Just sharing my experience with tires: Michelin Pilot Sport 2 versus Michelin Pilot Super Sport versus R compound Pirelli Corsa. As usual, any expert, or even non-expert :-), please correct me as needed.

For reference: My Turbo came with Michelin Pilot Sport 2, which I used for a year and a half/18,000 miles (rear tires changed once). The Pilot Sport 2 was followed by R compound Pirelli Corsa, after I had foolishly tried to follow a 997.2 GT3 on canyon roads - talk about bringing a knife to a gun fight. My Turbo blew away the GT3 in straight line acceleration - not even remotely close, but on the twisty roads of Los Angeles canyons, the *&^%$# GT3 left my babe in the dust. Indeed, the Pirelli Corsa did a lot to make my Turbo baby whole again, and I made note to self to NEVER again race another car on un-equal tires - tire is a critical part of the suspension system. At any rate, the Corsa has lasted about 14 months/12000 miles or so, which brings us to now. Because I drive my car daily to work and because I always want to experiment and compare, I will take a break from this love-hate relationship with the Corsa (see comments below). Conveniently, I need replacement tires at the same time that Michelin is going all out promoting their newest and latest, Michelin Pilot Super Sport.
Btw, it's important to note that my Turbo's stock suspension has been replaced with Bilstein B16 Damptronic, making the car stiffer than both 997.1 and 997.2 Turbo.

 

Comments about Pirelli Corsa:

1. R comp has superior traction, that's obvious. Not often discussed is the stiff side wall, and its very significant effect on the feel of the car and its handling capability. IMHO, the stiff wall "stabilizes" the soft Turbo suspension by reducing the excessive weight transfer (body roll). My Turbo with R compound feels more planted with the Corsa; it takes corners with confidence, perfectly flat, and perfectly beautiful. Pilot Sport 2 feels more comfortable of course, but also disturbingly rubbery soft. In my mind, there is no question whatsoever that into a corner at speed, the car leans more with PS2, than with Pirelli Corsa. Corsa feels stiff, heavy, and with much less body roll. In other words, planted.
2. Either I did not do my research well enough, or not enough has been discussed about the Corsa's road noise as it becomes old either. This tire did not age well with respect to stiffness, and particularly noise. All tires become noisier and stiffer as they age, but the Corsa seems to take an exponential leap towards the end of its life. The noise is excessive whether just rolling on the freeway, a loud, annoying din, or when hitting a sharp road bump, which results in a loud sickening crack, instead of a thump. When new, I don't think the noise was bad at all, towards the end, it is almost deafening. I will try Michelin Cup next and see if it's different; I've been told that it's better.
3. The usual warning about R comp tire: it's dangerous when road is wet, it's not good until it gets up to temp, and it runs lower pressure: 32 front/36rear hot, meaning in your turbo the tire alarm will be on always. Mark my words on this: that alarm sign gets on one's nerves!
4. I am almost too afraid to talk about rolling resistance :-), but for whatever reason, whether lower operating pressure or higher rolling resistance, the Corsa makes the steering feel heavier, and the car "feel" heavier, even though it is the lighter tire by weight. The heavier feel actually makes the Turbo much more satisfying to me to drive, because IMHO the stock steering feels too light, but YMMV. The question is whether tires with more traction have more rolling resistance (yes, right?), and if so, if I were to go for straight line speed, I think a street tire is faster than R comp.

 

Lower pressure will make a very noticeable difference in rolling resistance. When I raced karts, I could tell a difference in rolling resistance on a 200 lb kart by raising the tire pressure from 20 psi to 25 psi just by pushing the kart in neutral.

 

Nice review - thanks for the info on the Corsas especially.

I have tried the MPSC's and had a lot (three) punctures in a single driving season (street and some DE's). This was a major drawback that people don't seem to talk about much. Yes, they are much better on a road course than the PS2s, but they don't last too long and are prone to getting screws and nails in them.

I'm using Bridgestone RE-11's now for street and DE duty. They grip almost as well as the cups, but are a lot heavier. Haven't had any punctures in ~7k miles. I think the steering feel is not quite as precise as either the cups or the PS2's, but for me the better grip, durability and price outweigh that drawback.

 

Forgot to mention that the RE-11s don't have the same stiff sidewall as the corsa's or cups. But they seem to hold up very well in hard cornering nonetheless. I haven't noticed greater body roll with them but will compare soon when I try the cups one more time in a DE in a few weeks.

