The air-cooled engine is very simple, that is the main reason why it was used that often in motorcycles, stationary pumps, and petrol-engined road tools. In this guide, we’ll cover multiple topics about the air-cooled engine: how the engine works, the engine removal process, the engine reinstallation process,…
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Not only were these engines simple, but they were also very light due to the absence of radiators, water pumps, thermostats, coolant or hoses. The “flat-four engine” is an internal combustion engine with horizontally-opposed cylinders. This engine was easy to maintain: from an early stage of development, the arrangement of totally separate cylinders was used on most of these engines, to allow air to flow right around each cylinder. Because of this separate cylinders could be quickly removed from the engine, to gain access to the piston and con-rod.
Back in the days, air-cooled engines were as efficient as water-cooled designs, however, they had a few drawbacks. One of the disadvantages of these engines was that they were very noisy (caused by the absence of a water jacket and the noise of the cooling fan).
How the engine works
You don’t have to be a mechanic to understand the working of a VW Beetle engine. When you put a small amount of gasoline in a small, enclosed space and ignite it, an immense amount of energy is released in the form of expanding gas. If you can create a chain reaction that allows you to set off burns like hundreds of times per minute, and if you can capture that energy and make full use of it, then you have created yourself an internal combustion engine.
An internal combustion engine works in 4 phases:
- Intake stroke: cool air inlet through the piston at the top, the intake valve opens, and the piston moves down and sucks the gasoline / air mixture from the intake manifold into the cylinder. Only the tiniest drop of gasoline needs to be mixed with air in the carburetor for this cycle to work.
- Compression stroke: with the cylinder or barrel now filled with the fuel/air mixture, both the intake and exhaust valve are closed and the piston starts back up on the compression stroke. Compression of the mixture makes the rapid burning in the cylinder more powerful. The more you can compress the air mixture before ignition, the more power you have after ignition.
- Power stroke: just before the piston reaches the top of its stroke, the points open, generating a high voltage spike in the coil, which is fed to the spark plug, and the spark plug emits a spark to ignite the compressed gasoline/air mixture. The gasoline charge in the cylinder burns very rapidly, causing very high pressure in the cylinder, driving the piston down, transmitting the power of the chemical reaction in the cylinder through linear power via the piston and connecting rod to rotational power in the crankshaft.
- Exhaust stroke: once the piston hits the bottom of its stroke, the piston moves back up and the exhaust valve opens to vent the exhaust gases from the cylinder to go out the tailpipe.
The components of most air-cooling systems are quite simple. The cooling fan is positioned in a semicircular ducting. The ducting also covers the cylinder head and its interior is fitted with baffles that channel the flow of air over the engine cooling fins and through an oil cooler. Below the cylinders, the air is directed over a thermostat, which operates a valve via a lever. The valve controls the amount of air reaching the flywheel, thus maintaining the correct engine temperature. After passing over the engine and thermostat, the air is forced out of the rear of the car or passed through a heat transfer system that supplies hot water to the car’s heater.
One problem associated with the use of air-cooled engines is the provision of an adequate heating and demisting system for the car. Water-cooled engines always have a constant supply of hot water and it is easy enough to convert this into hot air. Air-cooled engines usually have an independent heater or harness the heat of the exhaust system. Some older models have heating systems that combine both of these methods.
The cylinder head and cylinders or “barrels” to use the motorcycle term, of an air-cooled VW Beetle engine are cast with fins. These fins spread the heat of the engine over a wide area. If a barrel is made without fins and is 15 cm (6in.) long, all its heat will be spread over that length. If the barrel is manufactured with 10 fins, each 5 cm (2in.) deep the same amount of heat will be dispersed over 100 cm (40in.). This will lower the overall barrel temperature and permit the air greater access to surfaces that most require cooling. The engine-driven fan directs a blast of cool air on to the fins. The fan is necessary because an air-cooled engine requires a very large airflow: 4,000 times more air than water, by volume, is needed to cool an engine, so the flow of air created by the car cannot be relied upon.
Definition piston: the pistons move back and forth in the cylinders in accordance with the strokes of the combustion cycle (4 phases) and are sealed to the machined sides of the cylinders by piston rings.
