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The common wheels found or that came on cups:
BBS E29- often confused as E88s but are the wheels that came on cup cars or at least 6-cups. They are aluminum. Back when the 6 cups were in grand am the race shops would send the centers to Porsche Motorsport to be machined for 5 lug. There are two types of E29 centers. Ones that came on the cups from 98-02 for four piston brakes and the ones that came on cups from 03-05 for six piston calipers. My wheels e29s: 1st pic
BBS E28- They are sort of like a magnesium E29s. They are pretty hard to find. Some were found on 964s but some were on early cup cars. I've seen them on later cars but they require a 3mm spacer as they were all made for four piston calipers. My wheels e28s: 2nd pic
BBS E88-they are billet aluminum and found on later cup cars. They have bigger brake clearances. They have scalloped spokes. Friends wheels e88s: 3rd pic
Sorry if there are any errors. I believe this is all right from my experience. Feel free to add on.
above infor are more or less correct. some aren't but they are minute errors so i wont bother correcting.
but the bottom line is pricing OP asked
E88 has many centers. 0288170, 0288107, 0288274, etc etc
first you need the right center (different offset, thickness of ctr section etc) for your car.
on average 18" E88 complete wheel are $6000/set plus minis $500.
so use that as ref for used prices.
^0288170: they are made specifically for 997 grand am cup cars
for 986, you might try 0288264 front
0288107 or 0288108 rear
on my 987 i run 9.5" front 10.5" rear. but LOTS of fender rolling and pulling.
i think 8.5-9: and 10" would fit your car.
i just found this....copied from Speedmerchants website
useful info
996 Cup 1999-2001
Front: Center 0229021 Outer Ring 1.0" Inner Ring 7.5" ET + 45.5
Rear: Center 0229021 Outer Ring 1.5" Inner Ring 9.0" ET + 58.5
996 Cup 2002-on
Front: Center 0229052 Outer Ring 1.0" Inner Ring 8.0" ET + 47.0
Rear: Center 0229052 Outer Ring 1.5" Inner Ring9.5" ET + 59.5
997.1 Cup
Front: Center 0229066 Outer Ring 1.0" Inner Ring 8.0" ET + 43.0
Rear: Center 0229066 Outer Ring 2.5" Inner Ring 9.5" ET + 43.0
997.2 Cup
0229066 / 1.5 / 8.0 ET + 36.5
0229066 / 3.0 / 9.0 ET + 30.0
997 Carrera S Less aggressive / standard brake
0288120 / 1.0 / 7.5 ET + 49.0
0288107 / 2.5 / 8.5 ET + 55.0
997 Carrera S More aggressive / PCCB
0288274 / 1.0 / 8.0 ET + 50.0
0288107 / 2.5 / 8.5 ET + 55.0
997 Turbo Less aggressive / Standard Brake
0288120 / 1.0 / 7.5 ET + 49.0
0288108 / 3.0 / 8.5 ET + 44.5
997 Turbo More aggressive / PCCB
0288274 / 1.0 / 8.0 ET + 50.0
0288107 / 3.5 / 8.5 ET + 42.0
997 GT3 non Center-lock
0288274 / 1.0 / 8.0 ET + 50.0
0288108 / 2.5 / 9.5 ET + 64 w/o factory 5mm spacer
0288107 / 2.5 / 9.5 ET + 68 for use with factory 5mm spacer (effective ET is + 63.0)
997 GT3 RS non Center-lock
0288274 / 1.0 / 8.0 ET + 50.0
0288108 / 3.0 / 9.0 ET + 51 for use with factory 5mm spacer (effective ET is +46.0)
997 GT3 Center-lock
0201182 / 1.0 / 8.0 ET + 43.0
0201183 / 1.5 / 10.5 ET + 61.0
997 GT3 RS Center-lock
0201182 / 1.0 / 8.0 ET + 43.0
0201183 / 2.0 / 10.0 ET + 48.0
Cayman Street / Track day car
0288120 / 1.0 / 7.5 ET + 49.0
0288107 / 2.5 / 7.5 ET + 42.0 for use with factory 5mm spacer (effective ET + 37.0)
Cayman Interserie car uses same rim combos with specific centers for lower ET’s
Cayman S aggressive track day fitment
0288274 / 1.0 / 8.0 ET + 50.0
0288107 / 3.0 / 7.5 ET + 36.0
Yes, the different part numbers on the centers denote different offsets for the mounting face to the wheel half face. It can also mean front mount or rear mount for how the halves mount together. The offsets go in 1/4" increments best I have seen.