Hi there,
If this post appears twice, it's down to a senior moment on my part.... heaven knows what I clicked but my first attempt seems to have gone west. Anyway I was working on my car today and remembered this thread so took a peek underneath the seat. I reckon if you bolted directly to the floor then you'd gain about an inch, maybe a touch more depending how you did it. The subframe isn't that deep but it is bolted to raised bobbins in the floor which I suppose you'd have to keep to unless you did some external stiffening of the floorpan.
Is it headroom you're trying to gain or just more space to make getting in and out easier ?
If it's headroom then you might consider another seat design, the modern Lotus seats are quite thin and no doubt a racing shell would fit ok.
If it's room to make life easier getting out, then how about either a smaller wheel or a more modern removable one ? The original owner of my car was a tall chap and he fitted the 10" wheel that's still on the car because apparently the OEM one was too cramped for him. And a lot of Elise S1 owners went down the removable wheel option because getting out of that car is certainly a learning curve in yoga positions....
Brian
ps - you'll love Goodwood, it's the biggest petrolhead thing over here. The only downside is our Great British Summer weather - bring an umbrella !
