How far from factory tire pressure should/can I run?
 

Hello all. I'm hoping that the autocrossers or just ones with experience can help me.
I just purchased a new set of tires for my 84 Carerra Coupe. I replaced a set of Dunlop Sport ZR SP 8000's with a budget set of Fuzion ZRI WR tires (which seemed to get many good comments on the Web). Same size.
These new tires seem to have softer sidewalls so I am going past the factory recommended settings of 29 PSI front and 36 PSI rear. Right now I have them at 35 PSI front and 38 PSI rear, but the handling in quick turns is still not the same as my old worn out tires. By pumping up the tires past factory spec, they have improved, but I wonder if how far I can keep increasing pressure to get the handling the way I want?
Thanks for all those that chime in.

BUMP!!

Well, since you're looking to increase the pressures for a limited application (autocrossing) and not for normal street use, I would use the max pressure rating on the sidewall of the tire as a guide as to how far you can go and experiment from there.

Tires really suffer from lower inflation pressures more than higher, so I wouldn't worry about it unless you start approaching the max rated inflation pressure on the tire itself.

Mike

Mike thanks for your comment. My concern hasn't even addressed the autocrossing component yet. This is just purely street driving. I will try your suggestion though. If anyone else has comments please let me know.

Don't forget the max tire pressure listed on the sidewall is a COLD pressure. In autocross your tires will get much hotter than on the street and the pressure rises much more than street driving and that may become to high for the tire. Remember pressure increases linearly with temperature.

Coupla things....

after having replaced a number of tires on my 911 over the years, one thing to remember is that the first X00 miles, new tires won't behave properly. I found I had to get almost 700 miles on one set of new tires before I got comfortable to track the car again for a DE.

plus....the tire pressures you use depend on the kind of track event. For pure track driving.... 31-32 front/ 34-35 rear ( cold) seems right. After a number of laps these increase ( hot) by 6 psi front and rear. If you autocross, you won't heat up the tires as much ....and you might want to start out close to the hot pressures ( 37 / 40).

- Wil

Wil,

I also have an '84 coupe, and only drive it on the street (right now anyway). Would you use factory specs, or recommend different? I do drive it fairly 'spirited', as my ride to and from work is fairly curvy. I'm very new to the car, so I'm still 'getting used to it'. It's a different animal compared to my other car ('92 Miata) and my previous car - '99 Audi A4 Avant. Thanks for any advice given.

Mike

try 32F/34R or something close to that - play around and re-drive the same road repeatedly

RsZk,
funny you should mention the sidewall issue. I just replaced some rear, worn to the belts Yoko 520's with ES100's. First drive home with them I was SO disappointed... lots of sidewall flex. I think the old tires (no longer available) either had stiffer sidewall construction or had hardened over the years. It's not just tread squirm. I'm going to try slightly increased pressure as well. I miss the solid feel of those old tires. Now when I make a slalom manouever there is one extra tail wiggle before it settles in again. Hopefully as the tread wears down and the tires 'mature' there'll be less of this. I'm thinking new, stiffer shocks in the rear may help as well, but I can SEE the sidewall flex when I push the car side-side at the rear. What a difference between two tires of the same size and manufacturer. Oh well...

Dentist90,
what you are describing are my exact symptoms. I've got the fronts at 38 PSI now and the rears at 45 PSI cold. I will test this configuration. Does anybody think that this is too much cold tire pressure? Max inflation is rated at 50PSI cold. Thanks.

Too much cold pressure...use what RandyWebb says for starters....

Wil

I replaced a set of Dunlops with ES100's and noticed the same thing, although I appreciate the softer sidewalls around town. I run 30/36 F/R around town for comfort, but for perfomance driving, 32/40 feels a lot better (though I can't say I've got exactly dialed in). I am running 205/55/16 and 245/45/16 on 16 x 7 and 9's on a TL, which is a little heavier than a standard Carrera. I hope this helps.

My owner's manual says to use 29/44 F/R, and I have come to believe that these pressures are too low in the frone and too high in the rear for my car (at least with modern tires).

wholberg, I know what will help my situation a bit.... that 491 option you have. Wider is better. Stock Carrera tires are same as your front, rears are 225/50/16 on 7J Fuchs. Factory recommends 36psi rear, Yoko says max. 44psi (w/ 900kg load ea.), so not a lot of room. I will try 40 for awhile and see how it feels. Then I'll just have to get a Turbo Look and some wider wheels! Lucky you.

Thanks Randy, Wil, and others. I'll try what Randy said, and go from there.

Thanks again,
Mike

At an autocross last weekend I ran 34/44psi. It seemed perfect for me under AX conditions.

What tires were you running? What size? I have ES100s, 205/55/16s f, 225/50/16s r.

Goodyear Eagle F1 GS D3. stock 16 sizes

Thanks very much for all the replies and information. I have backed off my cold inflation rate and am back to 32F and 38R for now.