mopar bob

I just looked at Tire Rack and those tires are not listed in 18" or 19". I wanted to make sure that I didn't miss this. They may have run out and will not be making any till later. My 997 came with a 10 x 18" wheel or 11 x 19" . My stock size is 265 x 40 x 18 or 295 x 30 x 19. I was hoping the the RE71r's would be shipped to the states but they are not. Not much out there at the moment as it was I had to time it correctly to get the back RS3's. Tire Rack only received 8 sets and they sold out in 1 day.

sjfehr

?? Don't limit yourself to OEM sizes, you'll be fastest if you stuff the widest tires that will mount without rubbing. Lower sidewall ratios help lower the car and improve gearing, too. Tirerack has P315/30ZR18 RivalS and 255/35R18, 265/35R18 and 275/35R18 RE-71R in stock. Front of mine's running his GT3 with RE-71R on the front and RivalS in the rear and doing really well with that setup.

mopar bob

I didn't think that I could put a 315 on a 10" wheel and some of the other Rivals that I looked at were .5" taller so I went with a tire that was .2" shorter. Next set I will be running the 19's and I will look at 315 or 325.

mopar bob

One thing that I should add is that I was also looking at wheel weight. Since I'm at over 5,200 ft I loose a lot of power. I'm looking at 5-6 lbs per back wheel, tire combo. I'm not sure going to a 315 over a 285 will offset the added weight but it might. This would be going from the 18 to the 19.

sjfehr

Oh, right, the 10x18, not 11x19. Yeah, 315 is probably a tad too stuffed for those wheels. 275 is the next largest size, and that's a bit small. Likely faster than a 295 R-S3, though.

PedalFaster

Junior Johnson has run 315s on his Z06's 9.5" front wheels with good success at several events this year, and I'd expect the penalty for running too much tire to be greater on the front axle because of the impact on steering response.

+1

sjfehr

Which begs the question: how the hell did Mike get 315s to set onto 9.5" wheels? Says loads that he's running that fast on a square 315 Rival S setup.

I thought I was overtired at 255/35-18 on 8x18" & 275/35-18 on 9x18" (RE-71R), but given just how much people are overtiring these in street and how fast they're going, I wonder now if I'm not overtiring enough? It sounded bad when I ran the numbers a few hours ago, but 315 on a 10x18" is really only a few mm worse than how I'm stuffed on my front tires, and I don't think I'm pushing the limits of the possible at all.

mopar bob

Does the heavier wheel and tire weight offset going to a much larger tire? If it offsets the added weight can I get a 325 or 335 on 11 x 19 in the back of a 997 C2 that is a narrow body? I would be using stock Lobster forks they are the lightest factory 19" wheel

sjfehr

Horsepower is less important at an autocross than a track, as we spend so much more time in sweepers, transitions, braking, and combinations than in flat-out acceleration.

Cornering: The extra weight of a larger wheel/tire has very small impact to cornering (1:1), and is typically more than offset by the larger slip angles permitted by the wider tire.

Braking: The extra weight of a larger wheel requires more energy during braking to reduce the rotational energy. On a typical Porsche, your braking grip is tire-limited, not brake-limited, and again, the extra rubber will help more than it hurts due to the higher slip angles possible in a wider tire.

Aceleration: It's only in acceleration that you're hurt. This is where the rule of thumb came from that every 1lb of rotating mass is worth 4lbs of sprung mass. But even so, it's only under full acceleration, where you don't have any power in reserve, that the rotating mass hurts you the most. I ran the calculations for my 986S a few years ago and determined adding 8lbs of wheel/tire was roughly equal to a 1hp loss in power for 6 seconds on a typical course- which I believe is more than made up by increases in speed elsewhere.

PedalFaster

I don't know, but I ran 275s on my 8" wide front wheels for most of last year -- that's as much of a stuff (and a bigger percentage stuff) than Junior's running.

Grip is more important than weight in autocross. Having said that, you're making an assumption here that may or may not be true -- that a wider tire will have more grip. That's true to a point, but that point varies by tire. As I mentioned above, I ran 275s on the front end of my car last year, but a buddy and I also did separate back-to-back tests between 245s and 275s, and neither of us found any discernable difference in either time or feel.

cretinx

You can run 305s or 315s on an 11" wide wheel . . . . . . 996 option coded 416

let them protest you if they wish.

mopar bob

Big 19's are in short supply but a 315 on a 18 x 10 today sounded real good in my EVO class. OK half the laps were VERY wet. I spun once in a slalom with a bump in the transition and it was very wet. OK that one was on me. I'm going to the 19's for next year.

mopar bob

It looks like Bstone is starting to fill out there RE71r 19" sizes. They only have up to 285 35 and smaller on the way next month but some just say pending. I want to see 315 or 325 by 30 for the back.