Fell Off Wagon

[Onbeachw320]

  Hi.  I got busy with life, and as you can tell, I'm not committed deeply enough that the Lotus is going to dictate my schedule.  In short, I have let the car go for a while.  Actually, we have been experiencing weather like we USED to have -- lots of rain, which we always need, and lots of yard work.  Probably more satisfying for now!  I need to do a 200 mile trip to see a marine supplier, and will when I attend a car show with the Corvette.  But thinking about the project (dealing with a new bodyshell and those firewall openings), I have a few questions:
  a) why are there areas cut out of the "firewall" in the first place?  What purpose do they serve?
  b) hardboard is original, but we have new options that hold up to moisture better, but...
  c) if this area is already fiberglass, why not fill in these openings with more fiberglass, and seal it up with a nice glassed in finish?
  d) has anybody here done this with fiberglass?  What do you feel about this?

[4129R]

The soft bit that falls apart is fibreboard, or compressed cardboard, about 19mm 3/4" thick.

Either cut it out, replace with ply, and glass over, or if not too bad, and not soggy, just glass over to keep it together.

My guess is it is not fibreglass to cut noise transmission down from the engine bay. Fibre glass alone would act like the skin of a drum.

[gideon]

The firewall was low density fiberboard (LDF) because its (i) cheap, (ii) light, (iii) insulating for heat and noise, and (iv) strong enough to provide some rigidity as a bulkhead.  It just doesn't last long when it gets wet.  A good solution is to build a sandwich structure which can address items ii, iii and iv above.  You need a lightweight core material with a fiber and resin layer on either side.  I reckon end-grain balsa would make a fine core material for this job, but you could go with a foam core, or even a honeycomb core if you're adventurous. 

As a pessimist I would try to choose materials that are fire resistant and don't emit clouds of toxic fumes when exposed to fire.

[Richard48Y]

My plan is still aluminum honeycomb.

[GavinT]

The reason the firewall openings are there is due to the way the body is manufactured. It's basically a top half and a bottom half joined together.
I made some simple PVC foam core panels which are plenty stiff and waterproof.

[JR73]

Most of the reasons have already been covered - cheap, light, insulating (heat and sound), and is installed the way it is because of the way the body shells are manufactured.

If you are removing the whole original firewall then you should find that there isn’t that much fibreglass holding it in place anyway - strips of chop strand around the perimeter where it meets the shell and across any joints. This is pretty easy to remove (some careful grinding may be required to get back to the main body shell fibreglass) and it also allows you to evaluate where is best for you to position the replacement as small gains are possible here and there in the cabin vs engine bay.
At Banks we used to position slightly more rear wards under the rear window and go straight down to the floor (not following the 45 degree ish angle of the inner shell lower down where it sits over the chassis on an S2 for example) as it allowed more space for either roll cages or a little extra space for taller drivers etc and you can gain a bit of stowage space behind the seats inside the cabin - helps to have the engine in place and anything else like the tanks (if retaining the originals) just to be certain that you are leaving enough clearance.
Used to use pre made flat monolithic fibreglass sheets that where cut to fit (approx 5mm thick) and properly glassed into place, sound deadening came from the carpet etc (doesn’t really matter what you do, the engine is right behind you so whisper quiet it will never be!).
I used to work in the foam core industry until recently and there are benefits of using a sandwich construction providing you are suitably skilled with composites, be sure to check the data sheets in order to select the correct core material and remember that you can reduce the skin thickness when using a core.
I wouldn’t recommend using a balsa core, especially if there are any holes to be cut through the sandwich as moisture can affect it (foam is the preferred modern alternative) and lead to failure. It also absorbs a fair amount of resin which adds weight vs foam alternatives.

Another option is plywood, Lotus used this on Esprit’s right up until the end of production.

[GavinT]

Interesting, JR.
I hadn't really thought of taking the firewall vertically down from the mid-point.
In my mind I'd assumed following the stock line was to properly feed loads from the shell into the middle hump over the chassis but if the original idea was to mitigate some sort of side intrusion, then that's probably all somewhat marginal in any case.

But you make a worthwhile point about cribbing small amounts of space.
My panels are 10mm foam and the intention is to fit them flush with the existing interior bodywork instead of laid on top. To respect the seat angle, the uppermost panel will also be moved back similarly.
I'm 6'2' and a bit so even half an inch of extra rearward seat travel is worth it.