Why did the 911 use a horizontally opposed engine from 1964-on?
It's an interesting design study. Other car makers to use a flat engine have been:
GM
Subaru
Tatra
Volkswagen
Toyota
Ford
BMW
Tucker
& others! (https://en.wikipedia.org/wiki/Flat_engine)
The first horizontally opposed air-cooled engine was patented by Carl Benz in 1896 and first used in production in 1900. (https://en.wikipedia.org/wiki/Flat-twin_engine)
A horizontally opposed engine has the following advantages:
A lower center of gravity - and in the 911 with its dry sump, it is as low as possible.
They are shorter than an in-line engine, allowing for efficient packaging
They have good engine balance (smoother/less vibration) with 6 cylinders here; they have perfect primary & secondary balance. (like a straight 6 does)
They do not have a "rocking couple" (the engine does not try to rotate on its mounts)
Since the arrangement is symmetrical there is no net turning moment along the access of movement by the pistons.
Well suited to air-cooling.
Disadvantages of a flat engine:
They are very wide which limits their placement in the car
They need a large intake manifold if they use a single carburetor
You need to duplicate ports if you use water cooling
You need more cylinders, heads, cams and so on to built a 2nd side.
Advantages of air-cooling
Simplicity: no water pump, no belt, no radiator, no styling impact to fitting a radiator and airflow into the body
Light weight (no radiator, water pump, water or water hoses)
No coolant to boil or freeze or cause rust
Simpler castings since you do not need plumb oil and water through the engine together and keep them separate
Disadvantages of air-cooling
You are limited in how much power you can make, since more power means more heat and there is a point at which you cannot get enough cooling
Air-cooled engines tend to be louder
You may or may not like the fan noise coming from the engine compartment
Adjacent - why did the 911 use a flat-plane crankshaft?
They can be lighter since they don't need heavy counterweights in a flat 6 engine thanks to the lack of shaking.
The engine can thus be lighter
The pistons and rods can also be lighter, since they don't have to balance out a big counter-weight
The engine is more responsive (less mass to spin around)
You can make more horsepower at higher RPMs (suck in more air, burn more fuel)
The above points also should indicate why the engines were also over-square - having a piston diameter larger than the engine's stoke. This also optimizes for high RPM power and responsiveness.
The simpler castings enabled in-part by air-cooling allowed Porsche to use 1 casting for all 6 cylinders, 1 casting for all 6 heads, & 1 casting for both rocker/cam housings. They could also create engine cases of Aluminum or Magnesium (instead of cast iron) since the crank was not trying to shake out of its bearings.
The overall design bias here is very race-car like - low center of gravity, low weight, high rpms and simplicity. The transaxle they chose is also more efficient than a car with a drive shaft so more of the power you make gets to the wheels.
The dry dump oil system enables the car to corner at high g-forces without cavitating the oil pump (pumping air instead of oil and possibly leading to engine damage or failure).
People sometimes call the 911 a street-legal race car and it can sound a bit egotistical, but there is rather a lot of evidence supporting the idea.
Dr. Porsche of course was the designer of the original VW which has a complex history with both the Tatra as well as the Nazi party of the 1930's and 40's. (links below) But it used a flat air-cooled 4-cylinder for its entire run (65 years!) and when Porsche launched his own car company after the war, the 356 had an air-cooled flat-four powering it that looked an awful lot like the VW engine. Going from a 4 cylinder to a 6 for their new flagship sports car in 1964 is not a big stretch to understand.
Just for fun, here is a Triumph Spitfire block and crank; heavy cast iron, 4-cylinders, a standard (heavy) crankshaft and 3 main bearings to hold it all in place. When it was originally 800cc and made 25hp that was probably fine. This is looking at the bottom of the block; the sump bolts over this. (the engine is upside down here)
And here is a 911 engine that has been "split" - the case (block) is made of 2 halves made of Aluminum (sometimes Magnesium), bolted together. Here you can the lighter crankshaft with smaller counterweights and 7 main bearings to take the load. The connecting rods are a gold color. That large shaft below the crank is driving the oil pump (lower LH side) as well as the camshafts (the chain to the right) The screened dome is the oil pickup.
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