If you're over six feet tall, as I am, and you have tried wearing a helmet in your 911 for track days, this article may be of interest. Especially if your 911 has a sunroof, as does my 1977 red 911S.
When I first acquired the car, I did what so many taller people do- I moved the seat to the rear and raked the seat back so my head would clear the headliner. Then I moved the seat forward so my legs were comfortably bent even with the clutch fully depressed. Then I raked the seat back some more, to regain adequate headroom. The result, as most of you know, is definitely not the ideal driving position- arms almost fully extended, with no leverage below the shoulder to allow you to maintain a 3-9 o'clock positioning of the hands.
When I bought a helmet for track days, the problem was increased significantly. I tried all kinds of combinations with the seat both forward and back to try to clear the headliner with my helmet on, to no avail. The best I could do was a position where the seat was farther forward than I like and the back was raked more than it should be. My helmet touched the ceiling slightly, but I couldn't possibly drive long distances with my arms so fully extended. A better way had to be found.
I searched for racing seats--some of which would have worked--but decided not to spoil the stock appearance of the car just for a handful of track days. I considered cutting out the seat rails to lower the entire seat closer to the floor, but I didn't want to start cutting things up, only to find that the change was unacceptable. I thought about removing some of the foam padding from the seat, to lower the cushion. I even bought some new foam, thinking it would be an inexpensive experiment to try reshaping the seat cushion. I could always put things back the way they were if it didn't work.
I began the project by removing the seat and detaching the 15-20 small clips that fasten the leather to the frame. Then I removed the entire foam cushion with the leather cover intact. To my surprise, the lowest part of the cushion was already quite thin- maybe 1-1/2 inches. So making a thinner cushion was not going to solve the problem. The solution lay in changing the steel springs that support the cushion.
There are six S-shaped steel springs running front to back, joined together by small staple-like clips. Each of these springs is attached by a simple bent metal clip to the front and rear seat frame tubes. Each of the springs is bent at 90 degrees, about one inch in front of the rear tube, forming a hockey stick profile with the elbow (or heel) of the hockey stick at the lowest point under your behind. To lower the height of the seat cushion, I removed the middle four springs from the seat and straightened the hockey stick. Then I bent them back to the same hockey stick profile, at a distance of about 3 inches from the rear. When I reassembled the array of springs, I clipped the middle four together as before, but moved the clips to the outer two springs well forward towards the front the of the seat. By doing this and by not changing the outer two springs profile, I maintained and even increased the bucket shape of the seat cushion. This might be a bit more challenging for any driver who's a bit broad in the beam. The rear edge of the cushion is now at least one inch lower than before and even lower when I sit in it. Reassembly of the seat cushion was simple and fast.
I now have adequate clearance for my helmet with a much more vertical seat back. My legs are in the proper position for long-term comfort and strength on the pedals and my arms are bent strongly at the elbow, allowing me to assume the correct driving position. The best part of this modification was the price- zero! Now I can focus completely on my driving, not my physical comfort. I hope this works as well for you as it has for me.