ncp

I'm thinking of trying these very new Dunlop Z1 Star Specs for track/street use. They come pretty highly rated according to Tire Rack tests and at about $750 with rebate, they're almost 1/2 the price of the PS2s and PS Cups. I'm planning to do at least 3 or 4 DE events this summer and I just want a grippy tire that I can beat on and not care about chunking... and at that price, I can rip through nearly 2 sets for the price of one of the Michelins.

Unfortunately, the closest the rears come in is a 275/35/18, which has a 25.6 inch overall diameter. The typical 285/30 tire normally run (Michelins) is closer to 25.1 inches. I realize I'd be giving up a little less than 1/2 inch in surface patch by going with the 275 which is not ideal, however, My Questions: 1) will I have any fitment issues with this tire size in the rear due to the larger diameter? 2) If I were to go with this tire size, do I need to increase the diameter of the fronts to more closely match the increase in the rear due to the viscous coupler and if so, how much of a spread is acceptable between front and rear anyway? 3) I realize the larger diameter of the tires will affect gearing, however, is a 1/2 inch difference diameter noticeable? and lastly 4) has anyone run these tires and if so, how do you like them?

Thanks guys, your input is appreciated.

Nick

JJayB

Nick,
You didn't mention what size front tires you'll be running. If its 225/40/18 with a 25.1 diameter you'll have a half inch diameter difference with the 275 Dunlaps. IMHO too many compromises in a race setting. I think viscouse couplers are about $5k installed.

ncp

Hey Jimmy, sorry, that's part of my question. How much of a difference between the front and rear is acceptable? There's a 235/40 that I can run at 25.4 diameter for the front vs. the 275/35 at 25.6 diameter for the rear. No matter what tires we run, there's usually going to be a very slight difference between most (not all) tires.

Thoughts?

JJayB

The highest variation I've run is the Hoosier R6 with a .3 differerence to the rear and it was more understeer than I care for. The best combination has been when the front diameter is slightly more (.1) Michelin PSC's. I've tried to keep it as close as posssible. I've seen guys run as much as a 4% difference and there cars drove like sh*t and then they want to remove the AWD for better handling. I've heard that 4% difference will not damage the viscouse coupler but I do not believe that.

rinlv1

JJay,

what kinda wheels do u have, bbs?

sorry for OT

LAT

What are the spec'd revs per mile for front vs rear tire.

The viscous can tolerate quite a bit, it is your ABS that can act up if the difference is too great and even then at very high speeds only, 300 kph +.

My 993TT set up in Germany by RS Tuning runs 285/35 18" and 235/40 18" which are 25.9 and 25.4 respectively and RS considers these the ideal for the 993TT

ncp

There's a 245/40/18 at 25.7 which will match up nicely with the 275/35 at 25.6, for a .1 larger diameter at the front. With that, any question of affecting the viscous coupler is moot. The questions now are a) will the 245/40's fit the front end okay??? b) do I want larger diameters (larger profiles) front and rear respectivley and how will the larger diameter (ratios) affect the overall gearing and c) how much at all will a 1/2 inch less rubber to the ground (footprint) at the rear affect handling?

ncp

LAT- unfortunately, Tire Rack (and Dunlop site) doesn't list revs per mile for this tire yet. Although the non-star spec Z1 revs are listed, they don't have the same sizes available so I can't answer that question, yet. So what I hear you saying is that a "larger" diameter rear, ie., higher revs up front is advised by RS Tuning - and a 1/2 inch spread at that between the two?

ncp

LAT - for the fronts, there's at 225/40 at 25.1 and a 235/40 at 25.4. Given the rears are a 275/35 at 25.6, which should I run up front?

LAT

ncp; You read this correctly and keep in mind 1/2" is diameter and therefore only 1/4" taller from the center.

I would use the 235/40 for the front.

Rears also wear at 2 to 3 times the rate of the fronts and if you take that into consideration your .2 of an inch which is .1 in radius will be chewed up in the first 1,000 miles.

Hope this helps

ncp

LAT- good point. Thanks for the advice.

malcolmd

I've have run 235/40 and 295/35s for some time in the past without any issues at all - this is a 2.69% difference wth the rear being larger. The decision at the time was largely driven by immediate need and unavailability of the preferred size at that time. I never felt the decision was a bad one subsequently. I had and still haven't had any issues with either handling or the viscus coupler. Hope this helps.

Malcolm.

ncp

Thanks, Malcolm.

JJayB

BBS Motorsport race wheels.

rinlv1

tks jjayb... I really like them wheels not to mention ur blackie....