Also consider the fact that the rears are supporting the weight of the entire drive line, so pressure changes will affect handling quite drastically. Always make sure you have enough support back there.

Also -- if you upgrade to solid or semi-solid engine mounts it will keep the engine motion in check and be even more stable at speed.

 

The formula is PV=nRT. See below a chart for that.

Note: That cold setting should be done when the tires are at ambient temperature. Using that curve I set 'cold's 33/37 and when hot on the road (spirited) I am usually at 39/43.

 

With all due respect... I think your issue is that the outside temp is low and you have not dynamically heated the tires. Raising the 'cold' rears you are aiding into that but that is not the right way.

If you know your garage temp is at 60F you should subtract roughly 1 PSI (0-0.7PSI from the chart above) from the correct value and then properly (dynamically by driving heating up the tires. If the outside temp is 51F it will take while... I use 'cold' 33/37PSI with M-PSS and the car is perfectly stable and precise. i start at 33/37 and on the road I am often at 39/43 (the correct hot tire pressures).

 

Makes sense. We don't really have miles and miles of winding roads here, so you kind of go in and out of spirited sections, creating more friction from turning or acceleration. So temps will fluctuate a bit on a normal drive. Tonight I made it a point to keep the heat in the tires at any opportunity. But also had a tire pressure in general higher than I had by quite a of.

 

Another thing to consider is that there is no way to put track heat into tires on the street since most spirited sections are pace and flow-style sections rather than the 100% accel / 100% braking / 100% corner load 100% of the time that you have on a track. (At least I hope it isn't!)

So that said, you might still be running your tires too low.

 

I hear ya, I was thinking the same thing regarding the front pressure, maybe bumping that up a tad would add a little more "feel" as well. I think the closest we got to track conditions was in arkansas, 20 mile sections here and there of winding roads, constantly on and off at maybe 8/10 or so. Hopefully we get some track time this year. Thank you again. I'll keep adjusting the pressure a little and see how they improve.

 

Some of us went through this a few years ago (with proper pyrometer measurements) and the conclusion was that the optimum cold (at 68F) pressures are 33/37. In the summer, short of city stop-and-go, one gets to 39/43 pretty quickly, and one does not want to be much higher than that. Of course, in the winter it takes longer and in the Midwest it may not be possible. And speaking about Midwest... in cold climates one should be running Winter rubber...

 

Not surprising. I spoke to one of the head tire engineers at Michelin @ Circuit of the Americas during the Lone Star LeMans about cold temps and MPSS. I was asking about that cause I could tell my car feels different when it was in the 30s here. He said anything 40s and below, he wouldn't advise being real aggressive with when driving on those tires. He also told me that driving really aggressive on the tires in below freezing temps could potentially damage the tread due to the rapid temp change and the compound used in the tire. Actually causing a tread block to break off. Of course that extreme case in very cold climate. He's not saying the tire can't perform, just not advisiing to push the PSS in temps that low.

I learned a lot listening to him and talking to him that day. He even went over the whole N rating for Porsche and why the PSS didn't get it. Nice guy.

 

Well...spill the beans already!!

 

I would go further. Summer tires should not be driven below 40F. Their tread compound is very hard at those temps, risking an accident and reducing tire life once they heat up and eventually cool down to those temperatures.

Please elaborate on what you heard re N-rating of M-PSS.

 

IIRC, it has to do with control/slip angle (at the very limit) performance of the tire. Porsche is the pickiest customer they have for OE tires. They are very particular on what they expect for control by a given tire at the limit (how controllable at the point of slip and bring it back under control). He was telling us that as the tire was going into production, Porsche came with a tougher specification. It was just too late for them to go back and adjust the tire for it. The tire still performs well, they just didn't meet the new specs Porsche wanted so no "N" rating. The question was asked will they get the "N" rating. From his response, it seemed like they were going to eventually adjust for it.

Yup adias, agreed.

 

I can speak to the control-at-limit nature of the PSS... They are spectacular. Incredible levels of front-end bite at the limit, and the rears break away so smoothly and predictably.

 

December 28th, 2014
Houston - Dallas, TX
Condition: heavy rain
Ambient temperature: 41*
Departure: 8:00 am
Average MPH: 80
Max MPH: 105

rain constantly until 40 miles from Dallas.

Michelins PSS thumb up.

 

Absolutely spot on. I live in the Northeast, just east of NYC. MPSS in temps below 45 just do not perform well due to the rubber composition. During the Fall, temps can fluctuate between mid 40s and mid 60s from day to day. Car handling changes dramatically on the colder days while runninng MPSS. That is why I switch over to the all season Contis for the Winter months (which cannot end soon enough for me.).

 

Don't know if Michelin will adjust now. Since 2014, all the 991s at my dealership come with Pirellis.