manufacturers Tire Pressure Placard. Since 09.03, the NHTSA has mandated that Manufacturers must state the Manufacturer’s Maximum Load Rating (Maximum number of Persons in Seating Positions and Cargo), the Original Tire Size (including Spare) and the Manufacturer’s recommended cold tire pressures per axle, nothing more, nothing less. Unfortunately, the new placards do not tell the Customer that the Manufacturer’s recommended cold tire pressures are intended for a Fully Loaded Vehicle where weight bias is in the Trunk area.

Due to the Underinflation (among other) issues that apparently caused some Tire failures on Ford Explorers, it appears NHSTA now prefers Tires inflated for Full Load use, all the time. It appears they also think that more information on any Tire Pressure label is distracting and confusing for customers, and they actually paraphrase those feelings in the ruling itself – see: http://dms.dot.gov/search/searchFormSimple.cfm and enter 13678 in the blank docket search field.

Remember that, while Higher relative Tire Pressures make the Car seem to come alive, consistent, long term use of anything more than 40 psi on even a Stiff Carcass Performance Tire increases the likelihood of accelerated Inner Tread Wear (results will vary). Interestingly, Tire Manufacturers did not petition against the Pressure Amendments because the Higher recommended Pressures are actually good for business.

The Accepted and Proven method in Autocrossing is that one try to determine and use the absolute Lowest Tire Pressure that is effective for events (using a pyrometer), depending upon Course Layout, Driving Experience, Ambient Temps, Concrete vs. Asphalt surface, etc. With the exception of pyrometer use, the above would serve well for Spirited Street (everyday) use as well.

It’s easy to go Up/Down with Pressures, no $$ are involved, so experiment and find a combination which you like. Judge the Drivability, Noise and Ride Quality you’re comfortable with. You might even jot down a couple different combinations to adapt to different Driving and Load Conditions. Hope this helps…

Happy Motoring!... Jim’99
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Old 03-14-2006, 04:34 PM   #11
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OMG , I just read this forum and had bad flash backs from college Calculus and Statics and Physics (I was a wanabe ME) my head really hurts now. Time for a glass of wine..

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Old 03-14-2006, 07:48 PM   #12
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Crap - you can't MAKE ME read this stuff. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore. I don't have to do math anymore.
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Old 03-15-2006, 10:22 AM   #13
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Valcax,
I'm in the south, and run 36 all round (cold) on the street.
On track days I start lower, usually 32 all round, which winds up 38-40 hot. If the tires go any hotter I bleed them down to 38-39.
(but be prepared for Michelin users to say they use lower pressures)

Loved reading the formulas, I got chills!
(yep, I'm an engineer)
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Old 03-15-2006, 10:29 AM   #14
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Jim, the education you provided here is fascinating. However, valcax just wants to know what pressure you are currently running in the tires of your boxster!

Valcax, I check and fill my tires with good old fashion compressed oxygen when the car is cold and the fronts get 29 and the backs get 36, just like the operating manual recommends. I'd try nitrogen if I had a bottle of it in my garage...

But if I was keeping bottles of stuff in my garage, it wouldn't be nitrogen! I'd probably go for Nitrous Oxide, which has a dual purpose. Making a car go fast and a driver go reaaaaallly slooooooow if inhaled properly.
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Old 03-15-2006, 10:52 AM   #15
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36 psi in the rear, 29-30 psi up front....

'nuff said...
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Old 03-15-2006, 11:36 AM   #16
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On my 19's I use the following cold:

39 Front

39 Rear

I need to keep the PSI level higher to keep my rims from getting dented from potholes, etc.

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Old 03-15-2006, 12:04 PM   #17
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33/37 cold

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