Someone mentioned engine mount bolt torques.  On my 360 -- with a fiberglass covered plywood firewall -- I had troubles getting the FAA-specified bolt torques, but I did get it done.  I since read that many builders double up the large area washers under the nut on the cold side of the firewall.  I pulled my engine mount off (for other reasons) and when I put it back, I got the next size longer bolt and did just that. 

The original washers -- I think they are AN970-7 -- were ever so slightly dished, indicating that there was a small amount of crushing of the plywood.  With the double washers, all of that is gone.  It would seem as though here is another place where a lower than normal torque spec might be appropriate, but these bolts are loaded up in tension so I'd prefer the double-washer approach.

I don't know what Legacy firewalls are made of, but I don't think it's plywood.  I will say that if the manual call for a less-than-normal torque, there's probably a good reason.

BTW, there are places where aluminum washers may be safely substituted for the steel ones.  On the hot side of the firewall, you need 1/4 inch spacers to provide a hard bearing surface for the engine mount so that you don't crush the fire proofing material.  Aluminum washers are fine there as the required strength is provided by the steel pad on the engine mount.  On the cold side of the firewall, however, you need to have steel to distribute the load.  When I swapped out the front area washers for aluminum ones, I saved 237 grams (one half pound).  If you are contemplating changing your old-style fiberfrax-plus-steel firewall for the newer quilted stuff, well, I did it and saved a total of three pounds.  I can provide the weight breakdown to anyone who's interested.

- Rob Wolf