Definition flywheel: the Flywheel is a round, heavy machined piece of steel attached to the front of the crankshaft. The flywheel serves a triple purpose. The machined face acts as part of the clutch. There are teeth all around the outer circumference which the starter motor engages to turn the engine over for starting. It also serves as a heavy rotating body bolted to the end of the crankshaft to maintain the revolving inertia of the engine through its cycles, changing the power pulses to a smooth power output. An air-cooled engine will accelerate quicker with a light flywheel (which are used most of the time for drag racing).
Definition crankshaft: this component is fastened to the flywheel with four pins and a gland nut. It runs in four bearings, called main bearings, which are massive and pressure-lubricated. The crankshaft has four cranks, or throws, which serve to translate the back and forth movement of the four pistons, through the connecting rods, into the circular motion of the crankshaft and flywheel assembly. The connecting rod bearings are also pressure lubricated. Both the main and connecting rod bearings are thin shells which can be replaced as they wear.
Definition fin: a cooling fin is a surface that extends from an object to increase the rate of heat transfer to or from the environment by increasing convection. The amount of conduction, convection, or radiation of an object determines the amount of heat it transfers.
Definition fan: a fan is a blower which circulates air down through the spaced between the fins to carry the heat to the atmosphere.
Definition baffle: the rate of heat transfer from the cylinder walls can be substantially increased by using baffles which force the air through the space between the fins.
Having to remove an air-cooled engine is usually the result of mechanical breakdown, or performance improvements.The removal process is rather easy when you have a few extra hands to help you out. Before you start, make sure you have these tools: a small trolley jack, four jack stands, a jack pedestal, and something to make two engine pedestals (bricks, boards,…).
- Disconnect the battery earth cable.
- Loosen the nuts on the rear wheels, as they will need to be removed.
- Raise the front of the car as far as the trolley jack allows and support it securely on jack stands.
- Remove the rear wheels.
- Return to the front of the car. Elevate the trolley jack using the jack pedestal. Raise the front of the car as much as possible and extend the jack stands or place blocks of wood securely under them as needed.
- Return to the rear and repeat the process. Continue raising the front and rear of the car until it has been raised high enough for the fan shroud to clear the lowest point under the rear of the car as the engine is pulled out.
- Remove the jack pedestal from under the car temporarily.
- Remove the air cleaner and any emissions hoses from the fan housing.
- Carefully remove the fresh air hoses and lay them aside where they will not be damaged.
- Remove the alternator/generator wires.
- Disconnect the wiring harness in the engine compartment, automatic choke, backup lights, oil pressure switch.
- Disconnect the accelerator cable and pull it from the front through the fan housing.
- Remove all of the rear tinware from around the engine pulley, and the small “box” shaped tinware pieces near the outer ends of the heat riser tube.
- Remove all of the rubber from around the rear of the engine.
- If your car is equipped with a vacuum-advance distributor with the silver vacuum canister on the side, loosen the distributor clamp with a 10mm socket and turn the distributor all the way to the left to get the vacuum chamber out of the way, otherwise, it will hit the rear apron as the engine is lowered.
- Remove the muffler as follows:
- Remove the cover plates over the intake manifold preheater pipe connections. [Dave – If I’m not mistaken – these must already have been removed to move the lower rear tinware – I think they are screwed to that rear tinware aren’t they – working from memory here.]
- Unbolt the preheater pipes from the muffler.
- Remove the nuts, bolts, and clamps that join the heat exchanger outlet pipes to the flanged pipes on the muffler.
- Remove the four nuts (two on each side) that hold the muffler to the rear cylinder heads. Note that on most VW models, there is a piece of metal carburetor preheater tubing on the right side (close to the cylinder head finning) which is attached to one of the exhaust flange studs.
- Pull the muffler to the rear and off of the car.
- Crawl under the car and remove the heater cables from the lever arm assemblies on the heater boxes and the flexible tubes off of the heater boxes.
- Remove the fuel line from the fuel pump and stuff a pencil into the end. Pull the fuel line forward through the firewall and attach a hose clamp firmly onto the engine end. Tie the fuel line back out of the way of the engine tinware. It helps to have the tank nearly empty to reduce any leaks from the fuel line.
- Rebuild the pedestal under the center of the engine and place the trolley jack on top of it.
- Make platforms of blocks and nominal 2-inch wood on either side of the pedestal that the jack is resting on, underneath the heater boxes. The total height of these platforms will need to be a couple of inches higher than the low point of the jack after the engine is off of it so that the jack and pedestal can be removed, leaving the engine resting on the two-block/wood platforms.
- Place a piece of 2×4 across the trolley jack, centered under the oil sump. Snug the jack up with the wood across the engine case for added support.
