In the quest for more helmet clearance for track days, I have gone to an aftermarket seat. The seat I chose was the Momo Start 2007. This is one of the least expensive FIA homologated seats I could find (the only less expensive was a Sparco sprint seat). The Start 2007 is a side-mount seat (side-mount seats generally can be mounted lower than bottom-mount seats). The FIA homologation means that the seat can be safely mounted without needing to be attached to a roll bar (a roll bar generally prevents use of the back seat in the RX-8). The Momo seat is quite tight for my behind. This seat is recommended for up to a 36" waist -- I'm a 33" and am wedging myself down into the back of it. I don't know how a 36" person would possibly fit (unless those extra 3" were in the front!).
I bought a "super low position seat mount" from MemoryFab. They don't make the bracket -- they are just an importer. The bracket actually came from JapanSeatRail.com (they show some sample seats with their bracket in RX-8s). I got a seat mount with dual-locking sliders on it. MemoryFab is slow as a Smart ForTwo, so leave plenty of time to get your stuff (mine took over 2 months after ordering). Directions for the seat rail, including translation from Japanese.
I routed the seat belt the same way as Derex'8 did.
Some pictures are below of the installation. The only tight spots are the shoulder wings on the seat interfere with the rear door handle if the seat is back too far (which isn't a problem for me in a comfortable driving position); and the seat belt receptacle is a bit hard to get at. I plan to go to Mazda and order another receptacle (in addition to the one that came with the car), so I can leave one permanently mounted to the new seat. I may also drill a new mounting hole a bit further up the strap on the receptacle to allow better access to the release button.
I believe I gained about 1-2" doing this, which actually allows me to sit in a reasonable driving position now with a helmet. My helmet still hits the roof where it slopes toward the door, so I would like to find another inch if I could.
The whole process took about 2 hours. Future removal/installation will probably only take 20 minutes now that the seat and rail are assembled.
Weight: the Momo Start 2007 weighs 20.0#, and the MemoryFab bracket weighs 9.5#. The factory seat with the seat rail bracket is 39.0#. So I have added (in the Lotus sense) about 10# of lightness to the car.
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Bracket from MemoryFab.
Momo Start 2007 seat, front view. Seat belt slots are plastic.
Side-quarter view of seat. Yellow is the FIA homologation information.
Mounting brackets included in MemoryFab kit (hardware included, too).
Mounting bracket on seat (seat upside down). Wire hanging off is the seat belt sensor connector.
View of how the seat belt receptacle was intially mounted. This was modified later (see below).
Another view of the receptacle mounting.
Seat in car. Note clearance with rear suicide door. This seating position was quite comfortable for me, both in arm-to-steering wheel distance, and in feet-to-pedals.
Close up showing how the seat belt was routed through the seat on the door side.
Update:
Went up to WHRRI for an OTD, which you can read about in Doug Guthrie's blog at the Detroit News:
To put it mildly, the seat was awesome. I don't think I realized how much support it provided until I took it back out and put the stock seat back in. With the stock seat back in, I felt like I'm riding in a SUV with a cushy seat and a "high" view of the road.
I made a few modifications since the original posting to get the seat a bit lower. In the end, I added about 2" total head room compared to stock, which allowed me to sit up in a reasonable position with helmet.
The changes I made from the original setup include:
1) Pushing the seat belt receptacle up through the seat. I had hoped I could run the buckle out through the right side of the seat and into the receptacle between the seat and the center tunnel, but there just isn't enough room, and the buckle required some contortions to get it through the slot in the side of the seat. I made an extension for the receptacle to allow it to come up through the slot in the side of the seat. Much easier to put on and remove the seat belt after this.
2) If you look back at the original pictures, the seat mounting frame was a 4-sided piece. I ended up cutting the cross-members out, which allowed the seat to move down about another 1/2". The picture of the seat upside down shows the modified bracket.
Things I would do different next time:
1) Buy the non-adjustable seat bracket. It will sit you lower, it costs less and you don't have to wait 8 weeks for delivery.
2) Get a seat with an adjustable lumbar. The Momo has a solid cover stretching over the whole thing. Some of the other seats I looked at had removable pad sections in the back, which allow for lumbar adjustment. Not this seat. Probably part of the big difference in cost between the Momo and some of the others.
Modified mounting mounting bracket (removed the L-R cross pieces). This allowed the seat to move down 1 set of holes on the vertical mounting brackets, gaining another 1/2".
Extension bracket to allow the seat belt receptacle to stick through the seat.
Seat belt receptacle in final position.