Well certainly, Adias. That is the question we're discussing. Should we go with the assumptions built into the traditional method or should we trust the computer, which according to the manual takes into account all the issues about which we always used assumptions.

If I gave that impression, I'm sorry. We don't do that on the track. Tires wouldn't last five laps if we did that. We set the pressures by measuring the temperature of the tires when we mount them and using a graph (or just a calculator) to see how much above or below the target pressure they are. Then we adjust that if necessary while sitting in the pits before the race. Never during.

The target pressure is determined at a baseline temperature, just as Porsche describes in the manual, and we have determined it by monitoring tire temperatures when the tires were working hard on test days. The gage pressure we want on race morning is determined by adjusting the target pressure for the actual temperature of the tire as we get ready to race. Usually, the tire temperature will be at ambient, but careful teams measure each tire itself and use that temp to adjust the gage pressure they set. The rest temperature might vary between the four tires just because of where they were sitting in the race garage.

This morning my on-board computer said the tires each needed one more pound of gage pressure after sitting two days in the garage in our second big winter storm. The gage pressure before that adjustment was 30 front and 36 rear. The computer is set to summer tires on 19" wheels with a light load, so the reference pressure at 68F is 34/40. They are now at 31/37 psig after I adjusted as the computer directed. That isn't the same answer the traditional method gives. As Steve said, the computer is taking into account that the tires will warm to a normal operating temperature once I hit the road. In this case, it is applying the curve in the manual and figuring the gage pressure will rise by three psig when the tires start working for a living. All of that is exactly what we do on race day.

By the traditional method, I would raise the gage pressure in the garage to 34/40. Then when they warmed up the tires would read 37/43, which is three psi above the recommended pressure with a light load. Clearly not enough difference to be damaging to the tire, just not the recommended pressure. The optimum pressure if we trust the tire engineers and chassis engineers at Porsche.

As I said, I certainly can't fault someone who prefers the traditional method, but it really will inflate the tires above their optimum pressure on cold mornings. And it will result in inflation below the optimum on a warm morning when the tires are at ninety degrees just sitting in the garage. The only reason it works at all is that road tires have to be designed for a broader range of tire pressures, because they know that not even Jay Leno keeps a race mechanic on hand to apply a compensation graph worked out by a race engineer he also doesn't keep on staff. (Well, I assume he doesn't. With all his cars, maybe that isn't a safe assumption.) The Porsche on-board TPMS does what a compensation graph does for race teams.

The difference on the race track is not the circumstances so much as the fact that true race tires are pared to the minimum weight that will survive the race laps required for the formula in question. At the extreme case, Formula One tires have a target life of something around a hundred miles, so they don't have any surplus beef to handle the difficulties a road tire must endure to be saleable. Race tires are more like airplanes. We don't build them to tolerate fender benders and keep on flying, and race tires are not designed to tolerate sloppy pressures. I've heard they set F1 and Indy tires to the nearest tenth psig and it wouldn't surprise me if that was true. You may be picturing Nascar, where they will indeed adjust tire pressures on the fly because their suspension has no useful adjustment range and they use tire pressures instead to correct handling problems.

Road tires have a design intent that must embrace people using different ways to set their pressures. Including the classic: "I let the dealer check them every time I change the oil." Changing the gage pressure according to the ambient temperature won't put a road tire outside its designed operating limits. It just won't be the optimum pressure. The pit lane method emulated by the computer shoots for the optimum pressure, to the nearest pound.

All that assumes the manual is correct and not just bragging about something they didn't implement. But this is Porsche we're talking about. They are an engineer's company if there ever was one. I'll bet they implemented it as described.

As I say, I'm going to believe the manual. Not only is this Porche, but our TPMS is behaving as they describe. But I still won't scoff at someone who prefers the traditional method.

 

I can't see any argument for this at all, Adias. None.

 

The reported deltas are not based on cold tire readings - there is nothing in the manual to suggest that - they are based on whatever the current conditions are. You can easily see this by adding or removing air while the car is still parked - those readings are real-time.

And as I would imagine, if my tire is 2lb low in the morning cold, then it is still 2lb low when warmed up - unless the relationship between low cold and low warm is not linear. I *think* it is linear enough based on what I read: 1lb/10degF. So if ideal cold pressure is 34, and actual is 32, then warmed up it would hit maybe 36 actual, but ideal pressure would be 38 - so the TPM delta is still -2. I could add 2lb on the way home, and the next morning cold, it should still be OK.