- With the engine supported by the jack with a piece of 2×4 under the oil sump, remove the four nuts/bolts that connect the engine to the transaxle. Each of these connections is different; they are described as follows:
- Upper right: This is a free bolt which also serves as the starter motor upper bolt. It is most easily disconnected by removing the nut inside the engine bay, in front of the fan shroud on the right-hand side, using a 17mm box-end wrench (ring spanner). Once the nut is removed, just pull the bolt forward and leave it in place.
- Upper left: This is also a free bolt located just inboard of the clutch lever at the top of the transaxle. The nut in the engine bay is captive on “doghouse oil cooler” engines, so the bolt must be removed from the front, as there is no access in front of the fan shroud. This is best done using a 6-inch extension on a 1/2-inch ratchet with a 17mm nut. Approach the bolt with the long extension above the clutch cable. A good light up under there is a great help.
Access is easier on the older “internal oil cooler” engines, as the nut is available in front of the fan shroud just as on the right side.
- Lower right and left: These two connection points are studs which come out with the engine. The nuts are located at the ends of the frame forks on the lower side of the transaxle. They are most easily removed with a 17mm box-end wrench (ring spanner).
- Pull the engine back on the trolley jack; wiggle it up and down and back and forth if necessary. Lower the engine VERY slowly while pulling it back enough to clear the gearbox drive shaft; don’t lower it too much until the engine is clear of the gearbox drive shaft.
- Lower a bit, pull back, lower a bit, pull back, etc. Support the engine with one hand on the fan housing to keep it from tipping either from front-to-back or side-to-side as you are lowering it.
- All of a sudden you’ll find the engine is supported only by the trolley jack.
- Gently lower the engine so that the heater boxes are resting on the block/wood platforms.
- Carefully raise the engine again and remove one block from each side, then lower the engine onto the side pedestals and lower the jack pedestal by one tier. Repeat the process until the engine is lowered as far as possible with the jack (it will still be resting on about three or four blocks). Remove the jack and any remaining jack pedestal pieces.
- Run the creeper about halfway under the engine.
- Manually lift one side of the engine and remove one of the remaining supports. Repeat on the other side and so forth until the engine is resting on the creeper.
- Pull the engine (on the creeper) out from under the car so it can be worked on.
After you have made the necessary adjustments to the engine, you must put it back in place. Reinstallation of the engine is essentially the reverse of the removal.
- Wipe off the mating surfaces of the engine and transmission. Lightly lubricate the manual transmission rear drive shaft splines with molybdenum disulfide powder. On engines equipped with Bosch starters, lubricate the starter drive bushing in the transmission case with multipurpose grease.
- Position the engine under the rear of the car.
- Position pedestal blocks under the heater boxes on either side. With a helper, lift one side of the engine and place another block under it, then the other side. Raise the engine high enough to operate the floor jack under it.
- Carefully raise the engine, placing one block at a time on each of the two pedestals on either side and adding layers to the jack pedestal as needed. Hold the engine with your hand on the top of the fan housing to keep it from flopping around while.
- Position the two bottom studs in the engine in the holes at the bottom of the bell housing. Continue to slowly raise the engine until it is level (on the same plane as the transaxle). Look carefully at the jack height to make sure that the engine is “lined up” with the gearbox shaft.
- Engage first gear, set the parking brake, then hand-turn the crankshaft until the splines mesh.
- Push and wiggle, push and wiggle — turn the crankshaft a bit if necessary. It may resist a little at first, then “pop” into place.
- If the rubber engine seal between the engine tine and the body is intact, check to make sure that it is aligned properly and securely positioned within the groove provided for it around the perimeter of the engine compartment.
- With the engine still supported by the jack, replace the four nuts/bolts that connect the engine to the transaxle.
- Torque all four mounting fasteners to 22 ft-lb.
- Clear away the two side pedestals that were used to support the engine as it was raised. Leave the jack pedestal in place, as it will be used to lower the car.
- Run the fuel line forward through the grommet in the firewall tin on the left side. Attach the fuel line to the fuel pump.
- Make sure that the accelerator cable guide tube is properly positioned through the fan housing and the hole in the top of the firewall tin. Run the accelerator cable into the guide tube and attach it to the carburetor.
- Attach the heater cables to the lever arm assemblies on the heater boxes.
- Attach the flexible couplings to the heater boxes and secure with hose clamps on both ends.
