What purpose does the oil expansion tank?

are they required?

do just allow extra oil for the motor?

why do they have one

it is not an "oil expansion" tank.
it is an oil tank
it is required because the 911 engine used in the 914/6 is dry-sumped.

meaning - the oil is pumped from the bottom of the engine to the tank by one oil pump, and oil is fed to the bearings from the tank by another oil pump.

search here for "dry sump" and you'll see all the benefits of this system.

six motors are all dry sumped, which means it does not have the typical oil pan to hold the oil...sooooo, the oil must be stored in another location...hence the oil tank...a benifit of a remote tank is being able to store more oil which is a good thing since the more oil you have, the less likely it is to break down from the heat and abuse in the motor.

for a dry sump engine, the oil pump sucks up the oil, takes it to a cooler and then the tank, the tank will have baffles in it to help eliminate foaming of the oil...the oil pump will then suck cool oil from the tank and force it into the oil galleys of the motor...I'm sure someone can give a better description, but you should get the general idea..

Lack of a great pool of oil in the crankcase allows the crank to be placed lower or to have more clearance. The 911 engine rides a little higher than the 914/4 for more clearance. Crank and tranny input shaft are in line. It also reduces the friction of the crank and so on moving through the oil. In the 914/4 the cam rides below the crank. In the 911 I think the intermediate shaft lies below the crank. Better for power.

Not only can it hold more oil w/o a big pan dangling below the engine, but the engine itself is shorter vertically, so it can be mounted lower in the car for a better CG. Also, because the pressure pump is fed from a relatively vertical baffled tank rather than a generally horizontal sump, oil slosh from hard cornering is much less likely to cause oil starvation. It's for these two reasons, primarily, that nearly all serious race engines are dry-sumped. Really serious (read: for cars with lots of downforce) race engines have multiple scavenge pumps so they can ensure they get the oil out of the tiny "sump" even if the car is running a 3-4G for several seconds, and all of the oil is held against one side of this "sump". F1 engines have 6-8 scavenge pumps since those cars regularly see > 3G in braking and turning, and over 1G even under acceleration.

Reading all of the other threads about AX cars having oil starvation problems, and how tuna cans and deep sumps usually hit something with dire results, is a clear demonstration that the 914 is a good candidate for a dry-sump engine.

Like this