How do the heat exchangers work?

I am just curious. How do the heat exchangers work? How to they keep exhaust gas from entering the cabin? I know how water cooled cars heat the cabin, but what about air cooled? Thanks!

The heat exchangers consist of solid tubes that connect the exhaust port to the muffler, surrounded by an outer sheet metal plenum. Air from the engine fan (supplemented by the heater blower moter) blows through the plenum, gets heated by the hot pipes, and the gets directed to the heater flapper boxes. When the flapper boxes are shut (heat off), the hot air from the heat exchangers gets vented under the car. As you lift the heat control knobs, the flapper boxes open to let some of the hot air into the cabin.

If the interior exhaust pipes are rusted out, then you will get exhaust into the cabin. (Another good reason to get the SSI exhaust). Also, I've noticed that if oil leaks from the lower valve covers, it can seep into the heat exchangers and give you a noticeable burnt oil smell.

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Bill Krause
'79 911SC Euro

Burnt oil smell??? I call it a 911 fragrance.

Also FWIW there were no heater blowers on the early cars. I don't recall when that was added, but it was after 71. In other words, in the early cars you don't get much heat unless you're moving, then you get a little. Plenty for those 'brisk' SoCal mornings. Not sure about Green Bay or Buffalo style cold...

Don't use the heat much here in Florida either, but I had a '74 Volks that melted an entire tool box into a puddle of plastic. The heat exchanger setup can be quite efficient! Funny as hell the hotfoot syndrome back seaters had- takes a while to heat a shoe, but once its hot........

Joe
'80 SC Targa

The best part about the heat exchangers, is that you get hot air almost immediately. We've already had some 20 degree days up here in Minnesota, so the quick heat is appreciated.



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Bill Krause
'79 911SC Euro