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Have a line on a new set of cup 1s in cup car configuration - 8s/9.5s w/ 21mm spacer for the rear. Any issues running these on a stock 964 (targa, but not sure that part matters). Pros, cons? Any advise would be appreciated! Also, what do I need for those tears w/ spacers to work? Longer studs, I imagine? Any idea which and from who? Thanks!
You will need longer studs for spacers > 5mm. I went with Titanium Porsche Products. Also got the open end lug nuts. Very high quality (and light, too!).
I'm running 7.5's and 9's Cup 1's on my Targa with 10mm spacers on the rear and 15mm spacers on the front. I had to install longer wheel studs. What are the offsets on your wheel set? Here's a shot of my Targa with the 10mm rear spacers. I could run a 15mm but 21mm might create a clearance issue. My 7.5" fronts are 65mm offset from a water cooled (instead of 55mm) - thus the 15mm spacer is really only moving the wheel out a net of 5mm. The 9" rears are 55mm and bolted up without spacers so the 10mm spacer is just to get a more aggressive look - not required to run the wheel. Tires are 225 front and 255 rear.
Lots of good threads on this if you search. You will need a longer rear stud. The OEM Cup rear stud is available as well. The biggest issue is the limited tire selection for a 9.5" rear wheel. You might consider going with a 9", as the tire choice will likely be the same and you don't need the spacers or studs.
You can run a 255/40 on a 9.5" wheel without any issue. The rounder sidewall profile as a result of the wider wheel might even benefit the handling a bit over the more square profile of a 255 on a 9" wheel, since rounder profiles tend to break away more gradually when they reach the limit.