The firewall is basically a separation between the engine compartment and the cabin or the rest of the aircraft. Its function is to make sure that in the event of an engine fire the aircraft structure and its occupants and protected from the heat and flames long enough to execute an emergency landing (FAR PART 23.1191).
Another function of the firewall is to have a convenient surface to install essential components for the operation of the engine. It also is the place where, among other items, engine controls are fed through to the engine and wires from pressure, temperature and flow sensors are routed back to the engine instruments or EFIS.
With composite aircraft the firewall is a combination of the composite structure, a ceramic blanket and a stainless steel sheet. This is different from aluminum aircraft and it also serves to protect the composite against high temperatures.
Installing the engine is sometimes seen as a 'marriage' between airframe and motor. Have some champagne and friends nearby when the job is completed!
From the US Federal Aviation Regulations: The following materials may be used in firewalls or shrouds without being tested as required by section 23.1191:
Most aircraft kit manufacturers supply you with a sheet of the correct material so you will be safe in that department. Remember when drilling steel, do it slowly with a sharp drill.
Metal aircraft usually have the firewall sheet mounted directly onto the aircraft fuselage, see Piper, Cessna. But when you have a composite aircraft like a Pelican PL, DynAero MCR then an extra sheet of fiberfrax high temperature ceramic blanket is installed between the firewall sheet and the aircraft bulkhead.
Please do not use asbestos. You may find it still in very old aircraft when restoring these. Be careful with it and remove it the proper way.
With the firewall installed and still clean without any parts its time to layout the items needed for the engine and to plan any openings where controls should come through the firewall. In your kit there will be layout plans for that for the most commonly used engines. If you install something different then you need to discuss this with the engine manufacturer to obtain any drawings for the engine mount and other requirements.
Drilling into the firewall steel will result in a hole(s) with very sharp edges. And thats not going to help fuel or brake lines when they need to pass through it. Even electrical wiring will not like these sharp edges. For this purpose grommets are required to properly install and pass these items through the firewall without chafing them.
You will need different sizes of these handy thingies. They will protect the wiring or lines passing through them and seal the firewall to protect the cabin. Some builders use in combination with these grommets a fire retardant sealant to prevent any leakage of fumes or gases into the cockpit, but do this after the firewall is finished and needs no more modifications.
If you are going to use AN fittings to pass fuel and other fluid lines through the firewall then this will result in a chafe free connection point for those items without disturbing anything on the other side of the firewall, plus it looks very professional!
This is going to be the first item to be installed on the firewall. Its bulky and large although not very heavy but it dictates where everything else is going to be placed. Some engine mounts include the nose gear of the aircraft, make sure that its wheel and all of that is ready too. You will need it after hanging the engine.
This is usually one of the first items that get installed on the firewall after the engine mount, it will also need the lowest position. Make sure that should you plan to fly with fuel containing ethanol its rubber parts are suitable for that and that the fuel connections are large enough for the expected fuel flow.
Some engine manufacturers (Rotax 912/4/5) ship their voltage regulator separately from the alternator, as such this item must be installed close to the battery with short thick wiring to keep voltage losses at a minimum.
The battery, being heavy and bulky, is also one of the items installed together with the engine mount. As mentioned before: these large items dictate where everything else is going to be installed. In some aircraft the battery is installed in the tail section, use thick wires for connection!
Next item of interest is the cabin heat box (in case of air cooled engines) or the hoses for the cabin heat radiator (water cooled engines). You will need heat resistant tubing toward the firewall. On the cabin side of the firewall there will be a deflector to control the airflow to the floor or to the windshield for defrosting.
This is installed at the highest point on the firewall, above the level of the brake cylinders in the cabin so that any air will find a way out. It also has a small hole drilled to let air in or out of the reservoir.
These will scratch you arms when working on the firewall as they sit on the edges. Use enough of these so that the top and bottom cowlings are secured properly but not so many as to prevent easy daily preflight inspections by the pilot.