 

Thanks guys for your comments.
For those who might be wondering, the reason I am spending time on tire stiffness is that it is very relevant to the comparison of street tire Michelin Pilot Sport to R compound Pirelli Corsa, and it is such an interesting idea that has rarely been discussed. At any rate, my interest in tire's sidewall stiffness stems from a comment Walter what's-his-name made a while ago. They were preparing that GT3 that Porsche was going to show off by driving it to the N'ring race track, race it, then drive "home." Somewhere along the line, they decided to replace the R comp on that GT3 with Slick, and WR made a comment to the effect that when you change to a stiffer tire, you must change to a sitffer spring as well, and vice versa. Otherwise, the softer component of the 2 will be "overloaded." In trying to understand the statement by WR, I could see that a tire acts in many important aspects like a spring. It compresses, then rebounds.

In a stacked spring setup found in some coil-over suspensions - 2 springs, one on top of another (main + tender), the final rate of the dual-spring setup is given by the formula: F= (A x B) / (A+B).
So if spring A's rate is 20, and B is 30, then final rate is 12, quite low.
But watch: if A is 20, and B is 80, the final rate is still only 16. So, no matter how high B is, the final rate takes on the characteristic of the softer of the 2 springs, A. The suspension system is only "as strong as its weakest link."

The bottom line? If you think of the tire as one spring, and your car's suspension system as another spring, one on top of the other, then you have seen the logic behind WR's statement. A soft tire could well screw up your nicely stiff, race track ready, GT2's system. It lowers the overall "spring rate" of the system, to a point that's lower than the lowest, namely the soft tire. This is why you don't want to put street tires on your precious GT2. And vice versa, you may not want to put stiff R comps on your softly sprung Turbo without first stiffening the springs.
I think I might have lost everyone? I myself need a couple aspirins now.

 

nice review Can. As far as straightline speed and acceleration the rolling resistance should be mitigated by the significant weight reduction of the Corsas. They are even lighter than the Pilot Cups. The Corsas also seem to have a softer sidewall than the Cups, which can be compensated for by increasing the air pressure. So your suspension tuning is spring, sidewall, and air presssure as well.

 

Can - with this Walter's recommendation of matching the stiffness of springs and tires, and taking 'stock everything' as the baseline, would this math then say combining bilsteins with street tires be a bad combination? I know you've seen a positive change in the car with the suspension upgrades, so assuming bilsteins are not sooo aggressive as to no longer being recommended with street tires. Curious.

Also, looking forward to hearing more on your experiences with the PSS. I put them on as well (and really like the tires), but don't have strong exposure to other tires for drawing a good comparison.

 

Very good point. To play devil's advocate: Your assumption is that stock suspension + stock tire is baseline and a perfect match-up, my counter is that stock suspension + stock tire is baseline, yes, but it's not perfect. The spring of stock 997.1 suspension is too soft and therefore Bilstein + stock tire is a "better" combination. (BTW, 997.2 Turbo has both stiffer spring and stiffer sway bar than our 997.1 Turbo. Many of the suspension revisions of 997.2 were to address the softness of 997.1 per Panorama article that I read.)

Above was just for the sake of (hopefully) interesting argument and to bring across the point of how "relative" everything in suspension setup is. Walter Rohrl's comment must be taken in the context of performance above all else. I am sure what they do is pick which tire they like to use, than test car on the tract with various spring rates, sway bar settings, etc.. The sweet spot would of course be the combination with the fastest time, comfort be damned. In real life, ride comfort enters the picture and makes the definition of "perfection" very much relative.

Having said all that, within reasons, anything you do to stiffen (and lower) the car, WILL make it faster, assuming a flat road surface - stiff cars don't like bumps because they go airborne more readily. Of the 4 combinations below, I am fairly certain as you do down the line, track time improves, and comfort decreases. Combination number 3 is stiffer and less comfortable, yet it is also the sweetest and best and "funnest" that I have ever experienced in my car with respect to performance. This is the very same combination that eclou (moderator) above has in his car - and he's not often wrong in these matters (cheers :-)). Once you have experienced it, it's very hard to go back.
1. Stock suspension + stock tire
2a. Stock suspension + R compound tire
2b. Bilstein suspension + stock tire
3. Bilstein suspension + R compound tire

 

