what does this fan do?
 

If I understand right, this fan (in pic below) is taking hot air from engine bay to the front. Then the smaller fan on each side of the foot-well blow hot air onto your feet.

Question is if this fan has the option to blow hot air outside to cool your engine bay (cool oil temp)?
Thanks

http://cdn4.pelicanparts.com/techarticles/Porsche_911/ENGINE-Heater-Blower-Motor-Replacement/images_large/pic01.jpg

It's job is to boost the pressure of the air going through the heater boxes for cabin heat and defrost. It take air off before it passes through the cooling fins of the motor or the oil cooler, so it does nothing to aid in engine cooling.

well? not really...it also helps to cool the engine down..here is some reading .
http://forums.pelicanparts.com/porsche-911-technical-forum/707552-engine-compartment-blower-fan.html

Not sure where the idea got started that the heater fan in some way cools down the engine compartment. The fan takes "engine cooling air" from the shroud and pushes it through the heat exchangers instead of the engine. Perhaps you could make a claim that cooling the heat exchangers might cool the engine in some way but that fan only runs when you pull the levers to turn on the heat.
The fan takes cool air from under the engine shroud in the engine bay and redirects it to the heat exchangers.
It is a common misconception that the air in the engine bay is hot. It is being pulled in at a very high rate so it hasn't had time to heat up.
This misconception probably comes from the fact that the air in the engine compartment heat soaks very rapidly once the engine stops running. Also when you open the engine compartment you will feel IR heat but remember that IR won't heat air. IR will only heat surfaces.

BTW - The engine compartment heater fan is the main fan to supply heat to the passenger compartment. The footwell blowers act as boosters to increase the quantity of heat but they aren't the primary supply of hot air.

I haven’t really noticed any improvement in cabin heat in my ‘84 Carrera over my old ‘75 Targa with SSI backdated exchangers. The simplicity and reliability of the old design seems preferable. If you needed more heat, you simply dropped down a gear for a bit!

I suspect the idea of the later 3-blower system was not to improve over the old system, but merely to make the volume of air less dependent on revs. So it would work in stop and go traffic, for example.

I remember an old Christophorus article chowing that the air movement wo the aux fan was a straight line paralling engine revs

with the aux fan the air move was an almost horizontal line, keeping heat constant. It also improved air movement at idle & you are trying to defrost the windshield

well?.... here is more about the extras cooling when engine is too hot....
Ivan

https://www.pelicanparts.com/techarticles/911_extra_cooling/911_extra_cooling.htm

My fan turns on when I turn the ignition on...75s uk car...mike fitz

can I summarize this to see if I get it correct?
So this fan has 2 features:
1- help blowing hot air from inside engine shroud to foot-well when the heat level is lift.
2- blowing heat from inside the engine shroud to the 2 heat valves underneath the car?

Tach articles to the contrary, I am unconvinced the relatively minuscule add-on blower contributes anything to engine cooling. I think the case could even be made (minor though it is) that it is stealing air from engine cooling (bypassing the engine) for occupant comfort.

In hot weather and slow traffic in notice a engine temp drop if I turn on the heater. Or am I imagining things?

Possibly, if blowing air over the exhaust system and cooling it results in less heat transfer back up. But none of the air pushed by that blower passes over the cylinder heads, barrels, or oil cooler(s).

something like that

Ivan

For my car, it sits out back of the shop providing a spider habitat.....

Mine too.

I thought it was an extra Kg or two it didn't need over the back axle.

Those are both the same thing. If the heater valves are open the air is dumped under the car. When they are closed it forces the air up to where the footwell blowers live.

The reason the air flow isn't shut off is that if there is no airflow the heat from the heat exchangers will rise up through their inlets and start cooking stuff. The plastic tube will melt almost immediately.

If interested, a circuit can be added to The engine-mounted blower to continue blowing after engine OFF with a temp sensitive relay (many Asian cars use for continuous radiator cooling after engine shutdown). Relays can also be sourced for time OFF delay.

When engine is off, blower will push shroud air downward through the heat exchangers, then out to atmosphere through the closed (to the cabin) heater valves. Any reduction of hot air is good.

An interesting idea but the functioning of the heat control valves might be a problem.

The heater valves simply dump the hot air to the atmosphere when they are opened. And opened means that "heat" is off. The valve has no mechanism to block heat from continuing to go into the cabin. This has always bugged me as I really hate heat unless it is actually cold out.

I've wondered if there is pressure induced into the ventilation system that would prevent continuous low level heating. Maybe the cowl air intake (a high pressure area when moving) provides the pressure needed to stop any heat from coming forward. The 911's ventilation system is such a hodgepodge I don't know if anyone understands it enough that they could answer that. :confused:

This is one of the cable-controlled heater valves directing hot air from the engine to the cabin (65-89). Correct. In the hot air closed, "heat is off" position as shown, hot air from the heat exchangers is directed to atmosphere. In the "hot air open position", hot air from the heat exchangers is directed to the cabin.

http://forums.pelicanparts.com/uploa...1563990385.jpg

It's a mystery whether hot air in the closed position bleeds into the cabin rather than completely directing to atmosphere. However with engine OFF, if the engine-mounted blower is temporarily active (as in many modern cars), residual engine heat can be exhausted instead of being trapped inside the engine/engine compartment.

Sherwood