How does it work - heat exchangers

[Nico911]

Hi guys.

I'm trying to figure how does hot air ends up in the cabin on a early 911 (let's say we're talking about a 3,2 Carrera). I mean, I guess the air is heated but the the engine (while cooling it) but I don't get what the heat exchangers are for... ?

Thanks for clarifying that for me.

Nico

[dporsche74]

that big ol fan in the engine bay .. blows the air over the exhaust headers up thru the rockers inside the car. all types of swtiches and leavers to STOP it .. yes it gross. unless it new and oil free.
996 has a nice heatting system

[dale914]

There's tons of descriptive info on how airplane heaters work. Same thing. That may be easier to understand

[Arne2]

Heat exchangers are air chambers that encase the exhaust headers. On a 3.2 the air is forced down into the exchangers to be heated by the headers, and from there up into the tunnel and into the passenger compartment. The function of the exchangers is to keep the air that is being heated separate from the outside air, trapped in close proximity to the headers.

[tirwin]

And as long as you have no exhaust leaking in, it works great. Almost too great...

[Nico911]

Originally Posted by Arne2

Heat exchangers are air chambers that encase the exhaust headers. On a 3.2 the air is forced down into the exchangers to be heated by the headers, and from there up into the tunnel and into the passenger compartment. The function of the exchangers is to keep the air that is being heated separate from the outside air, trapped in close proximity to the headers.

Great ! I'm learning...

So is there a continuous flow that brings air to the heat exchangers ?
And how the air that is forced in by the fan to cool the engine exits ?

[Eric Hahl]

The levers in between the seats allow you to direct the air into the cabin heating system or to bypass the system and dump the hot air out underneath the car.

[tazzieman]

Originally Posted by Nico911

Great ! I'm learning...

So is there a continuous flow that brings air to the heat exchangers ?
And how the air that is forced in by the fan to cool the engine exits ?

As long as that fan operates , air is forced past the exhaust and along the duct towards the cabin. Where it goes next depends upon whether you have the levers beside the seats up or down.

The other pathway for fan air is over the cylinders and out under the car.

HE : imagine a sealed hot water pipe. Thread a larger diameter pipe over the top and blow through one end. What comes out the other is what heats the cabin.

[James Brown]

it's called a air-to-air heat exchanger. cool air traveling past hot pipes, like a crude intercooler in reverse. just like the old VW's or big jets. simple and it works

[1986 930]

Let me assure you that you can cook yourself right out of an older Porsche with those heat exchangers in just about any winter climate unless you are down in the negative degree F range. I would roll down the windows with the heat maxed out on cold Midwest days when I lived in Oklahoma years ago. (20-30 degrees F)


They work very well and give you that wonderful smell of an old Porsche. (That also extends to air cooled VW Bugs which I am a big fan of as well. Yes, I know, sacrilege!)


The neat thing about '89 or older 911s is that you can put the heat on your feat and some fresh air on the uppers and be real comfy since the heat and fresh air vents work independently.

[psalt]

So is there a continuous flow that brings air to the heat exchangers ?
And how the air that is forced in by the fan to cool the engine exits ?[/QUOTE]

The belt driven fan blows some forced air through a duct to the electric heater fan and some (most) over the cylinder fins to cool the engine. The air that cools the engine exits out the bottom of the cylinders and oil cooler and is not the air used for cabin heating. The cabin heating air flows past the electric fan, through hoses, through the heat exchangers, up to the heater valves and is either dumped out or directed to the cabin based on the levers by the hand brake. An important point to recognize is that the exhaust pipes in the heat exchangers are cooled by a large constant flow of air, even when the heater is not on. Heat block off plates are sometimes marketed as "improving cooling" which is nonsense, cylinder head temperature and detonation increases when you block off the flow to the enclosed header exhaust pipes. Porsche knew this and some later models have a thermostatic switch to turn on the electric booster fan to prevent overheating.

[911pcars]

Originally Posted by psalt

So is there a continuous flow that brings air to the heat exchangers ?
And how the air that is forced in by the fan to cool the engine exits ?

The belt driven fan blows some forced air through a duct to the electric heater fan and some (most) over the cylinder fins to cool the engine. The air that cools the engine exits out the bottom of the cylinders and oil cooler and is not the air used for cabin heating. The cabin heating air flows past the electric fan, through hoses, through the heat exchangers, up to the heater valves and is either dumped out or directed to the cabin based on the levers by the hand brake. An important point to recognize is that the exhaust pipes in the heat exchangers are cooled by a large constant flow of air, even when the heater is not on. Heat block off plates are sometimes marketed as "improving cooling" which is nonsense, cylinder head temperature and detonation increases when you block off the flow to the enclosed header exhaust pipes. Porsche knew this and some later models have a thermostatic switch to turn on the electric booster fan to prevent overheating.

Paul,
Good reminder to all. What is equipped on a race/track car (e.g. open headers, minimal/no air filters, roll bar next to bare head, etc.) may not be conducive on a street car.

Sherwood