billhart

So, somebody help me here without my contributing more fuel to the debate. Simply for street driving, aggressive but not 200mph top speeds, i always check my pressures before going out. I tend to follow the manufacturer's recommendations on inflation, certainly not the maximum pressures indicated on the sidewall. I think, for the Michelin Pilot Sports on the GT-2, these are 32/39 F/R. Now, in course of street driving, these tires will heat up, and while i haven't rechecked the hot pressures, i would assume them to be anywhere from 2-6 psi higher than cold pressures.
But, and here's the question: Everybody, even Bentley, seems to be saying that you work backwards from the recommended pressure and inflate so that the tire reaches that, only after its hot. Is that right? For aggressive street driving as well? (I am less concerned about the ultimate gas mileage or tire longevity, per se, than i am about extracting the best overall performance from the car, recognizing that tire wear, particularly if it is uneven, plays greatly into that). Thanks.

Bentley

You are correct in surmising that 'you work backward'. The formula is easy: Cold temperature is ascertained; calculation is made as to increase in pressure due to road friction and anticipated ambient temperatures; and finally, you remove some air from the tires based on your pressure gain estimate earlier.
Gentlemen, this is not rocket science.

wallacethecat

Food for thought...

I road race bikes, in general we always set our cold temperatures to 30 psi front and rear...never ever set psi at factory cold pressure settings and then reduce as tire warms, thats horseshiz...good way to die young.

Same goes for cars. Shiz, Viken already gave you guys the best advise, so take it.

As for day to day street driving, use the factory settings, or at least a couple pounds shy...you need to use these pressures because public roads are so bad and running track pressures on the street just increases your chances for bent rims.

just my two cents, Bentley you need to make a booty call...lots of pent up frustration.

Dirt Track Racer

i would like to nominate this thread for dumbest thread of the year. Well thats already taken by Frank, but this could be the first runner up.

Anyway, this thread started about DE tire pressures and now people are talking about street pressures. I would like to pose a question to the members of this forum: If you dont want to listen to the advice of the experts, why post the question.

If you were told by a DE instructor that you want 50 or 500 psi cold in your fancy 996tt tires before a DE, he is wrong. There is no debating this fact. Many of us have many thousands of hours of track time in many cars including 996tts and we all recognize this as wrong. If all the experts think its wrong, and a noobie thinks it right, then why ask the question.

I will tell you as a fact that it is wrong and Viken is right. Not because he is omniscient or smart or even sexy (how the hell would I know), but because his answer jibes with what all the experts consistently post as advice. Do a search on other forums for tire pressures, you will get the same answer. Are you really so snobby that you think your fancy 996TT needs higher pressures than all other porsches ever made?

Still not a believer, you want more proof, call Kevin Buckler at The Racers Group and ask him what pressures his LeMans winning cars use. I would bet you its nowhere near 50 psi - hot or cold.

I mean, no reason to trust the judgement of a guy that spends more time wiping his *** at 205 mph in a 996 than any of you will ever spend at any speed over 70 mph in your 996s.

I mean, why trust Viken, a moderator of the whole Rennlist site and the most knowledgable Porsche guy there is?

Lets all trust Bentley who spouts off about subjects he clearly knows nothing about. Then lets complain to the experts when they end up in a wall at Watkins Glen because they blew a rear tire in the esses due to over inflation.

When the hell are you people going to wake up to the good advice that is presented to you?

DTR

TT X50

Ypshan- Thx for rasing the question.
Going to a DE/track this Sat -- first time with the just broken in '03 TTX50 ( P-Zeros)and want to start right.

- What does the the phrase " overpressure " in the P-manual on Tire Pressures for Cold Tires 36 & 44 p.s.i. "overpressure" mean??

- the P manual warns not to exceed pressure listed on the sidewall which everyone noted wasn't to to exceed 51 psi.

- My P-Zeros went up 8 to 10 lbs hot after myu stints at a few DE's on a Boxster and a 360 Spider.

- I'm still in the learning stage in all this and accept Viken's, Stemers, Patricks, Rufracers, etc comments.
I just don't want a failure. But I'm confused as heck why I've gotta depart from Porsche's numbers in their manual, or have hot temps exceed the tire max.

--Lastly, if I learn to use a tire thermometer, wouldn't this be a good way to check the eveness of the temp across the tire, overinflation and contact patch?

Thanks guys

Patrick

I believe "overpressure" means above atmospheric pressure, as apposed to absolute. Since your tire guage measures that way, it really has no meaning.

