Heat exchanger ? SSI?
 

I have been in the process of getting my 70 911E back on the road, after getting the MFI working again I now need to decide what to do with the HE's. They are both in bad shape and although I am not interested in heat I think they are a major contributor to some backfire issues. I have now been thinking about getting a set of SSI's and other then the stainless not rusting out are there other advantages. The car will not be driven that much but I hope to keep putting into better condition over time.

Apart from the stainless/rust issue you mention, SSI's are basically headers that also allow you to retain heat. They should improve your performance some but on the 2.2 or 2.4l engine I don't know exactly how much. (I think more so on the bigger engines up to 3.0)
The SSI's are also basically a better copy of the factory steel HE's so their design is an improvement over original.

When I bought my replacement engine last year, it came with stainless steel heat exchangers in great condition and I'm very happy with them. Having heat is great and they look nice. I think you'll like heat exchangers if you can find a nice, used set or go for broke and get shiny new ones!

Get a set of SSIs.... look on Pelican for sale .. someone will sell some... then get a M&K exhaust.....

you will never be sorry... the best thing I've done for my SC in the 20 years I've owned it... the boost in power ...and the sound... its like heroin...

Buy the SSI used.... save the $$$

If the SSI are grimy when you get them.. get some easy-ff oven cleaner ... steel wool.. rubber gloves... mild steel /brass detail brush... sunny day and have at it.... they are stainless steel and clean up nice!!!

like Tony said the SSI are a better version of the factory units you already have but I doubt you'll see much if any performance gain from them over the factory units on a 2.2 motor.

What makes you think they are the cause of your backfires?

I think part of my backfire issue is leaking exhausts? I have since tighten things down a bit more and not only is it quiter but it does seem to have less backfire. I think I also have an issue with the MFI micro switch. In looking at all the pictures I think the plunger is gone from my switch so this is also probably contributing.

I agree with Bobby when it comes to a power improvement, although I do believe SSI's can provide a marked improvement in heating performance. The layer of oxide that forms on the stainless SSI exchangers is not as pronounced when compared with the more mild steel OE exchangers. This oxide layer forms an insulating barrier and impacts negatively the thermal conductivity of the steel.