I think the correct approach is to set a pre-registration limit for each of the lots(and events) recognizing that there will always be some same-day entrants. If your goal is 130 in the West lot, you could set pre-reg as 100 (or 90 or 80) and if by Wed. before the meet that number is met, no X-cars for that event, posted on forum and everyone knows.
I think there should be no distinction between affiliate members and regular members. If two from the same family, houshold whatever want to run, they are both welcome, in Porsches unless the pre-registration is too low.
I think Kary is right, that autocrosses are a difficult venue to add X-cars because of popularity. A more reasonable approach would be to get qualified in a Porsche at autocross and then run the QDE's and big track events in the X-cars (if they pass tech).
If tech is a real concern, one option would be to have the car inspected at one of the shops before any events are run.
Also, I feel we should not be too particular about what constitutes a Porsche for running in events. A Beck spyder to me meets the spirit of the the law if not the letter (as well as the 914 with Chevy V8)
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I think if you looked real hard at some of the modified cars you would find more non-Porsche parts than factory. How much fiberglass is required before it is still a Porsche
Greg