Gutting: With mine, obviously, all the interior trim is gone, but I did retain the complete stock dash. The console (around the shifter, plus the armrest) isn't all that light, and I don't think there's much you could hack out of it to pull weight, either.
EVERYTHING that was bolted into the interior that isn't required for operation of the car is gone... Parking brake, seat belts, etc. were all removed. I cut the package shelf out of the rear, but left the crossbrace on advice I got from Poe, but I did hole-saw that. Behind the dash, there is... air. I gutted the glovebox lid, pulled the cover off the passenger-side airbag module, so for aesthetics, I took about a 1.5lb hit. All the HVAC stuff is gone, firewall holes plugged/plated-over, etc. I used an FR500S chassis harness, and custom-decontented the dashboard harness, probably pulled 5lbs of wire and misc connectors out of it. Lightweight battery in the stock location, custom power distribuition with nothing heavier than 4ga wire, and limited amounts of that, as well. For further weight reduction for me, it would be a K-member, cutting the spare tire well out, and de-contenting the remaining factory wiring harnesses. I could cut out the rear parcel-shelf brace, and start trimming metal here and there, but I would prefer not to. I'm built to the AI rule-set, so there is no actual weight target, just a power-to-weight ratio to hit.
Rear bar attachment points: Both the Autopower and the Maximum Motorsports 4-point bars bolt to the inner wheelwell, not the frame or shock tower. IF you're doing a custom cage (strongly recommended!), then the sky's the limit. My rear stay mounting plates are on the frame horizontally, and wrap up vertically around the front half of the shock "towers," and have a cross-bar between them. The tubes themselves land on the plates, and the crossbar spans the tubes, roughly 3/4" above the level of the tub, to allow for wire passage. For safety's sake, I would opt NOT to span the rear shock towers with a bar, and then land the backstays on that. Depending on how you read the NASA CCR, that kind of setup could be deemed illegal.
Steve Poe uses an interesting setup as an alternative... He builds a plinth box around the shock mount, holesaws the top for shock access, then lands the tubes on the faces of the box.
Things I would think about if I were you, in terms of cage design:
1) Landing point location: You want to create a cocoon around you with the cage. Put your A-pillar bar points as far forward into the footwell as you can, and realize that you can take full advantage of the vertical wall surface there! There's no reason at all that you can't shape the SIDE of the tube at the plate to clear bumps and protrusions in the kick panels, and weld directly to the walls, IN ADDITION to the required plate on the floor area. For the main hoop, land it on the main crossbeam under the rear seats. Hugely strong, and that puts it in a place where it won't impact the door aperature (see note #3!). Unless you can think of a reason not to (possible class rules, etc.) think about welding plates directly between the main hoop and the walls of the chassis, then hole-saw them for weight reduction. Gusseting like this REALLY locks the cage into the chassis, and minimizes the possibility of the chassis collapsing around the cage structure.
3) Door openings: Do your design with an eye towards keeping easy entry and exit. You'll need to do some personal risk assessment here, and balance between intrusion protection and on-fire exit time. Ideally, from a crash-intrusion standpoint, you'd have bars all the way up to the window opening, so that if somebody T-bones you right in the door, you have a LOT of steel keeping their engine out of your lap. From a fire standpoint, you don't want ANY door bars! The more in your way, and the more bending, twisting and slithering you have to do to get out, the longer it will take. Personally, I leaned more towards the fire scenario. My door bars were put in at a pretty steep angle, but from a flat, side impact scenario, they cover me pretty well. Also, the doors in the S197 are VERY wide, so you may want to think about adding an additional mount point to support part of the span and give it extra rigidity.
4) Extra bars: Think about what the "basic" cage will and will not protect against, and where the weak areas are, and consider adding extra bars to the design. As was menioned previously, footwell intrusion bars are a great idea. I also opted for an FIA bar running between the A-pillar tube and the top of the door bars, lined up directly with the extra bottom mount point for a solid load-path. This was done primarily with an eye toward rollover protection, helping to prevent the roof from crushing downward. Triangles are strong, rectangles not so much, so wherever you have a rectangle, try to make triangles out of it. I added gusset bars between the main hoop and the halo, as well as between the A-pillar tube and the windshield bar. I also added the diagonal in the rear stays, pretty well visually lined up with the main hoop diagonal for reduced vision impact.
Here's what the door area looks like:
You can see the FIA bar, the extra mounting point, and the gusset bar pretty plainly. That design does have some drawbacks, but they're (hopefully) very-low-percentage scenarios. Airborne car, on the way down, lands in the window opening: FIA bar isn't in tension or compression, so it will buckle, and that leaves a pretty large opening for parts to come in. I'm more concerned, personally, about fire, though, and I can be out of this car in under seven seconds with that setup. In just about any "normal" crash scenario, I feel pretty safe in this cage.
I'll try to grab pix of the rear landing plates when I can.
