Imo000

WOW! That's all? But now that I think of it, it makes sence. Most of the time the tire seats first and then it's pumped up to the proper pressure. Most shops use a standard of 36 psi on all tires. I have been perhaps a litle ignorant about proper tire pressure and have always pumped all tires up to 36psi. 36 seemed to me as a good general inplation level.

heinrich

How bout now, time to bug out of the thread yet?

Bill51sdr

Oh, I don't know. I'm going to lunch, we'll see who's left when I get back.

fabric

Has anybody considered that it's because the tire widths are different? I keep hearing weight distribution. I still don't think the different width explains the large difference, but it should be considered. Most front wheel drive cars don't have as much of a difference either, so I don't think weight distribution is the only factor that we should be focusing on.

ANd while we can argue that tire wear is not something we should be worrying about, it is a sign that perhaps the inflation value is sub-optimal. Nobody would advocate that when the shoulders are wearing that you shouldn't put in more air. The only reason to over-inflate is fuel-economy, and as Heinrich rightly points out, over inflation results in less traction.

One last point - Porsche is not infallible. While deferring to their engineering is sensible, blind faith in it is not.

dr bob

[Flame suit ON]

My --guess-- is that the 44 PSI rear was to allow a bigger, more stable patch, and avoid that dreaded oversteer at all costs. I agree with the suggestion that high speed equals higher heat, especially when the presure is lower. There are times when I want some tread and sidewall flex for improved handling and ride quality. If I was cruising at 150+ on the Autobahn, I'd be willing to trade some of that ride quality for tire (and passenger) survival.

For most of the driving we do, including the very sort-duration blasts to 120+, a slightly lower pressure is fine.

-----

I try to keep in mind that there have been over three minor improvements made in tires since my car was asssembled, maybe four since it was penned and tested in prototype. My guess is that the testing wasn't done on SoCal freeways between 25 and 45 MPH with the occasional blast up to 75. They might have been driving a little faster, and needed sidewall stability at that speed that came easily with a few extra PSI in the tires.

------

The chalk method is not a bad way to get at least a ballpark inflation number. Better is a pyrometer or IR heat gun IMMEDIATELY after a series of high speed corners. For most the chalk will do fine, though. Drive the car the way you normally do, and see how far up the sidewall you are scrubbing in serious corners. If toyu are past the edge of the traed and into the sidewall, you may need more air. Is 32 enough? Not for me and the way I drive, with the roads I drive on and the tires I have. Is it enough for you? Might be, if you have stiffer sidewalls, weigh less, drive different roads using a different technique when cornering.


I have the option of measuring and tracking tire wear and tire temperature regularly. I find what works for me and my combination of factors, along with my expectations for ride quality, handling, steering sensitivity, tramlining, and high-speed handling and balance. Each of you has the same option. From that info you can decide what tire pressure to use. Me, I have 37 front and 40 rear. It's what works for me and my combination of circumstances. Perfect for you and the way you drive? It might be. It could happen. But you'd still end up testing and tweaking. After all, it's the easiest single suspension adjustment you can make that will affect handling, ride, and wear. All at the same time.


OK, back to your regularly scheduled 'mine is bigger than yours but you can't really tell 'cuz it isn't fully inflated' discussion.


[flame suit /off]

heinrich

I put my flame suit on I put my flame suit off ..... I do the hokey pokey and I .......

heinrich

There you go. THAT's the correct datum. Thank you sir

Tahoe Shark

Uh Oh, H agrees with me.

Please don't take this example to mean that if you run the stock tire size you can use these pressures, you can not. This example is for the stated tire sizes only no others.

Richard S

Here, I updated Tim's chart, so everybody will know their correct tire pressure....and top speed.


Now, being serious, I remember an article in Panorama a couple years ago which had a Q&A session with a Porsche engineer. His recommendation for tire pressures was +1psi for the tires under the engine vs. the tires at the other end. Tim's Michelin data seems to reinforce this.

Rich

heinrich

Rich

Bill51sdr

Hey H, I think I see the reason for Porsche's 36/44 psi spec's and it's in the data Michelin sent to Tim. Notice the tire pressure recomendation goes up with speed. Since the later versions of our cars are capable of quite high speeds, it seems the pressures designated are for high-speed stability.

Haven't read the previous responses yet, so if i'm repeating someone here...

Bill51sdr

No blind faith here Chris, just using their recommendation as a point of reference. What works for me clearly may not work for someone else. that, more than anything else in this thread should be pretty clear!

hacker-pschorr

Sorry, I should have been more specific. My cold pressure is around 34 on either size wheel in the summer.

Bill51sdr

Rich, was this engineer referring to all P-cars or just the rear-engined cars?

Richard S

Bill, as far as I recall, it was all models. I would really have to dig through the old issues to find it, best guess would be an old TechTactics article.

Rich