Makes sense...thanks Can. And good point that the 997.2 suspension changes try to address the softness of stock 997.1, all while retaining street tires. I bet it's been posted somewhere so will try digging, but wonder how large the stiffness difference is for each jump between 997.1 --> 997.2 --> Bilstein. Eventually I'll have to try finding a member around Dallas who's put on Bilsteins so I can experience this first hand (since as you know these upgrades on are my mind )

 

Yes it's been posted, and you don't have to dig far. It's right in the Bilstein thread in my signature. I drove the 997.2 and posted my impression. I believe I posted 997.2 spring rates as well somewhere in that interminable thread.
Essentially, 997.2 stock is stiffer than 997.1 stock, but overall 997.2 is still fairly soft. A Bilstein modified 997.1 is stiffer than 997.2 stock and feels more sporty.

 

In the past, when someone stated changing to a "cup" tire (aka R compound Pirelli Corsa or Michelin Cup) means x seconds improvement on the Nurburgring time, I would think it was all about better traction. It is now apparent to me that I was mistaken, that there is another parameter besides traction that is responsible for the usually surprisingly significant improvement, and this other parameter is the tire'e sidewall stiffness. The increased in suspension stiffness, merely from switching to stiffer tire, IMHO is not too dissimilar from changing to a stiffer spring. And as you already know, on a flat surface, stiffer cars take corners faster. This increased stiffness is very significant, and very obvious even to amateur's driver like me. That's why I have used the word "transformation" to describe the switch from street tire like Pilot Sport, to R compound tire like Pirelli Corsa. If you ever want to emulate the feel of a GT3 or GT2 in your Turbo, besides stiffening the springs, a change to R compound tire *is* an absolute must. There is no way around it.

Anyway, from a tire discussion standpoint, the 2 parameters, traction and sidewall stiffness, intertwine, and an all out performance tire needs to have both. It is useless to have one without the other.
1. Assuming you have a tire with very stiff sidewall. The stiff sidewall will allow the car to take corners faster because there is less body roll/weight transfer. Without an appropriate increase in traction, car will slide sideways. It won't body-roll, but it will slide sideways as the tire's traction cannot hold the centripetal force. In other words, stiff sidewall is useless if tire does not have appropriate increase in traction.
2. OTOH, assuming you have a tire with superior traction, but its sidewall soft. When this car speeds into a corner, the soft sidewall causes car to body-roll, limiting cornering speed. Even though the tire has superior traction, this traction is never "tested" because max speed in corners is limited by body rolls. In other words, superior traction is wasted if tire sidewall cannot support against weight transfer/body roll.

I think this should be my final thought on sidewall stiffness as I have this feeling it's driving people away from this thread . I promise that the discussion so far *is* quite relevant to the Michelin Pilot Sport versus Pirelli Corsa comparison. Seasoned veterans among you probably already know what I am leading to, whether number 1 or number 2 above is the Michelin Pilot Super Sport, strictly from a performance standpoint. No doubt, it is the more comfortable and more quiet tire. A much better daily driver's tire.

 

Good points! A lot of people have no clue that the sidewall is a major part of the suspension. Stiffer + stickier are a great combo.

 

I agree that tires are one of the most important factors. Try taking on slicks with normal tire with a street inflation.

The best tires I ever used on the track were 18" Hankook RS2/3 after Mr. White from this board recommend I run 31PSI front and 33 PSI rear HOT. The car had grip for days. It was AMAZING! As an added benefit they feel like marshmallows on the street at 38 PSI rear and 33 PSI front. The 888s also get great marks although I have never personally used them. IMO great tires aren't the stiffest, but the ones with lots of grip and a lot of feedback via sidewall flex (without being too soft side-walled). This why hardcore track guy actually go to -1 setups 18" on the 997T. More sidewall gives more feedback and a smoother more predictable breakaway limit that you can feel.

 

Ari/HC, thanks for your inputs. Yes until recently I was not aware how very very significant this contribution is.

HC, of course they are good points. I learned from you!
Having the Pirelli Corsa for a year then switched back to the street tires has shown me the light. I was amazed how much less body roll there is with the R comp Pirelli Corsa - it's as if someone has tuned my suspension. I have been told by reliable sources that the Michelin Cup is even stiffer than my Corsa!

Have you noticed that the GTR's stock tire is run-flat? As run-flats have stiff walls, I would predict a harsh ride but significant advantage at the track. Matching run-flat with sticky rubber would give these tires some characters of R comps. It seems they are sacrificing comfort and tire wear for faster times.