HORIZONTALLY-OPPOSED (BOXER) CONFIGURATION
because the cylinder head is the highest point; spark plug replacement and valve train access is very easy. Vibrations are minimal because the outside two pistons move in the opposite direction of the inside two pistons.
All this makes the four-cylinder inline engines incur low manufacturing costs.
However, secondary vibrations are not balanced, which limits the size of the engine. Rarely will an inline-four exceed 2.5 litres with some having 3 litres. These bigcapacity engines will often require balancing shafts to cancel the vibration caused by the secondary imbalance.
Another disadvantage is a higher centre of gravity compared with some layouts, reduced somewhat by angling the cylinder block. The long crankshaft means the engine is also not as rigid as other shorter crankshaft layouts. Not a common layout, except when the air-cooled Beetle was a volume seller, but an excellent configuration from an engineering point of view. It is especially attractive due to the easy packaging it allows. Subaru places it under the front bonnet whereas the Beetle and Porsche has it in the rear.
Primary and secondary vibrations are well balanced, meaning the boxer is a smooth engine. This also allows for lighter components such as the pistons and the crankshaft, resulting in less power-sapping inertia. The boxer has an inherently low centre of gravity which allows for better handling.
However, boxers are very wide engines. A shorter stroke and clever cylinder head design reduces this disadvantage somewhat. Two cylinder heads (and sometimes two cylinder blocks) make the engine more complex and more expensive to manufacture. The offset required in the crankpin produces a slight vibration due to rocking couple.
Maintenance can be challenging, if packaging is tight. Removable panels in the engine bay are a regular feature. The Beetle just requires a few bolts removed before the engine drops out, thus this is not a disadvantage in its case. Conversely, doing a valve adjustment on an Impreza is a challenging prospect. the inline-six also makes design, manufacturing, construction and maintenance simple.
The simple design also makes V12s and Flat-12s a simple extension of the six, as they are simply two inline-sixes patched together.
Packaging can be difficult due to the length of the crankshaft. Front -wheel drive? Not an option. Just like an inline-four, the inline-six also suffers from a high centre of gravity.