So I see the delta's remain constant all day and the pressures go up and down as the car is driven or parked. To me, that is what I would expect. And I would expect if the delta tells me to add 2lb, that it should not matter if the tires are hot or cold.

I also get your point that there must be some minimum tire pressure. Maybe the car is smart enough to know that as well. It would take more cold than we are likely to have here to find out.

At the moment, it is 64F in the garage, the TPM Delta (0,0,0,0), the AccuTire, and the Owner's Manual all agree with 34/38. The TPM Pressure reads 33/38 - so it is off by 1lb in the fronts which is how it has been since I got it.

I expect we get more cold in Feb - I'll take a look at it then and see if it still all works together.

 

So at 64F the TPMS says it's all correct !?! Geez, I though the manual said the ref temp was 68F.

You made my point. Thanks!

 

You are correct. I have corrected my post.

 

According to the famous chart on Page 154, 20 degrees C (36 F) temperature difference produces approximately 3 psi difference, so the difference in 64 F and 68F F would only cause a 0.33 psi difference. I wouldn't expect that to show up with the TPMS readings in whole numbers. It will be more interesting (and telling) to compare when the temperature is at least 56 F or lower to see what it shows.

 

Thank you - I felt or hoped it was obvious that 64F was a reasonable approximation for 68F. Seemed a good opportunity to establish a baseline.

Will be more interesting if Atlanta dips back into the 20s and I'm afraid it will soon enough

And I'll report back to see what my car says about all that (when the bigger concern won't be pressure as much as lack of winter tires).

 

If the ambient temp dips into the 20s you better adjust the cold pressure to the recommended 33/39 or drive at your peril. Simple physics.

 

Plus minus 1-2 psi should not be critical unless you are tracking the car. I just use a digital gauge when the tires have cooled overnight.

 

Boys,

Thanks for the advice on this...... ...and the physics dissertation?!?! (OMG!).

I left the car at the dealer. Apparently, I had 2 issues - a sensor had to be replaced and my summer/winter tire settings needed to be reset. The service manager did say they've seen a number of people bring in cars with a variety of TPMS issues, and that sometimes they take a few minutes before giving accurate readings.

Anyway, I wanted to thank everyone that contributed and helped me out. I'll get my baby back tomorrow. I can't wait to get my butt back into the seat

 

Enjoy your ride worry free.

 

Enjoy your beautiful car, Amy. We just bought an 09 C2S coupe and love it. Let's hope this weather cleans up soon, so you can enjoy your cab.

Incidentally, I was very impressed yesterday. We ventured forth into the biggest rain storm of the last decade or so here in California. Coming down a quiet multi-lane road, I saw a flashing light near an airport entrance. In another mile, it resolved to a highway patrolman sitting in the third of four lanes next to the entrance. I slowed naturally, so we were only going about 57-58 mph when we passed him, glancing on his other side to see who made the mistake that called him there. Turns out it was the weatherman.

Suddenly, yikes! We're at sea! At least it felt that way. Turns out something like a hundred yards of the road was underwater to a depth of at least a couple of inches. Felt like six inches but I know better. Six inches would have slowed us much more and would have felt like water skiing. You haven't much time to worry at such moments, but that road narrows to two lanes from eight right after the airport and I kept worrying that we'd rotate off line and dart sideways into the airport sign as soon the tires got traction.

As it was, we threw a rooster tail whose like I haven't seen since the year it rained seven inches at Sears Point while we were testing a Barber Ford. The FF was doing that sort of thing if you entered water stretches with any rotation on the body at all.

The Porsche was different. Much heavier with the same size tires for one thing. With tire pressures this high [See! This is on topic!] It did not start rotating on its axis or anything dramatic like that. It just slowed perhaps ten mph in that distance and doused the highway patrol car thoroughly.

When we made it through the intersection without incident, I crossed my fingers that he hadn't had his window cracked, because he might have been seriously annoyed to get several gallons of water in his lap. Not that we were going much over the 55 mph limit there, but he probably expected drivers to slow to twenty and creep through the water after seeing his lights.

Ah well. Very stable car in water, even water much too deep for a bloody desert. When will it stop raining for all love?

 

Try to fill your tires with Nitrogen.

 

Is that a general recommendation? Or for those planning a water skiing adventure like mine?

 

hummmm...maybe~~