- Reconnect the wire harness in the engine bay:
- Black wire from the ignition switch to the (+) terminal on the coil;
- Black wires from the (+) terminal on the coil idle to the idle cut-off switch, automatic choke, and backup (reversing) lights;
- Separate wire from the “oil” light in the instrument cluster to the oil pressure switch.
- Reconnect the alternator wires:
- Heavy red wire from the B+ terminal on the alternator through the firewall to the battery;
- Green wire from the D+ terminal on the alternator to be spliced with the blue wire that runs forward to the “Alt” light in the instrument cluster.
- Reinstall the muffler as follows:
- Place gaskets over the four bolts (two on each side) where the muffler attaches to the rear cylinder heads.
- Note: Use two gaskets on each side to assure a tight seal.Place tightening ring and donut gasket on the heat exchanger outlet pipes.
- Position the muffler in place on the rear cylinder heads and the heat exchanger outlet pipes.
- LOOSELY attach the nuts on the rear cylinder heads.
- Bolt the pre-heater pipes to the muffler using new gaskets, then tighten the rear cylinder head nuts as tightly as possible.
- Attach the heat exchanger outlet pipes to the flanges pipes on the muffler with the clamps, bolts, and nuts. Make sure the donut gasket is properly positioned before tightening the nuts completely.
- Install the cover plates over the intake manifold pre-heater pipe connections.
- Reinstall the rear engine tinware.
- Reinstall the fresh air hoses.
- Reinstall the air cleaner and the crankcase breather tube.
- Reconnect the grounding strap on the battery.
- Adjust the clutch.
- Add engine oil.
- Adjust the valves.
- Replace the wheels.
- Lower the car.
- Tune the engine.
As you know already, you can adjust a Beetle to your needs. You can repaint the body, change the steering wheel, install new running boards and lots of other cool stuff. The ultimate part of your project is the VW Beetle engine swap. The possibilities for an engine swap aren’t endless, but they are certainly vast.
A very popular engine to swap is the Subaru EJ25 or EJ257, especially when you want some big power. The 2.5-liter turbocharged engine comes from the Subaru WRX STI.
Some other engines you could swap into a Beetle:
- Mazda 20B-REW engine
- Mazda KF-ZE 2.0L V6 engine
- Honda K20Z1 engine
- Toyota 4-AGE Redtop engine
Consider these tips when selecting a used swap engine:
- First, look for signs of a head gasket leak. Head gaskets can leak internally and externally. An internal leak might show coolant in the oil or a sludgy film in the overflow bottle. An external leak will show wetness around the base of the cylinder head as it meets the block.
- Check the mileage. Subaru engines last a long time, there is even a half a million-mile club for them. However, settle for en engine with a little less.
- Look for missing parts or cut wires. Often time a wrecking yard will have multiples of the same cars but make sure you’re not missing anything critical.
- Spin the engine over using a wrench and socket, see if compression is good. Also, feel for valves hitting the head or a broken rod.
- Pull the cam belt covers off and look at the condition if the belts, and if the timing belt has jumped teeth – SOHC notches on crank and cam pulleys all face straight up, DOHC has marks on the belt that line up with cam and crank pulley notches.
Engine swap kit
You will find it difficult to find a fully prepared engine swap kit. Yet we can set you on the road with the materials you will need fur a full engine swap. Here’s an overview:
- Engine, harness, and ECU
- Adapter plate and flywheel
- Oil and filter
- Timing belt kit with water pump and tensioner
- Plug for brake boost line
- 90 degree ⅝” for heater hose
- Belt for alternator
- MAF sensor to air filter 3”
- Air filter
- Fuel pump
- Fuel filter
- Fuel line high pressure 5/16, or 3/8 with 5/16 adapters
- Larger fitting and return line for the fuel tank
- Cooling lines
- Drain cock for easily bleeding system
- O2 sensor nut bung
- Vacuum caps
- EGR block off (exhaust gas recirculation and it is a federal standard for emissions)
- Throttle cable connector and bracket
- Vehicle speed sensor
- Exhaust (or performance exhaust)
- Metal oil separator
- Metal thermostat housing
- Intake manifold flip
- Hose clamps, vacuum line, wire shrouding, electrical tape, and random hardware
In case you need air cooled VW parts for your Beetle or Bug, visit our parts section for more information. Submit your parts request and it will be sent to multiple parts suppliers so that you get the best offer.