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I bought the 4" from Ecklers. They told me this was the same padding and style that the 78 pace cars used. Supposed to be better (more) padding than the 2".
I had 2" foam and seat covers in my 80... I just purchased 4" foam with 4" covers. If you are going to get new covers GET THE 4" FOAM!!! The seats are 10000 times better with the larger foam. More back support.
I would also suggest finding someone with 4" inch foam in ther car and sit in it. For me, at least, the 4" foam feels better on my back.
If you don't know anyone. You could try someone here on the forum or look in the news paper and see if any one is selling a late model C3. Try their seats.
Better to try them out then spend 1000 bucks on new foam, seat covers and mounting and at the end wished you had gotten something else
Otherwise the 4" foam offers more support and then the 2".
Robert:
Thanks for the advice. Unless someone tells me something totally contrary, you've convinced me into purchasing 4" foam. As you can see, I'm not hard to convince. Thanks: Kloppy