924Board.org

[Rasta Monsta]

I know that many folks tinker with tire pressures for tracking, often running higher pressures than for street.

Is there a general rule about what pressures to use? Can anyone suggest a starting point?

TIA


rasta
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[924RACR]

What tires you running? What size?

Usually we tune for hot pressures, as indicated by the tire manufacturer (at least for race tires). We run Hoosiers, and typically see 6-7 psi gain from cold to hot, so we start them around 31psi, and end up hot around 37-38psi.

Don't do the same unless you're running Hoosier race rubber - you'll likely shred your tires, possibly wreck your car in the process.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[fiat22turbo]

You need a tire pyrometer to know for sure.

Too much pressure and the center is warmer than the outside. Too little and it is cooler than the outside portions, etc.

Much like adjusting corner weights you're trying to get the numbers close to the same all around or at least side to side.

I'd check with your tire manufacturer or vendor for base starting pressures and work from there.

Hope this helps,
Stefan

[Rasta Monsta]

Tires are Yokie Avid V4s, P205/55R16 89V in the front, and P225/50R16 91V in the rear.

TIA


rasta
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[924RACR]

OK, so not really true track tires - probably want to err on the higher side - might want cold pressures in the higher range (35+?). But yes, your manufacturer or vendor is the guy who needs to answer that question for you.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[edh]

I know on most cars people recommend sticking more air into a street tyre, I just don't think it works that well for the 924/944. I think they start to slide more & get very noisy.

I've found on my 924 and 944's that you want to aim for a pressure when hot pretty much the same as normal street pressures. That means dropping a few psi to start off with & keep checking as they come up to temp.

You're not after the ultimate time here, so don't worry too much - experiment a bit & see what feels best. Just make sure you're not rolling onto the sidewalls.

oh - and try & keep the weight in the nose when you turn in - easy to create understeer & scrub off the edges of your front tyres. (no need to ask me how I know )
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current - '90 944S2, '00 986S,
ex - '90 944 turbo '86 924S, '88 924S

[morghen]

good topic...i'm going to the track soon and i'm very interested in geting a good lap record and not wrecking my car in the proces.

So it pays to use normal tire pressure if you are running street tires ?
I'm using 225/45/17 Marangoni zetta linea on the fronts, runing 2.0 BAR
and 245/40/17 Continental ContiSport Contact with 2.0 bar on the rears.

From what i've tested on the road...i can say that the car tends to slide equaly front and rear on a warm day and on colder days the tail wigles a little. So i'm preety happy with this setup. Last year i trashed my front piece three times because the front slid and then i lost control of the rear end too and climbed onto a sidewalk (yes here race tracks have sidewalks )

[Rasta Monsta]

I was able to find a very general starting point, for when your tires are so different from factory tires that the factory specs lose their meaning.

Starting Pressure=VehWght/100

For my car, this translates to 28 PSI.

Then add 2 PSI per corner, and two more PSI on the "heavy end" of the car.

For my perfectly balanced '31, this adds up to 30 front and 30 rear. . .

And of course, a probe-type pyrometer is required for fine tuning these "starting point" pressures.

Can anyone recommend a decent one? I found the Longacre AccuTech Economy Pyrometer 50635 on EBay for $85, that is about the cheapest one I could find.

TIA



rasta
_________________
Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[924RACR]

That's good - a newer version of the one I have.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype