Rinty

+1, on 17s.

Ed Hughes

There are no absolutes. Plenty of times, I added pressure at the rear to decrease oversteer. But, you can cross a threshold when it goes the other way. At the same time, the fronts were adjusted based on what they were doing, again typically a soft tire provided understeer in tight turns.

At the end of the day here, it is a balance of pressure at the front and the rear, and what one likes in a car.

Edward

Exactly. Increasing/decreasing pressures does what it is "supposed" to do within the tire's operating window ...beyond these pressures (too high or low), things just go wonky (technical term). This is very easy to illustrate on the track when you go too far on one end and see your cornering feel/grip head the other way.

As for the factory's "recommendation" and the seemingly inexplicable variances between its 17" and 18" spec, I can guess that this has more to do with the factory using "N-spec" tires as they rolled off the production floor. These are the tires Porsche tested with, contracted with, and sold with. So perhaps for safety/litigation reasons, they are printing specs that they can corroborate under exacting test conditions ...lest someone get "testy" and want to file a suit.

Fast forward to now: no one cares about "N" when tires have come so far (dang our cars are gettin old ...anyone remember the ubiquitous Pirelli P7 from waaaay back ), and we simply abide by a reasonable tire-pressure window. Lots of lattitude in that window, and achieving that balance one is comfortable with will vary with each car's suspension mods/wear, tire choice, and of course driver's skills/preference.

Edward

Edward

Oooh, just realized the OP was talking about Hankooks. Now I had posted my personal quibble with the Ventus V12 (IMHO, of course, to which many others disagree), and did indeed find the tires felt/behaved much better --worlds better-- with 40psi at the rear (this on 17" Cup IIs).

Granted the OP is speaking of different tires, but maybe the 'kooks share sidewall softness, to which somewhat higher pressures will help. Just food for thought.

Edward

nile13

That's why I've never paid any attention to tire pressure stickers on cars. Unless you are using the exact tire it's been tested with and want exact handling it came from the factory with, that information is irrelevant. Especially on older cars.

NC TRACKRAT

Ed, I'll grant you, tire pressure, alone, can't correct a large suspension deficiency but the OP was inquiring about a stock system and standard pressures as specified by the Factory. Variance from "stock" (tire size, aspect ratios, sway bar diameters and links, shock valving, corner balancing, etc.) will change the entire equation and tire pressure alone, may not have enough input to solve a particular problem. My track suspension is sufficiently tuned to the point that 1 lb. higher or lower in the front or rear will change handling characteristics one way or the other.

Ed Hughes

I was only speaking in terms as to what can induce or reduce under and oversteer tendency.

vincer77

I was just at the Petit Le Mans and some of the Michelin engineers came and gave a short presentation on tires for the PCA. After the presentation, I asked them questions regarding tire pressure. For some background, I am running 17" Cup 2s. When I bought the car, it had Michelin Pilots (N-spec). When the rears wore, I got cheap and went replaced them with Continental DWS (non N-spec). I have hated the handling since. I have been able to help the handling by increasing the rear pressure to 40 psi.

My question was what is the safe range to play with pressures. Their answer was to stay between the manufacturer recommended pressure and the max pressure stamped on the tire. Running less pressure can lead to tire over heating - recall the Ford Explorer. Running higher could exceed the stress limits of the tire.

The reason the 18" tires require higher pressure is that the load capacity of a tire is proportional to the amount of air in the tire. Since an 18" tire has less volume, it requires a higher pressure.

As far as N-spec goes, I am now a believer in the standard. Not that a tire has to be N-spec to be good for our cars, but if it carries the N-spec, you know it will be good for the car. A rear-engined 911 puts unique stresses on a tire - particularly the rears. So as far as I am concerned, from now on, I will only use N-spec tires on my car.

In my case, another mistake I may have made is mixing tires on my car.

il pirata

I agree with the Michelin comment regarding 18" tires requiring higher pressure. Another reason for the higher tire pressure that Porsche put in the manual etc for the 993 is to cover most situations...think driving at high speeds in the rain as an example.

Comparing an all season tire (Conti DWS) to a summer high performance tire (Conti DW or Michelin Pilot PS2 or Super Sport) there is no way way you will get the same handling. FWIW Tire Rack rated the Conti DW better than the Michelin PS2 which does not have a N-spec rating and is about half the cost. For me N-spec for street use does not mean much...you can get very good tires for significantly lower cost...you will have to be driving way to fast on the street to have any chance on feeling any difference.

nile13

What exactly do you hate in handling?

