Potomac-Greg

I put "hot" in quotes because I really don't know temps just pressure.

I was at a track event (Summit Pt WV) where it was fairly cool (hi 50s to low 70s) and broken clouds. I am running RA1s 235/40/17s up front and 255/40/17s in back. My starting pressures were 30 to 31 psi; and my ending pressures after 20-25 minutes sessions were consistently 39 to 40 psi, and that's after a cool down lap and a trip through the paddock.

I was surprised that the pressures went that high. I was targeting high 30s hot. The tires worked fantastically, but it got me wondering what kind of pressures I would need to start with on a hot and sunny day (26 lbs?). Any thoughts?

TedA

In general with the RA1s I start at roughly 28 psi all around looking for 36 psi hot. As a rule pressure are not consistant corner to corner after the first run of the day bu tsettle out afterwards.

Starting pressure may change based on ambient temperature.

jtsporsche

i would guess that there are so many factors (track temp, track layout, driving style, etc.) i wouldnt know where to begin the calculations. lol

OT: Ted A- are you anywhere near the tidewater peninsula?

TedA

Chesapeake.....

jtsporsche

cool are you going to the fest?
im hopeing to be able to. and id love to get a bunch of virginianers and caravan up there.

TedA

Nope. I will be at the FSR DE at VIR that weekend.

Potomac-Greg

Thanks. I guess all I really need to know was that I could start in the 20s.

TedA

Just give them a lap to come up to pressure and you should be good to go. I have started cold as low as 26 psi.

shiners780

I have run the same size RA1's. First session out stone cold they pick up 8-10 pounds of pressure. Same as Ted I typically start out around 28, being careful on the first couple laps not to roll them over. If they get over 40 pounds they get slicker 'n ****, you'll know immediately when that occurs.

TedA

Ditto......

L8_apex

To add warm-weather experience at the PCA DE this past weekend in AZ. Ambient temp was mid to upper 80's and had to bleed tires after every 20 minute session. Street tires - but they would pick up 4 - 6 pounds each time.

xsboost90

really! mine usually end up at 40psi hot-starting at about 30-35 psi and dropping pressures after they come up to temp - i thought they needed to be 40psi to work proper- i'll start dropping initial pressures now...

schwank

Last year I ran into this too... I am going to go for 28-30 start pressures this year. Last year at 34 they would get squirrley after a bit... just too hot.

M758

Nah... that is about right. It does not make much of any difference what the outside temp is the presure gain is always about the same on my 944 spec car. It can be 50F or 100F. Start around 31-32 ish and come off around 38-39-40 ish. The pressure rise is due to driving action and is not impacted that much by overall air temp. I have noticed that the pressure rise is dependant on moisture content of the air in the tire. More moisture in the tire will lead to a greater pressure rise. I race in the desert with very low moisutre content in the air. I would not be surprise to see that other wetter areas may need lower starting pressures to get to the same hot pressure.

Yes over 40 psi and they get a little slick. For my what I have been doing lately is to start in the morning 31 or 32 and run a session. Then bleed off to 38 hot at all 4 corners. From that point I don't change then any more. This seems to work out a little better as during the races the grip stays more consistant during the run. When I used to set pressure before the race the car would tend to go off just a bit as race wore on. The problem however was that it was tough to know if it was me not driving as well or a slight shift in the car.

What is interesting is ted said 36 psi hot. I have always run 38-39 hot. I have tried 36 psi hot before and I never seems to work, but he is not first person to use 36 on RA-1's in a 944. I think I will try that in my next event and see how it plays out. It just might get me that 0.5 sec I have been looking for.

TedA

I used to shoot for 38 psi hot, but somebody who I believe to be alot smarter than myself, with alot more experience with RA1s said that based on the weight of the car I should be looking at a lower hot pressure. That's what I have done since. Tires feel more "grippy" than running at the higher pressure where they tended to skate a little. YMMV.