Since I started this thread I've been deeply occupied with rebuilding my 3.0 engine, but now I'm at a point where I've got everything pretty well figured out regarding how to backdate the heat in a 911 SC. I've got access to my car (1983) and my brother's car (1979) so it covers a pretty good range over the life of the SC. In reality, the cars aren't that much different. We did remove the air injection system on the '79, which does affect the ductwork to an extent, and I'll show how that affected the process on the '79. I'll be using photos gleaned from the forum, as well as my own photos of my '83 (both with engine in and out of the car) and the '79 as we backdated the heat on both cars. I started this thread mostly to do my own research and get some help from this community along the way. I felt moved to post this, to consolidate the info I learned earlier in the thread, and to give something back here since I've learned so much in the past year. I should add that I don't know much about 911's. I've owned one for less than a year, but hopefully this will be helpful to another novice like me. I'm simply sharing my experience. Experts and experienced folks are more than welcome to chime in and correct any errors. I welcome it in fact. Here goes....
First off let's define what backdated heat is. I learned that a lot of the time "backdated heat" and "backdated exhaust" can be confusing terms, especially since both the heat system and exhaust system are linked through the heat exchangers. For the SC, the stock heat system includes an electric blower placed in the engine compartment that turns on when you raise the heat levers between the seats inside the cabin. The levers close off the heater boxes under the car and allow hot air to enter the cabin by means of the engine cooling fan blowing through the fiberglass fan shroud, out the hot air duct, up a long air duct, through the electric blower, down through two hoses which passes through the engine tin (and the right one takes a long route to get there), then into the heat exchangers and over the hot exhaust pipes, out the heat exchangers, through more hose, then finally into the cabin. Once you pull up on the levers to the maximum, a switch turns on the electric blower which gives a boost to the air coming into the cabin from the cooling fan. So why would you want to backdate the heat? Two reasons I've found. The first is: easier access to the engine. Without the electric blower and its extra ducting, one can work on the left side of the engine much easier. Second, I've read that because of the extra ducting and circuitous route the air from the cooling fan has to take with the stock setup, the effectiveness of the hot air is lessened, even though you'd think the electric blower would compensate for that. Backdating the heat system on an SC does away with the electric blower and its maze of hoses and ducts and replaces it with two older-style hot air ducts that flank the cooling fan.
So what is backdated exhaust? It doesn't have much to do with backdated heat. Instead, it means using heat exchangers and a muffler from the earlier cars, 1974 and earlier (or something close to that.) Whether you backdate your heat or not, the heat system connects to stock or earlier heat exchangers pretty much the same way: via two hoses at the rear of the engine. You can backdate the heat and exhaust independently from each other.
How the systems work:
Stock:
Here you can see the route the air has to take to get from the cooling fan to the warm pipes in the heat exchangers. Lots of twists and turns, not to mention getting through the very restricted passage of the squirrel-cage-style electric blower (yellow arrow). All heated air must pass through this blower. This is a '79 with the air-injection pump still in place in the lower left hand corner and the mounting bracket for the A/C compressor still in place at the lower right. We'll be removing both of these on the '79, but leaving the A/C in the '83.
Here's the same car with backdated heat.
To get the job started, first remove everything highlighted below.
I removed the large plastic duct at the left first. It connects to the hot air duct directly behind the distributor, and at the left side of the electric blower. Below I've removed the plastic duct.
Loosening the various hose clamps is next. I was careful to not damage too much of the ducting. If its in good shape it can be reused.
Now I remove the duct that goes to the right heat exchanger. I also removed the two hose mounting brackets on top of the shroud. They are not needed with the backdated heat.
It may be difficult to remove the hose out from underneath the A/C bracket. I remove the bracket anyway since I'll need to access the shroud behind it. I remove five fasteners (yellow arrows). The left most arrow points to a bolt that joins a bent steel bracket from the engine case to the compressor mount. It's a little tricky to get to. The two lower bolts are pretty easy to access. The top two arrows point to two smaller nuts that hold this aluminum bracket to the top of the cam housing. I use a small 10mm swivel socket and some extensions to get to the front one.
With the A/C bracket out of the way, I remove the hose. The yellow arrow points to a coupler that joins the two sections of hose. It's got to come off. We thought we could reuse this section, but we ended up up removing the entire hose down to the heat exchanger.
Next I remove the block off plate from the right side of the cooling fan. Arrows point to the three screws/bolts that need to come off. I was careful not to loose the wire retaining tab on the top most screw.
Next I disconnected the electric connector for the electric blower and then removed the blower motor itself. It'd held onto its mounting bracket with a metal strap thats pretty easy to remove. You'll have to use some clever movements to get the blower past the fuel lines without damaging them, but it will come out. The bracket has some of the CIS components mounted to it, so it has to stay (yellow arrow.) I've also taken the air-injection pump off in the '79 in the pic below. I also removed the bracket later on.