Precisely. But if memory serves, Explorer was spec'd for 26psi and Firestone put max tire rating at 35psi. Most tires were actually overinflated (but not to 35 psi). Still proved to be fatal.

Interesting. So they feel that sidewall stiffness has nothing to do with it? Or, in case of 993 where 44 psi rear pressure is spec'ed for both 16" and 18" tires and 36 psi spec'ed for 17" tire, do they feel that air is getting magically contracted and than squeezed again when going from 16 to 17 to 18?

I happen to disagree with every assessment in this paragraph.

Bingo!! Here comes the real issue

stickypatch

On the subject of mixing tyres, with 255/40/17 at the rear, and 205/50/17 in front, I could find no matching sets other than Michelin Super Sport or Hankook K110. Not even Pirelli made a matching set (that I could get here in NZ). So, it was a choice between a non-matched set, Michelin all round at a price approaching 10 percent of what I'd paid for the car itself, or the Hankooks.

If I went to 225/45/17 in front then a matched set would be easier, but it's a daily driver so I'm going to leave that for a future experiment perhaps.

nile13

225/45 is the much better match to 255/40 to get the balance of the car a little more neutral and understeer less.

stickypatch

So there would be no issues with the inside of the tyre rubbing the wheel well on full lock, or anything like that?

vincer77

Sorry, I was running the all season version of Pilots.

The rear end feels like it wants to come around on initial turn in. It doesn't lose grip. but then again I do not feel confident enough to really push it on the street.

Did not know that.


Just reporting what they said. Wish I had known that about 16" pressure at the time.

Not much to disagree with here.
You don't agree that N-spec tires are good for a 911?
You don't think a 911 puts unique stresses on a tire?
You do not agree that I am a believer in N-spec for my car?


Probably right, but not sure I want to buy Contis for the front to test the theory when I know the Pilots work in the rear.

nile13

No, I don't think that a random N-spec tire is good for a random 911. There were a lot of N-spec tires over the years. Somebody above mentioned the P7s and I remember them well along with P6s. You _really_ wouldn't like them today, I assure you. And they were very much N-rated.

No, I don't believe that 911s ask for anything unique from its tires. My autocross appliance is asking for something unique from the tires and they last about 90 miles (sic). 911 on the street? No, not at all.

I do agree that you believe that any N-spec tire is good for your car. I don;t agree that any N-spec tire is good for your car

More seriously, should you buy a pair of front Contis now that you have rears? I'm far from sure. How much driving have you done on rear Continentals? What's your tire pressures front and rear? Have you tried adjusting it?

You are saying that car feels like it wants to come around on initial turn-in. That would indicate to me a terrible oversteer, to a point of danger. I personally don't believe this to be the case. Let me explain, because this is important in keeping the car in once piece and yourself healthy. I believe that what you are feeling is the softness of rear sidewalls. The rear tires are rolling their shoulders and you feel like the rear is stepping out while in reality it just dives a bit with now lowered rear spring effective rate of the inner corner. The solution for this would be more rear pressure, but, again, in some moderation. If you overdo it you'll get real oversteer and will be quickly visually reminded in every corner where you just came from. Which would be a patently bad thing.

So... let us know what pressure you are running in rear tires. For something like DSW (I will keep my opinions to myself despite all the Tirerack accolades) I would expect that you'll end up close to 40 psi by the time it's all said and done in order to feel anything resembling ... I wanted to say "good handling", but I can't.

And an aside. We all buy 993. Presumably to drive and enjoy them. They are not Accords or Corollas. They deserve very good summer only highest performance tires in order to handle at least as well as intended (better if properly set up due to the fact that tires today are leaps and bounds better than in 1995). And I say this as a daily driver of a 993 in Boston, which means rain, snow and crappy roads. Today there are about 3 tires that suit the 993 best - Toyo R1R, Dunlop Direzza Z1 StarSpec and Hankook RS-3. These, in my opinion, are the best street tires on the market today. I don't have enough knowledge about Michelin Pilot SuperSport, but the regular PS/2s weren't even close to the above three from both first hand and non-first-hand experience.