And I agree a pyrometer would provide a lot of info. I just haven't invested in one yet.

billhart

Dirt: Sorry if my question offended. But, a long time ago, i learned that there are no stupid questions. Thus, i don't question the knowledge of those with expertise, or demand verification, but i do appreciate clarification of that advice as it applies to street driving. In effect, the advice is, disregard what the owner's manual says, underinflate the tire with the expectation that its hot pressure should be equivalent to the cold pressure indicated by the manufacturer.

To the extent i have raised a dumb question, i note that your response does not even purport to provide an answer. Or is it that obvious?

Dirt Track Racer

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by billhart:
<strong>To the extent i have raised a dumb question, i note that your response does not even purport to provide an answer. Or is it that obvious?</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Bill, I did not furnish a response or answer because it was answered in no uncertain terms in this very thread above. In addition, it was answered probably hundred of times in the archives of this and all the other rennlist boards.

My response was not directed at your question exactly, but rather those above your post in the thread. In your case, you could do one of two things, leave the pressures where the PCNA lawyers tell you it should be in the manual. That should be just find for any type of street driving - even high performance. You could also do what most do and lower the pressures some from the factory lawyer recommended pressures. But for street driving, the benefits are smaller than for track driving. Plus, with the lower street pressures, you run the risk of damaging an expensive wheel more easily. If I were you, and I'm not, so take this for what it is, I would run my tires on the street at about 34 cold front and 38 cold rear. That is a little less than the laywer recommended pressures, but not as low as we would recommend for a track day.

This advice should only be a starting point too. I would try that and go find your favorite exit ramp and see if you like the difference. If you dont, then put it back where you started. If you do, then great. Just dont put it in the weeds.

DTR.

billhart

DTR: Thank-you.

ypshan

I went by Paul Miller today and the tech persons there swear by the factory settings. They confirmed that the max psi of 51 is cold and the tire with 44 psi can safely heats up to over 51 psi. The contact patch will be smaller with higher pressure so less grip for cornering. However the straight line acceleration will be better.

I've posted a question to the PCA tech committee. Will report what they advise.

1AS

We clearly have a disconnect. Dirt Track Racer and Viken are right, and those who don't take their advice will duplicate my experience or worse. Remember, I saw sky-high pressures after only 3 hard laps on a 1.8 mile track (12 corners).The Porsche pressures are probably safest for street driving, I'd guess, because there are predictable risks that would be unacceptable if someone tried to take an off-ramp while the pressures were in the low 30's. Even fast street driving is nothing like track driving. If you started at low 30's on the street, you'd probably stay in the low 30's for most of our American driving. How often do you get to really push for 24 or 36 consecutive hard corners separated by only short straights on the street? Never, I'd say. Remember, the tt is a heavy car with speed potential and tons of grip, most of which is never used on the street. My surmise is that the recommended pressures are only given for street use. This is a mistake nobody needs to make. Bentley's recommendation re adjustment is somewhat coy, as anybody going out the first time could easily do a few laps and destroy the tires before they ever had a chance to check pressures. AS

docjackson1

i certainly appreciate the input of the various board members who have posted their qualifications as experts. it seems obvious to me that the recommendations from viken, et al, need to be heeded. it is obvious that in the street, 36/44 appears to be the way to go. on the track, it seems that lower cold pressures are needed to prevent tire failure. thanx for the education.

ypshan

I forgot to mention that when I posted my question to the Porsche service people, I told them that it's for track use instead of street use. And I specifically mentioned that I have a safety concern running at such high psi.

Steve N.

Billhart: lest you be confused by some of the numbers floating around re recommended pressure in the owners's manual, the TT pressures are quite a bit higher than the GT2 numbers - may be associated with the higher weight of the TT.

I wonder what the practice is at the PDE? These guys have a ton of experience with street tires on the track - albeit in Carreras, not TT's.

Viken

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Steve N.:
<strong>Billhart: lest you be confused by some of the numbers floating around re recommended pressure in the owners's manual, the TT pressures are quite a bit higher than the GT2 numbers - may be associated with the higher weight of the TT.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">No, the extra weight has nothing to do with this. Every 18" tire equipped 911 from the mid '90's on is supposed to ride on 36/44 psi. That includes 2WD, AWD, Turbo and Cabriolet models. This alone should tell you that these pressures are less than ideal for these cars in terms of handling and they only serve excessive understeer and too much middle tread wear on the rear.

A front to rear differential of 8 psi's is ridiculous. If you insist on staying close to factory specs, at least try to bring the rears down to below 40. If you don't like it, go back to factory specs but never exceed them.