Replacing Tires: Tire Pressure Monitor? Wider Tires?

[jyl]

I hope I can get some comments before I order my new tires. There is a bit of urgency as the car is now sitting on the spare.

Car is 1989 3.2 with 16" Fuchs, 7" and 8", stock except for moderate lowering, fenders not rolled. Driven about 5000 miles/yr, in SF Bay Area (rainy winters, some crummy roads), and the car will do a few DEs and several AXs per year. Currently have Dunlop SP8000s.

First, I'm thinking about getting the wireless tire pressure monitoring system sold by Tire Rack ($280 including upgraded display module). Has anyone had any experience with this product? I've talked to two of Tire Rack's recommended installers who were negative on it, and said the sensors are heavy and bulky and make it hard to install the tire and balance the wheel. But the website says the sensors are only 1.5 oz and Tire Rack says the clamp (for the steel band that holds the sensor) effectively counterweights the sensor anyway. Can anyone educate me?

Second, I've tentatively settled on Bridgestone SO-3s in the stock sizes of 205/50-16 and 225/55-16. I've read the threads about fitting 225s on the front and 245s on the rear. My question is whether, for DE and AX use, is it better to have the stock size or the larger size with the resultant change in the sidewall shape? Is any benefit of more tread width offset by a "pooched-out" sidewall?

[RallyJon]

Two things specific to S-03s: they are wider than nominal and they don't have the stiffest sidewalls around. The 225/50 is almost a 235, and the 205/55 is almost a 215.

IMO, those sizes are good for 7s and 8s. If you've ever driven with soft sidewall tires on a too narrow rim--it's not a pleasant experience. The ultimate grip may be about the same, but the car feels wobblier on turn-in.

Of course, I'm biased. Here is my car with the same width S-03s on the same width 15" wheels. Sidewall support looks about perfect to me.

[Rot 911]

Originally posted by jyl
Second, I've tentatively settled on Bridgestone SO-3s in the stock sizes of 205/50-16 and 225/55-16.

I will only comment on the above comment. After much research on this board and elsewhere I have decided to go with the SO-3 to replace my SP8000's. As to size, I think you have your stock sizes a bit confused. They should be 205/55 /16 for the front and 225/50 /16 for the rears.

[widebody911]

I think it's a neat idea, but I'm not sure how accurate it is. If it was accurate to +/- .1 psi, or maybe even .5psi it would be great for the track. For a street car, I don't know that it would be useful.

[jyl]

Oops, I did get my sizes mixed up. OK, I'm going for the stock sizes in the SO-3s.

The tire pressure monitors are only claimed accurate to +/- 1.5 psi. I think they'd mostly be useful to warn of dangerously low pressure, not to "fine-tune" exact air pressure. I got interested after reading reports of 911 accidents caused by tire deflation and then blowouts - I recall a PP member and his wife almost went under an 18-wheeler a few months ago, and wasn't there a 993TT that rolled after the right rear went flat at a trackday? I don't seem to be able to feel low pressure in the 911's tires - I recently drove 2+ hours at speeds from 50-90mph and then discovered only 15 psi in the right rear. These incidents have gotten me a bit spooked.

Also, one mechanic also told me not to use metal valve stems. He says they leak. I'd thought metal valve stems were better because they don't require valve stem supports. Any ideas?

[Rot 911]

Just go with short rubber valve stems. The are only about 1" long,

[jyl]

So short stems don't need support, right?

[To the guffawing one] Knock it off, we're talking valve stems here!

[Don Wohlfarth]

John, I don't think you're checking your tire pressures often enough. (How many Exploder/Firestone problems because owner's didn't check pressures?)
What have you checked about the tire that only had 15 psi? If it leaked once, pretty good chance it will leak again.
Track event with car rolling, could have been low pressure and over heating tire, just as easily have been a blowout after off track or running over curbing. (Most track drivers check tire pressure after every run looking for the perfect hot pressure.)
Personally, I wouldn't rely on a tire monitor as it's something else that could go wrong on my Porsche. Might consider it on another car that doesn't see quite as much spirited driving.
I checked Pelican for the hand held electronic tire pressure gauges, they don't carry them. Originally bought 2 from PerProd but they no longer carry them. Did web search and found gauge for $16, thanks Google.
European Car did a test a couple years ago where they tested 15-20 tire guages, everything from givaways to high dollar liquid filled. Every electronic guage was within .5 #. Other guages were off 3-5 #.

[carnut169]

A good tire guage is probably your most important tool if you plan on using the car on a track. I purchased the Longacre memory guage that measures down to .1, but that might be overkill for you. The slicks we run on the formula cars are very sensitive to temp and pressure... still, every tire will perform best at a certain pressure and temp. At the track fill the cold tires up to aprox 5 lbs below what you want to be at w/ hot tires. Immediatly check the pressure when you exit the track (don't even drive to the paddock), then adjust to ideal pressure. Unless they leak you should be set for the day (still check after each run). As the tires cool the pressure will decrease but will go back up as soon as you hit the track again. Want to be really "trick"? Get a bottle of Nitrogen- it does not change w/ heat as much as reg compressed air b/c it does not have the moisture.

[jyl]

Yes, I wasn't checking pressure often enough. I'm sensitisized to this issue now! I used to do it weekly, now I'll do it before any road trip too.

The leaking tire turned out to have a nail and a screw in it. And turns out it was previously patched too. Maybe I drove by a construction site on my way out of town. I've scrapped the tire.