VW engine cooling system components.

An air-cooled VW engine means that the heat of combustion in the cylinders and heads is carried away by the flow of air. By the suction of the fan in the front of the fan shroud, the cooling air is drawn from the outside through slots under the rear window (in the engine lid in later models), forced primarily down to the engine cylinders and heads & secondarily to other engine components then finally through the spaces between the engine fins to carry the heat to the atmosphere.

Various VW cooling System components:
1. The cooling fan.

fan/alternator assembly

This is a centrifugal fan (sends air from the center outwards - like a centrifuge) located in the fan shroud which is bolted to the front of the generator or alternator (depending on what you have) driven by the engine crankshaft pulley via a V-belt. As the fan turns, it displaces air and produces about 5 psi positive pressure inside the shroud. This causes low pressure in front of the fan inlet which allows fresh air to be drawn into the fan forcing it inside the shroud.
Related notes:
a. The VW beetle belt is on with ½" deflection (measurement of belt tension). The fan turns at 1.75x engine speed, meaning if the engine is at 1000 rpm, the fan is at 1750 rpm; hence. a “power” crank pulley is to be avoided because of this as it slows down the fan speed (also generator speed).
b. The number of deck lid air slots increased over the years with greater displacement of the larger fans. This is why there will be overheating issues with a late model 1600 dual port and doghouse cooler with an early model (no vent) decklid. It’s either a vented decklid should be installed or use stand-offs to prop open the decklid. 

3. Fan shroud

The fresh air drawn/forced by the fan inside the shroud is turned downwards by internal vanes to the oil cooler, the cylinder heads and cylinders, the fresh air tubes and a nozzle at the upper right all the way down through the engine cooling fins.

4. Oil coolers- Oil lubricates the engine but also helps cool it. As the oil travels through the various parts of the engine, heat transfers to the oil, which travels through the oil cooler. This heat is then transferred to the air passing through the face of the cooler, and is expelled through ducting under the engine compartment. 

Introduced in the 1971 model year on all upright air-cooled engines, the "doghouse" fan shroud (so called because of the two additional pieces of tin that were added to the front of the shroud to accommodate the new cooler)  provided a more efficient system of cooling the VW engine. The new design re-located the oil cooler (found previously within the shroud, over the number 3 cylinder) by offsetting it away from number three towards the front of the car.  Coupled with a  high volume fan this set up provided more cooling to the VW engine. As a side note, the doghouse fan itself is not interchangeable with non-doghouse shrouds and vice versa.

VW Oil Cooler

Oil Cooler "dog house".

5. Thermostat and flaps
The overall operating temperature of the engine is dictated by the thermostat, which actuates the flaps based on temperature.
a. The O.E.M. VW thermostat looks like this:

b. Flaps and flap mechanisms

This is seen in the bottom of the fan shroud. The flaps on the left and right are pulled closed by the thermostat when the engine is cold via the rod in the left side of the picture. The right and left sides are connected by the cross linkage and are under tension by a spring on the crossbar that is constantly trying to pull the flaps in the open position. When installed on the engine, the rod and thermostat are on the right side of the car. 

Fan House tributaries not for cooling the engine:

5. A nozzle that pumps air into the charcoal canister to scavenge collected fuel tank vapors, and send them to the air cleaner to be burned.

# 3 connects to the top corner of the shroud and fresh air blows
thru the cannister to purge it.  #2 goes to the Air cleaner.

More for Heating the VW Cabin than cooling the engine.

6. Fresh air tubes- cardboard/foil hoses that connect the outlets on either side of the fan housing to the heater boxes. These carry fresh air from the engine fan down through the engine tinware and connect with the heater boxes providing air to be heated for use throughout the vehicle.

Below shows the fresh air outlets and tubes the general flow of air. The yellow shows the flow of air towards the fresh air outlets and tubes.The red shows air moving toward the thermostat flaps where it will be directed at the heads and cylinders.

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